From f375440bd4fcbaf2fbbb51b4afb7ecda8d50895b Mon Sep 17 00:00:00 2001 From: John Kerl Date: Sun, 15 Feb 2026 09:24:38 -0500 Subject: [PATCH] Support YAML I/O (#1963) * docs/src/proposal-yaml-io.md * initial step * Add `--y2c` etc * test cases * docs * more testing * more * more test cases * git rm docs/src/proposal-yaml-io.md * make dev * ylistwrap -> yarray --- Makefile | 11 +- docs/src/features.md | 6 +- docs/src/features.md.in | 6 +- docs/src/file-formats.md | 51 +++- docs/src/file-formats.md.in | 14 +- docs/src/glossary.md | 6 +- docs/src/glossary.md.in | 6 +- docs/src/index.md | 4 +- docs/src/index.md.in | 4 +- docs/src/manpage.md | 45 ++- docs/src/manpage.txt | 45 ++- docs/src/mk-flag-info.rb | 25 +- docs/src/reference-main-flag-list.md | 31 +- docs/src/reference-main-separators.md | 3 +- docs/src/reference-main-separators.md.in | 3 +- go.mod | 2 +- man/manpage.txt | 45 ++- man/mlr.1 | 45 ++- pkg/cli/option_parse.go | 284 +++++++++++++++++- pkg/cli/option_types.go | 4 + pkg/cli/separators.go | 4 + pkg/input/record_reader_factory.go | 2 + pkg/input/record_reader_yaml.go | 153 ++++++++++ pkg/mlrval/mlrval_yaml.go | 202 +++++++++++++ pkg/mlrval/mlrval_yaml_test.go | 196 ++++++++++++ pkg/output/record_writer_factory.go | 2 + pkg/output/record_writer_yaml.go | 91 ++++++ pkg/terminals/help/entry.go | 10 + pkg/terminals/regtest/regtester.go | 3 + .../c2y/cmd | 1 + .../c2y/experr | 0 .../c2y/expout | 50 +++ .../d2y/cmd | 1 + .../d2y/experr | 0 .../d2y/expout | 50 +++ .../j2y/cmd | 1 + .../j2y/experr | 0 .../j2y/expout | 50 +++ .../m2y/cmd | 1 + .../m2y/experr | 0 .../m2y/expout | 50 +++ .../n2y/cmd | 1 + .../n2y/experr | 0 .../n2y/expout | 50 +++ .../p2y/cmd | 1 + .../p2y/experr | 0 .../p2y/expout | 50 +++ .../t2y/cmd | 1 + .../t2y/experr | 0 .../t2y/expout | 50 +++ .../x2y/cmd | 1 + .../x2y/experr | 0 .../x2y/expout | 50 +++ .../y2c/cmd | 1 + .../y2c/experr | 0 .../y2c/expout | 11 + .../y2c/input | 50 +++ .../y2d/cmd | 1 + .../y2d/experr | 0 .../y2d/expout | 10 + .../y2d/input | 50 +++ .../y2j/cmd | 1 + .../y2j/experr | 0 .../y2j/expout | 10 + .../y2j/input | 5 + .../y2m/cmd | 1 + .../y2m/experr | 0 .../y2m/expout | 12 + .../y2m/input | 50 +++ .../y2n/cmd | 1 + .../y2n/experr | 0 .../y2n/expout | 10 + .../y2n/input | 50 +++ .../y2p/cmd | 1 + .../y2p/experr | 0 .../y2p/expout | 11 + .../y2p/input | 50 +++ .../y2t/cmd | 1 + .../y2t/experr | 0 .../y2t/expout | 11 + .../y2t/input | 50 +++ .../y2x/cmd | 1 + .../y2x/experr | 0 .../y2x/expout | 59 ++++ .../y2x/input | 50 +++ test/cases/io-yaml-io/0001/cmd | 1 + test/cases/io-yaml-io/0001/experr | 0 test/cases/io-yaml-io/0001/expout | 3 + test/cases/io-yaml-io/0001/input | 7 + test/cases/io-yaml-io/0002/cmd | 1 + test/cases/io-yaml-io/0002/experr | 0 test/cases/io-yaml-io/0002/expout | 5 + test/cases/io-yaml-io/0002/input | 5 + 93 files changed, 2108 insertions(+), 122 deletions(-) create mode 100644 pkg/input/record_reader_yaml.go create mode 100644 pkg/mlrval/mlrval_yaml.go create mode 100644 pkg/mlrval/mlrval_yaml_test.go create mode 100644 pkg/output/record_writer_yaml.go create mode 100644 test/cases/io-format-conversion-keystroke-savers/c2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/c2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/c2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/d2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/d2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/d2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/j2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/j2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/j2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/m2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/m2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/m2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/n2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/n2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/n2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/p2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/p2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/p2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/t2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/t2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/t2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/x2y/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/x2y/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/x2y/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2c/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2c/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2c/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2c/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2d/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2d/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2d/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2d/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2j/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2j/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2j/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2j/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2m/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2m/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2m/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2m/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2n/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2n/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2n/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2n/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2p/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2p/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2p/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2p/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2t/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2t/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2t/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2t/input create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2x/cmd create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2x/experr create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2x/expout create mode 100644 test/cases/io-format-conversion-keystroke-savers/y2x/input create mode 100644 test/cases/io-yaml-io/0001/cmd create mode 100644 test/cases/io-yaml-io/0001/experr create mode 100644 test/cases/io-yaml-io/0001/expout create mode 100644 test/cases/io-yaml-io/0001/input create mode 100644 test/cases/io-yaml-io/0002/cmd create mode 100644 test/cases/io-yaml-io/0002/experr create mode 100644 test/cases/io-yaml-io/0002/expout create mode 100644 test/cases/io-yaml-io/0002/input diff --git a/Makefile b/Makefile index ec2b817cb..91a7187a0 100644 --- a/Makefile +++ b/Makefile @@ -92,19 +92,20 @@ staticcheck: # * turns *.md.in into *.md (live code samples), using mlr from the $PATH # * note the man/manpage.txt becomes some of the HTML content # * turns *.md into docs/site HTML and CSS files -dev: - -make fmt - make build +dev: fmt build make check make -C man build make -C docs/src forcebuild make -C docs @echo DONE -docs: build +docs: fmt build make -C docs/src forcebuild make -C docs +man: fmt build + make -C man build + # ---------------------------------------------------------------- # Keystroke-savers @@ -124,4 +125,4 @@ release_tarball: build check # ================================================================ # Go does its own dependency management, outside of make. -.PHONY: build mlr check unit_test regression_test bench fmt staticcheck dev docs +.PHONY: build mlr check unit_test regression_test bench fmt staticcheck dev docs man diff --git a/docs/src/features.md b/docs/src/features.md index ae1222a3f..4cca80a80 100644 --- a/docs/src/features.md +++ b/docs/src/features.md @@ -17,7 +17,7 @@ Quick links: # Features Miller is like awk, sed, cut, join, and sort for **name-indexed data, such as -CSV, TSV, JSON, and JSON Lines**. You get to work with your data using named +CSV, TSV, JSON, JSON Lines, and YAML**. You get to work with your data using named fields, without needing to count positional column indices. This is something the Unix toolkit always could have done, and arguably always @@ -25,7 +25,7 @@ should have done. It operates on key-value-pair data while the familiar Unix tools operate on integer-indexed fields: if the natural data structure for the latter is the array, then Miller's natural data structure is the insertion-ordered hash map. This encompasses a **variety of data formats**, -including but not limited to the familiar CSV, TSV, JSON, and JSON Lines. +including but not limited to the familiar CSV, TSV, JSON, JSON Lines, and YAML. (Miller can handle **positionally-indexed data** as a special case.) * Miller is **multi-purpose**: it's useful for **data cleaning**, **data reduction**, **statistical reporting**, **devops**, **system administration**, **log-file processing**, **format conversion**, and **database-query post-processing**. @@ -42,7 +42,7 @@ including but not limited to the familiar CSV, TSV, JSON, and JSON Lines. * Miller is **pipe-friendly** and interoperates with the Unix toolkit -* Miller's I/O formats include **tabular pretty-printing**, **positionally indexed** (Unix-toolkit style), CSV, JSON, and others +* Miller's I/O formats include **tabular pretty-printing**, **positionally indexed** (Unix-toolkit style), CSV, JSON, YAML, and others * Miller does **conversion** between formats diff --git a/docs/src/features.md.in b/docs/src/features.md.in index 13ea25bb2..b3d674f67 100644 --- a/docs/src/features.md.in +++ b/docs/src/features.md.in @@ -1,7 +1,7 @@ # Features Miller is like awk, sed, cut, join, and sort for **name-indexed data, such as -CSV, TSV, JSON, and JSON Lines**. You get to work with your data using named +CSV, TSV, JSON, JSON Lines, and YAML**. You get to work with your data using named fields, without needing to count positional column indices. This is something the Unix toolkit always could have done, and arguably always @@ -9,7 +9,7 @@ should have done. It operates on key-value-pair data while the familiar Unix tools operate on integer-indexed fields: if the natural data structure for the latter is the array, then Miller's natural data structure is the insertion-ordered hash map. This encompasses a **variety of data formats**, -including but not limited to the familiar CSV, TSV, JSON, and JSON Lines. +including but not limited to the familiar CSV, TSV, JSON, JSON Lines, and YAML. (Miller can handle **positionally-indexed data** as a special case.) * Miller is **multi-purpose**: it's useful for **data cleaning**, **data reduction**, **statistical reporting**, **devops**, **system administration**, **log-file processing**, **format conversion**, and **database-query post-processing**. @@ -26,7 +26,7 @@ including but not limited to the familiar CSV, TSV, JSON, and JSON Lines. * Miller is **pipe-friendly** and interoperates with the Unix toolkit -* Miller's I/O formats include **tabular pretty-printing**, **positionally indexed** (Unix-toolkit style), CSV, JSON, and others +* Miller's I/O formats include **tabular pretty-printing**, **positionally indexed** (Unix-toolkit style), CSV, JSON, YAML, and others * Miller does **conversion** between formats diff --git a/docs/src/file-formats.md b/docs/src/file-formats.md index 8a09dac54..c43cd27f7 100644 --- a/docs/src/file-formats.md +++ b/docs/src/file-formats.md @@ -17,7 +17,7 @@ Quick links: # File formats Miller handles name-indexed data using several formats: some you probably know -by name, such as CSV, TSV, JSON, and JSON Lines -- and other formats you're likely already +by name, such as CSV, TSV, JSON, JSON Lines, and YAML -- and other formats you're likely already seeing and using in your structured data. Additionally, Miller gives you the option to include comments within your data. @@ -62,6 +62,16 @@ JSON Lines (sequence of one-line objects): Record 1: "apple":"1", "bat":"2", "cog":"3" Record 2: "dish:egg":"7", "dish:flint":"8", "garlic":"" +YAML (single document = object or array of objects; multiple documents with ---): ++---------------------+ +| apple: 1 | +| bat: 2 | Record 1: "apple":"1", "bat":"2", "cog":"3" +| cog: 3 | +| --- | +| dish: 7 | Record 2: "dish":"7", "egg":"8" +| egg: 8 | ++---------------------+ + PPRINT: pretty-printed tabular +---------------------+ | apple bat cog | @@ -408,6 +418,18 @@ won't reject your input data for lack of outermost `[...]`, nor will it reject y of newlines. The difference is on _output_: using `--ojson`, you get outermost `[...]` and pretty-printed records; using `--ojsonl`, you get no outermost `[...]`, and one line per record. +## YAML + +Miller supports YAML as an I/O format in the same spirit as JSON: input can be a single YAML document +that is an object (one record) or an array of objects (one record per element), or multiple YAML +documents separated by `---` (each document is an object or array of objects). Output is either one +YAML document per record (with `---` between documents) or a single YAML document that is an array +of all records, controlled by the same kind of list-wrap behavior as JSON. + +Use `--yaml`, `--iyaml`, or `--oyaml` (or `-i yaml` / `-o yaml`). Nested structures and types are +handled like JSON: nested objects become dotted keys when flattened; types are preserved through +the stream. + ## PPRINT: Pretty-printed tabular Miller's pretty-print format is similar to CSV, but with column alignment. For example, compare @@ -708,20 +730,21 @@ While you can do format conversion using `mlr --icsv --ojson cat myfile.csv`, th 
 FORMAT-CONVERSION KEYSTROKE-SAVER FLAGS
 As keystroke-savers for format-conversion you may use the following.
-The letters c, t, j, l, d, n, x, p, and m refer to formats CSV, TSV, DKVP, NIDX,
-JSON, JSON Lines, XTAB, PPRINT, and markdown, respectively.
+The letters c, t, j, l, d, n, x, p, m, and y refer to formats CSV, TSV, JSON, JSON Lines,
+DKVP, NIDX, XTAB, PPRINT, markdown, and YAML, respectively.
 
-| In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown |
-+----------+----------+----------+----------+-------+-------+-------+-------+--------+----------|
-| CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    |
-| TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    |
-| JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    |
-| JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    |
-| DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    |
-| NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    |
-| XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    |
-| PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    |
-| Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          |
+| In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown | YAML   |
++----------+----------+----------+----------+-------+-------+-------+-------+--------+----------+--------+
+| CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    | --c2y  |
+| TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    | --t2y  |
+| JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    | --j2y  |
+| JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    | --l2y  |
+| DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    | --d2y  |
+| NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    | --n2y  |
+| XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    | --x2y  |
+| PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    | --p2y  |
+| Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          | --m2y  |
+| YAML     | --y2c    | --y2t    | --y2j    | --y2l | --y2d | --y2n | --y2x | --y2p  | --y2m    | --y2y  |
 
 -p                       Keystroke-saver for `--nidx --fs space --repifs`.
 -T                       Keystroke-saver for `--nidx --fs tab`.
diff --git a/docs/src/file-formats.md.in b/docs/src/file-formats.md.in
index 2ed581b19..0ac5223eb 100644
--- a/docs/src/file-formats.md.in
+++ b/docs/src/file-formats.md.in
@@ -1,7 +1,7 @@
 # File formats
 
 Miller handles name-indexed data using several formats: some you probably know
-by name, such as CSV, TSV, JSON, and JSON Lines -- and other formats you're likely already
+by name, such as CSV, TSV, JSON, JSON Lines, and YAML -- and other formats you're likely already
 seeing and using in your structured data.
 
 Additionally, Miller gives you the option to include comments within your data.
@@ -168,6 +168,18 @@ won't reject your input data for lack of outermost `[...]`, nor will it reject y
 of newlines. The difference is on _output_: using `--ojson`, you get outermost `[...]` and pretty-printed
 records; using `--ojsonl`, you get no outermost `[...]`, and one line per record.
 
+## YAML
+
+Miller supports YAML as an I/O format in the same spirit as JSON: input can be a single YAML document
+that is an object (one record) or an array of objects (one record per element), or multiple YAML
+documents separated by `---` (each document is an object or array of objects). Output is either one
+YAML document per record (with `---` between documents) or a single YAML document that is an array
+of all records, controlled by the same kind of list-wrap behavior as JSON.
+
+Use `--yaml`, `--iyaml`, or `--oyaml` (or `-i yaml` / `-o yaml`). Nested structures and types are
+handled like JSON: nested objects become dotted keys when flattened; types are preserved through
+the stream.
+
 ## PPRINT: Pretty-printed tabular
 
 Miller's pretty-print format is similar to CSV, but with column alignment.  For example, compare
diff --git a/docs/src/glossary.md b/docs/src/glossary.md
index 774975c41..b9e5a5e22 100644
--- a/docs/src/glossary.md
+++ b/docs/src/glossary.md
@@ -448,6 +448,10 @@ record is an object written on a single line, without need to be wrapped an
 outermost list. This format helps people interoperate with non-JSON-aware
 tools in the [Unix toolkit](#unix-toolkit) which generally operate on lines.
 
+## YAML
+
+A human-readable [file format](file-formats.md#yaml) for structured data supported by Miller. Input can be a single document (object or array of objects) or multiple documents separated by `---`. Output can be one top-level array document or one document per record.
+
 ## key
 
 The string index in a [map](#map). Also, the name of a field in a [record](#record).
@@ -464,7 +468,7 @@ trying to define a [local variable](#local-variable) `if = 3` will result in a p
 A subsequence of a text file in between line-ending symbols such as the special linefeed character.
 Tools in the [Unix toolkit](#unix-toolkit) generally operate on lines; Miller is designed to do
 that (using the [NIDX format flags](file-formats.md#nidx-index-numbered-toolkit-style)), as well
-as non-line-oriented formats such as [CSV, TSV, JSON, and others](file-formats.md).
+as non-line-oriented formats such as [CSV, TSV, JSON, YAML, and others](file-formats.md).
 
 ## local variable
 
diff --git a/docs/src/glossary.md.in b/docs/src/glossary.md.in
index b8eb8f417..e47dc611b 100644
--- a/docs/src/glossary.md.in
+++ b/docs/src/glossary.md.in
@@ -432,6 +432,10 @@ record is an object written on a single line, without need to be wrapped an
 outermost list. This format helps people interoperate with non-JSON-aware
 tools in the [Unix toolkit](#unix-toolkit) which generally operate on lines.
 
+## YAML
+
+A human-readable [file format](file-formats.md#yaml) for structured data supported by Miller. Input can be a single document (object or array of objects) or multiple documents separated by `---`. Output can be one top-level array document or one document per record.
+
 ## key
 
 The string index in a [map](#map). Also, the name of a field in a [record](#record).
@@ -448,7 +452,7 @@ trying to define a [local variable](#local-variable) `if = 3` will result in a p
 A subsequence of a text file in between line-ending symbols such as the special linefeed character.
 Tools in the [Unix toolkit](#unix-toolkit) generally operate on lines; Miller is designed to do
 that (using the [NIDX format flags](file-formats.md#nidx-index-numbered-toolkit-style)), as well
-as non-line-oriented formats such as [CSV, TSV, JSON, and others](file-formats.md).
+as non-line-oriented formats such as [CSV, TSV, JSON, YAML, and others](file-formats.md).
 
 ## local variable
 
diff --git a/docs/src/index.md b/docs/src/index.md
index bcb69c8ed..e7d72c341 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -16,7 +16,7 @@ Quick links:
 
 # Introduction
 
-**Miller is a command-line tool for querying, shaping, and reformatting data files in various formats, including CSV, TSV, JSON, and JSON Lines.**
+**Miller is a command-line tool for querying, shaping, and reformatting data files in various formats, including CSV, TSV, JSON, JSON Lines, and YAML.**
 
 **The big picture:** Even well into the 21st century, our world is full of text-formatted data such as CSV. Google _CSV memes_, for example. We need tooling to _thrive in this world_, nimbly manipulating data which is in CSVs. And we need tooling to _move beyond CSV_, to be able to pull data out and into other storage and processing systems. Miller is designed for both of these goals.
 
@@ -39,7 +39,7 @@ pretty-print your data horizontally or vertically to make it easier to read.
 mlr --json head -n 1 myfile.json
-The `sort`, `head`, etc., are called *verbs*. They're analogs of familiar command-line tools like `sort`, `head`, and so on -- but they're aware of name-indexed, multi-line file formats like CSV, TSV, and JSON. In addition, though, using Miller's `put` verb, you can use programming-language statements for expressions like +The `sort`, `head`, etc., are called *verbs*. They're analogs of familiar command-line tools like `sort`, `head`, and so on -- but they're aware of name-indexed, multi-line file formats like CSV, TSV, JSON, and YAML. In addition, though, using Miller's `put` verb, you can use programming-language statements for expressions like 
 mlr --csv put '$rate = $units / $seconds' input.csv
diff --git a/docs/src/index.md.in b/docs/src/index.md.in
index 25073a3f1..6ade85f28 100644
--- a/docs/src/index.md.in
+++ b/docs/src/index.md.in
@@ -1,6 +1,6 @@
 # Introduction
 
-**Miller is a command-line tool for querying, shaping, and reformatting data files in various formats, including CSV, TSV, JSON, and JSON Lines.**
+**Miller is a command-line tool for querying, shaping, and reformatting data files in various formats, including CSV, TSV, JSON, JSON Lines, and YAML.**
 
 **The big picture:** Even well into the 21st century, our world is full of text-formatted data such as CSV. Google _CSV memes_, for example. We need tooling to _thrive in this world_, nimbly manipulating data which is in CSVs. And we need tooling to _move beyond CSV_, to be able to pull data out and into other storage and processing systems. Miller is designed for both of these goals.
 
@@ -23,7 +23,7 @@ GENMD-SHOW-COMMAND
 mlr --json head -n 1 myfile.json
 GENMD-EOF
 
-The `sort`, `head`, etc., are called *verbs*. They're analogs of familiar command-line tools like `sort`, `head`, and so on -- but they're aware of name-indexed, multi-line file formats like CSV, TSV, and JSON. In addition, though, using Miller's `put` verb, you can use programming-language statements for expressions like
+The `sort`, `head`, etc., are called *verbs*. They're analogs of familiar command-line tools like `sort`, `head`, and so on -- but they're aware of name-indexed, multi-line file formats like CSV, TSV, JSON, and YAML. In addition, though, using Miller's `put` verb, you can use programming-language statements for expressions like
 
 GENMD-SHOW-COMMAND
 mlr --csv put '$rate = $units / $seconds' input.csv
diff --git a/docs/src/manpage.md b/docs/src/manpage.md
index f14427fe8..b3f893c2d 100644
--- a/docs/src/manpage.md
+++ b/docs/src/manpage.md
@@ -94,6 +94,16 @@ This is simply a copy of what you should see on running `man mlr` at a command p
          Record 1: "apple":"1", "bat":"2", "cog":"3"
          Record 2: "dish:egg":"7", "dish:flint":"8", "garlic":""
 
+       YAML (single document = object or array of objects; multiple documents with ---):
+       +---------------------+
+       | apple: 1            |
+       | bat: 2              | Record 1: "apple":"1", "bat":"2", "cog":"3"
+       | cog: 3              |
+       | ---                 |
+       | dish: 7             | Record 2: "dish":"7", "egg":"8"
+       | egg: 8              |
+       +---------------------+
+
        PPRINT: pretty-printed tabular
        +---------------------+
        | apple bat cog       |
@@ -407,6 +417,7 @@ This is simply a copy of what you should see on running `man mlr` at a command p
        --itsvlite               Use TSV-lite format for input data.
        --iusv or --iusvlite     Use USV format for input data.
        --ixtab                  Use XTAB format for input data.
+       --iyaml                  Use YAML format for input data.
        --json or -j or --j2j    Use JSON format for input and output data.
        --jsonl or --l2l         Use JSON Lines format for input and output data.
        --nidx or --n2n          Use NIDX format for input and output data.
@@ -423,12 +434,14 @@ This is simply a copy of what you should see on running `man mlr` at a command p
        --otsvlite               Use TSV-lite format for output data.
        --ousv or --ousvlite     Use USV format for output data.
        --oxtab                  Use XTAB format for output data.
+       --oyaml                  Use YAML format for output data.
        --pprint or --p2p        Use PPRINT format for input and output data.
        --tsv or -t or --t2t     Use TSV format for input and output data.
        --tsvlite                Use TSV-lite format for input and output data.
        --usv or --usvlite       Use USV format for input and output data.
        --xtab or --x2x          Use XTAB format for input and output data.
        --xvright                Right-justify values for XTAB format.
+       --yaml or --y2y          Use YAML format for input and output data.
        -i {format name}         Use format name for input data. For example: `-i csv`
                                 is the same as `--icsv`.
        -o {format name}         Use format name for output data. For example: `-o
@@ -456,20 +469,21 @@ This is simply a copy of what you should see on running `man mlr` at a command p
 
 1mFORMAT-CONVERSION KEYSTROKE-SAVER FLAGS0m
        As keystroke-savers for format-conversion you may use the following.
-       The letters c, t, j, l, d, n, x, p, and m refer to formats CSV, TSV, DKVP, NIDX,
-       JSON, JSON Lines, XTAB, PPRINT, and markdown, respectively.
+       The letters c, t, j, l, d, n, x, p, m, and y refer to formats CSV, TSV, JSON, JSON Lines,
+       DKVP, NIDX, XTAB, PPRINT, markdown, and YAML, respectively.
 
-       | In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown |
-       +----------+----------+----------+----------+-------+-------+-------+-------+--------+----------|
-       | CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    |
-       | TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    |
-       | JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    |
-       | JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    |
-       | DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    |
-       | NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    |
-       | XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    |
-       | PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    |
-       | Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          |
+       | In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown | YAML   |
+       +----------+----------+----------+----------+-------+-------+-------+-------+--------+----------+--------+
+       | CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    | --c2y  |
+       | TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    | --t2y  |
+       | JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    | --j2y  |
+       | JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    | --l2y  |
+       | DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    | --d2y  |
+       | NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    | --n2y  |
+       | XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    | --x2y  |
+       | PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    | --p2y  |
+       | Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          | --m2y  |
+       | YAML     | --y2c    | --y2t    | --y2j    | --y2l | --y2d | --y2n | --y2x | --y2p  | --y2m    | --y2y  |
 
        -p                       Keystroke-saver for `--nidx --fs space --repifs`.
        -T                       Keystroke-saver for `--nidx --fs tab`.
@@ -488,6 +502,10 @@ This is simply a copy of what you should see on running `man mlr` at a command p
                                 the default for JSON Lines output format.
        --no-jvstack             Put objects/arrays all on one line for JSON output.
                                 This is the default for JSON Lines output format.
+       --no-yarray              Do not wrap YAML output in a single array document;
+                                emit one YAML document per record with `---` between.
+       --yarray or --ya         Wrap YAML output in a single top-level array
+                                document. This is the default for YAML output format.
 
 1mLEGACY FLAGS0m
        These are flags which don't do anything in the current Miller version.
@@ -827,6 +845,7 @@ This is simply a copy of what you should see on running `man mlr` at a command p
                pprint   " "    N/A    "\n"
                tsv      "     "    N/A    "\n"
                xtab     "\n"   " "    "\n\n"
+               yaml     N/A    N/A    N/A
 
        --fs {string}            Specify FS for input and output.
        --ifs {string}           Specify FS for input.
diff --git a/docs/src/manpage.txt b/docs/src/manpage.txt
index 39ff434e7..31faedd7d 100644
--- a/docs/src/manpage.txt
+++ b/docs/src/manpage.txt
@@ -73,6 +73,16 @@
          Record 1: "apple":"1", "bat":"2", "cog":"3"
          Record 2: "dish:egg":"7", "dish:flint":"8", "garlic":""
 
+       YAML (single document = object or array of objects; multiple documents with ---):
+       +---------------------+
+       | apple: 1            |
+       | bat: 2              | Record 1: "apple":"1", "bat":"2", "cog":"3"
+       | cog: 3              |
+       | ---                 |
+       | dish: 7             | Record 2: "dish":"7", "egg":"8"
+       | egg: 8              |
+       +---------------------+
+
        PPRINT: pretty-printed tabular
        +---------------------+
        | apple bat cog       |
@@ -386,6 +396,7 @@
        --itsvlite               Use TSV-lite format for input data.
        --iusv or --iusvlite     Use USV format for input data.
        --ixtab                  Use XTAB format for input data.
+       --iyaml                  Use YAML format for input data.
        --json or -j or --j2j    Use JSON format for input and output data.
        --jsonl or --l2l         Use JSON Lines format for input and output data.
        --nidx or --n2n          Use NIDX format for input and output data.
@@ -402,12 +413,14 @@
        --otsvlite               Use TSV-lite format for output data.
        --ousv or --ousvlite     Use USV format for output data.
        --oxtab                  Use XTAB format for output data.
+       --oyaml                  Use YAML format for output data.
        --pprint or --p2p        Use PPRINT format for input and output data.
        --tsv or -t or --t2t     Use TSV format for input and output data.
        --tsvlite                Use TSV-lite format for input and output data.
        --usv or --usvlite       Use USV format for input and output data.
        --xtab or --x2x          Use XTAB format for input and output data.
        --xvright                Right-justify values for XTAB format.
+       --yaml or --y2y          Use YAML format for input and output data.
        -i {format name}         Use format name for input data. For example: `-i csv`
                                 is the same as `--icsv`.
        -o {format name}         Use format name for output data. For example: `-o
@@ -435,20 +448,21 @@
 
 1mFORMAT-CONVERSION KEYSTROKE-SAVER FLAGS0m
        As keystroke-savers for format-conversion you may use the following.
-       The letters c, t, j, l, d, n, x, p, and m refer to formats CSV, TSV, DKVP, NIDX,
-       JSON, JSON Lines, XTAB, PPRINT, and markdown, respectively.
+       The letters c, t, j, l, d, n, x, p, m, and y refer to formats CSV, TSV, JSON, JSON Lines,
+       DKVP, NIDX, XTAB, PPRINT, markdown, and YAML, respectively.
 
-       | In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown |
-       +----------+----------+----------+----------+-------+-------+-------+-------+--------+----------|
-       | CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    |
-       | TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    |
-       | JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    |
-       | JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    |
-       | DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    |
-       | NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    |
-       | XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    |
-       | PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    |
-       | Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          |
+       | In\out   | CSV      | TSV      | JSON     | JSONL | DKVP  | NIDX  | XTAB  | PPRINT | Markdown | YAML   |
+       +----------+----------+----------+----------+-------+-------+-------+-------+--------+----------+--------+
+       | CSV      | --c2c,-c | --c2t    | --c2j    | --c2l | --c2d | --c2n | --c2x | --c2p  | --c2m    | --c2y  |
+       | TSV      | --t2c    | --t2t,-t | --t2j    | --t2l | --t2d | --t2n | --t2x | --t2p  | --t2m    | --t2y  |
+       | JSON     | --j2c    | --j2t    | --j2j,-j | --j2l | --j2d | --j2n | --j2x | --j2p  | --j2m    | --j2y  |
+       | JSONL    | --l2c    | --l2t    | --l2j    | --l2l | --l2d | --l2n | --l2x | --l2p  | --l2m    | --l2y  |
+       | DKVP     | --d2c    | --d2t    | --d2j    | --d2l | --d2d | --d2n | --d2x | --d2p  | --d2m    | --d2y  |
+       | NIDX     | --n2c    | --n2t    | --n2j    | --n2l | --n2d | --n2n | --n2x | --n2p  | --n2m    | --n2y  |
+       | XTAB     | --x2c    | --x2t    | --x2j    | --x2l | --x2d | --x2n | --x2x | --x2p  | --x2m    | --x2y  |
+       | PPRINT   | --p2c    | --p2t    | --p2j    | --p2l | --p2d | --p2n | --p2x | -p2p   | --p2m    | --p2y  |
+       | Markdown | --m2c    | --m2t    | --m2j    | --m2l | --m2d | --m2n | --m2x | --m2p  |          | --m2y  |
+       | YAML     | --y2c    | --y2t    | --y2j    | --y2l | --y2d | --y2n | --y2x | --y2p  | --y2m    | --y2y  |
 
        -p                       Keystroke-saver for `--nidx --fs space --repifs`.
        -T                       Keystroke-saver for `--nidx --fs tab`.
@@ -467,6 +481,10 @@
                                 the default for JSON Lines output format.
        --no-jvstack             Put objects/arrays all on one line for JSON output.
                                 This is the default for JSON Lines output format.
+       --no-yarray              Do not wrap YAML output in a single array document;
+                                emit one YAML document per record with `---` between.
+       --yarray or --ya         Wrap YAML output in a single top-level array
+                                document. This is the default for YAML output format.
 
 1mLEGACY FLAGS0m
        These are flags which don't do anything in the current Miller version.
@@ -806,6 +824,7 @@
                pprint   " "    N/A    "\n"
                tsv      "     "    N/A    "\n"
                xtab     "\n"   " "    "\n\n"
+               yaml     N/A    N/A    N/A
 
        --fs {string}            Specify FS for input and output.
        --ifs {string}           Specify FS for input.
diff --git a/docs/src/mk-flag-info.rb b/docs/src/mk-flag-info.rb
index 3f54fe08a..cbe109817 100755
--- a/docs/src/mk-flag-info.rb
+++ b/docs/src/mk-flag-info.rb
@@ -15,19 +15,22 @@ for section_name in section_names
   if section_name =~ /.*conversion.*keystroke-saver*/
     # The markdown in this section looks a lot better when hand-crafted (thanks Nikos!).
     puts <= recordsPerBatch {
+				readerChannel <- recordsAndContexts
+				recordsAndContexts = make([]*types.RecordAndContext, 0, recordsPerBatch)
+			}
+		} else if mlrval.IsArray() {
+			records := mlrval.GetArray()
+			if records == nil {
+				errorChannel <- fmt.Errorf("internal coding error in YAML record-reader")
+				return
+			}
+			for _, mv := range records {
+				if !mv.IsMap() {
+					errorChannel <- fmt.Errorf(
+						"valid but unmillerable YAML: expected map (object); got %s",
+						mv.GetTypeName(),
+					)
+					return
+				}
+				record := mv.GetMap()
+				if record == nil {
+					errorChannel <- fmt.Errorf("internal coding error in YAML record-reader")
+					return
+				}
+				context.UpdateForInputRecord()
+				recordsAndContexts = append(recordsAndContexts, types.NewRecordAndContext(record, context))
+				if int64(len(recordsAndContexts)) >= recordsPerBatch {
+					readerChannel <- recordsAndContexts
+					recordsAndContexts = make([]*types.RecordAndContext, 0, recordsPerBatch)
+				}
+			}
+		} else {
+			errorChannel <- fmt.Errorf(
+				"valid but unmillerable YAML: expected map (object); got %s",
+				mlrval.GetTypeName(),
+			)
+			return
+		}
+	}
+
+flush:
+	if len(recordsAndContexts) > 0 {
+		readerChannel <- recordsAndContexts
+	}
+}
diff --git a/pkg/mlrval/mlrval_yaml.go b/pkg/mlrval/mlrval_yaml.go
new file mode 100644
index 000000000..9410600bb
--- /dev/null
+++ b/pkg/mlrval/mlrval_yaml.go
@@ -0,0 +1,202 @@
+// ================================================================
+// YAML decode/encode for Mlrval and Mlrmap.
+// Converts between YAML native types (from gopkg.in/yaml.v3) and Miller's
+// record model. YAML maps become Mlrmap; keys are stringified (YAML allows
+// non-string keys). Used by the YAML record reader and writer.
+// ================================================================
+
+package mlrval
+
+import (
+	"fmt"
+	"io"
+	"sort"
+
+	"gopkg.in/yaml.v3"
+)
+
+// MlrvalDecodeFromYAML decodes one YAML document from the decoder into an
+// *Mlrval. Returns (nil, true, nil) on EOF. The decoded value can be a map
+// (one record), array (array of records when elements are maps), or scalar.
+func MlrvalDecodeFromYAML(decoder *yaml.Decoder) (*Mlrval, bool, error) {
+	var doc interface{}
+	err := decoder.Decode(&doc)
+	if err == io.EOF {
+		return nil, true, nil
+	}
+	if err != nil {
+		return nil, false, err
+	}
+	if doc == nil {
+		return NULL, false, nil
+	}
+	mv, err := mlrvalFromYAMLNative(doc)
+	if err != nil {
+		return nil, false, err
+	}
+	return mv, false, nil
+}
+
+// mlrvalFromYAMLNative converts a YAML-decoded value (map[interface{}]interface{},
+// []interface{}, or scalar) into *Mlrval.
+func mlrvalFromYAMLNative(v interface{}) (*Mlrval, error) {
+	if v == nil {
+		return NULL, nil
+	}
+	switch val := v.(type) {
+	case map[interface{}]interface{}:
+		return mlrvalFromYAMLMap(val)
+	case map[string]interface{}:
+		return mlrvalFromYAMLStringMap(val)
+	case []interface{}:
+		return mlrvalFromYAMLArray(val)
+	case string:
+		return FromString(val), nil
+	case bool:
+		return FromBool(val), nil
+	case int:
+		return FromInt(int64(val)), nil
+	case int64:
+		return FromInt(val), nil
+	case uint64:
+		if val <= 1<<63-1 {
+			return FromInt(int64(val)), nil
+		}
+		return FromFloat(float64(val)), nil
+	case float64:
+		return FromFloat(val), nil
+	case float32:
+		return FromFloat(float64(val)), nil
+	default:
+		return FromString(fmt.Sprint(val)), nil
+	}
+}
+
+func mlrvalFromYAMLMap(m map[interface{}]interface{}) (*Mlrval, error) {
+	keys := make([]string, 0, len(m))
+	for k := range m {
+		keys = append(keys, yamlKeyString(k))
+	}
+	sort.Strings(keys)
+	out := FromEmptyMap()
+	for _, keyStr := range keys {
+		var v interface{}
+		for k, val := range m {
+			if yamlKeyString(k) == keyStr {
+				v = val
+				break
+			}
+		}
+		valMv, err := mlrvalFromYAMLNative(v)
+		if err != nil {
+			return nil, err
+		}
+		out.MapPut(FromString(keyStr), valMv)
+	}
+	return out, nil
+}
+
+func mlrvalFromYAMLStringMap(m map[string]interface{}) (*Mlrval, error) {
+	keys := make([]string, 0, len(m))
+	for k := range m {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	out := FromEmptyMap()
+	for _, k := range keys {
+		valMv, err := mlrvalFromYAMLNative(m[k])
+		if err != nil {
+			return nil, err
+		}
+		out.MapPut(FromString(k), valMv)
+	}
+	return out, nil
+}
+
+func yamlKeyString(k interface{}) string {
+	switch t := k.(type) {
+	case string:
+		return t
+	case int:
+		return fmt.Sprintf("%d", t)
+	case int64:
+		return fmt.Sprintf("%d", t)
+	case float64:
+		return fmt.Sprintf("%g", t)
+	default:
+		return fmt.Sprint(k)
+	}
+}
+
+func mlrvalFromYAMLArray(a []interface{}) (*Mlrval, error) {
+	out := FromEmptyArray()
+	for _, elem := range a {
+		mv, err := mlrvalFromYAMLNative(elem)
+		if err != nil {
+			return nil, err
+		}
+		out.ArrayAppend(mv)
+	}
+	return out, nil
+}
+
+// MlrmapToYAMLNative converts an Mlrmap to a Go value suitable for
+// yaml.Marshal: map[string]interface{} with nested maps/arrays.
+func MlrmapToYAMLNative(mlrmap *Mlrmap) (interface{}, error) {
+	if mlrmap == nil {
+		return nil, nil
+	}
+	out := make(map[string]interface{})
+	for pe := mlrmap.Head; pe != nil; pe = pe.Next {
+		v, err := mlrvalToYAMLNative(pe.Value)
+		if err != nil {
+			return nil, err
+		}
+		out[pe.Key] = v
+	}
+	return out, nil
+}
+
+// mlrvalToYAMLNative converts *Mlrval to a Go value for yaml.Marshal.
+func mlrvalToYAMLNative(mv *Mlrval) (interface{}, error) {
+	if mv == nil {
+		return nil, nil
+	}
+	switch mv.Type() {
+	case MT_ABSENT, MT_VOID, MT_NULL:
+		return nil, nil
+	case MT_STRING:
+		s, _ := mv.GetStringValue()
+		return s, nil
+	case MT_INT:
+		i, _ := mv.GetIntValue()
+		return i, nil
+	case MT_FLOAT:
+		f, _ := mv.GetFloatValue()
+		return f, nil
+	case MT_BOOL:
+		b, _ := mv.GetBoolValue()
+		return b, nil
+	case MT_ARRAY:
+		arr := mv.GetArray()
+		if arr == nil {
+			return []interface{}{}, nil
+		}
+		out := make([]interface{}, 0, len(arr))
+		for _, elem := range arr {
+			v, err := mlrvalToYAMLNative(elem)
+			if err != nil {
+				return nil, err
+			}
+			out = append(out, v)
+		}
+		return out, nil
+	case MT_MAP:
+		m := mv.GetMap()
+		return MlrmapToYAMLNative(m)
+	case MT_ERROR, MT_PENDING:
+		return mv.String(), nil
+	default:
+		return mv.String(), nil
+	}
+}
diff --git a/pkg/mlrval/mlrval_yaml_test.go b/pkg/mlrval/mlrval_yaml_test.go
new file mode 100644
index 000000000..0c40d58e3
--- /dev/null
+++ b/pkg/mlrval/mlrval_yaml_test.go
@@ -0,0 +1,196 @@
+// ================================================================
+// Tests for YAML decode/encode (MlrvalDecodeFromYAML, MlrmapToYAMLNative).
+// ================================================================
+
+package mlrval
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"gopkg.in/yaml.v3"
+)
+
+func TestMlrvalFromYAMLNativeScalars(t *testing.T) {
+	mv, err := mlrvalFromYAMLNative("hello")
+	assert.NoError(t, err)
+	assert.True(t, mv.IsString())
+	s, _ := mv.GetStringValue()
+	assert.Equal(t, "hello", s)
+
+	mv, err = mlrvalFromYAMLNative(42)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsInt())
+	i, _ := mv.GetIntValue()
+	assert.Equal(t, int64(42), i)
+
+	mv, err = mlrvalFromYAMLNative(3.14)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsFloat())
+	f, _ := mv.GetFloatValue()
+	assert.Equal(t, 3.14, f)
+
+	mv, err = mlrvalFromYAMLNative(true)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsBool())
+	b, _ := mv.GetBoolValue()
+	assert.True(t, b)
+
+	mv, err = mlrvalFromYAMLNative(nil)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsNull())
+}
+
+func TestMlrvalFromYAMLNativeMap(t *testing.T) {
+	// gopkg.in/yaml.v3 unmarshals into map[string]interface{}
+	m := map[string]interface{}{
+		"a": 1,
+		"b": "two",
+		"c": true,
+	}
+	mv, err := mlrvalFromYAMLNative(m)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsMap())
+	rec := mv.GetMap()
+	assert.NotNil(t, rec)
+	assert.Equal(t, int64(3), rec.FieldCount)
+	av := rec.Get("a")
+	assert.True(t, av.IsInt())
+	ai, _ := av.GetIntValue()
+	assert.Equal(t, int64(1), ai)
+	bv := rec.Get("b")
+	assert.True(t, bv.IsString())
+	bs, _ := bv.GetStringValue()
+	assert.Equal(t, "two", bs)
+}
+
+func TestMlrvalDecodeFromYAML(t *testing.T) {
+	input := "a: 1\nb: 2\n"
+	decoder := yaml.NewDecoder(bytes.NewBufferString(input))
+	mv, eof, err := MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.False(t, eof)
+	assert.NotNil(t, mv)
+	assert.True(t, mv.IsMap())
+	rec := mv.GetMap()
+	assert.Equal(t, int64(2), rec.FieldCount)
+	v := rec.Get("a")
+	vi, _ := v.GetIntValue()
+	assert.Equal(t, int64(1), vi)
+	v = rec.Get("b")
+	vi, _ = v.GetIntValue()
+	assert.Equal(t, int64(2), vi)
+
+	// EOF on next decode
+	_, eof, err = MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.True(t, eof)
+}
+
+func TestMlrmapToYAMLNativeRoundTrip(t *testing.T) {
+	rec := NewMlrmapAsRecord()
+	rec.PutReference("x", FromInt(10))
+	rec.PutReference("y", FromString("hello"))
+	rec.PutReference("z", FromBool(true))
+
+	native, err := MlrmapToYAMLNative(rec)
+	assert.NoError(t, err)
+	assert.NotNil(t, native)
+
+	out, err := yaml.Marshal(native)
+	assert.NoError(t, err)
+	decoder := yaml.NewDecoder(bytes.NewReader(out))
+	mv, eof, err := MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.False(t, eof)
+	assert.True(t, mv.IsMap())
+	back := mv.GetMap()
+	xv := back.Get("x")
+	xi, _ := xv.GetIntValue()
+	assert.Equal(t, int64(10), xi)
+	yv := back.Get("y")
+	ys, _ := yv.GetStringValue()
+	assert.Equal(t, "hello", ys)
+	zv := back.Get("z")
+	zb, _ := zv.GetBoolValue()
+	assert.True(t, zb)
+}
+
+func TestMlrvalDecodeFromYAMLArrayOfMaps(t *testing.T) {
+	input := "- a: 1\n  b: 2\n- a: 3\n  b: 4\n"
+	decoder := yaml.NewDecoder(bytes.NewBufferString(input))
+	mv, eof, err := MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.False(t, eof)
+	assert.True(t, mv.IsArray())
+	arr := mv.GetArray()
+	assert.Len(t, arr, 2)
+	assert.True(t, arr[0].IsMap())
+	assert.True(t, arr[1].IsMap())
+	r0 := arr[0].GetMap()
+	av := r0.Get("a")
+	ai, _ := av.GetIntValue()
+	assert.Equal(t, int64(1), ai)
+	r1 := arr[1].GetMap()
+	av = r1.Get("a")
+	ai, _ = av.GetIntValue()
+	assert.Equal(t, int64(3), ai)
+}
+
+func TestMlrvalDecodeFromYAMLMultiDoc(t *testing.T) {
+	input := "a: 1\nb: 2\n---\nc: 3\nd: 4\n"
+	decoder := yaml.NewDecoder(bytes.NewBufferString(input))
+
+	mv1, eof, err := MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.False(t, eof)
+	assert.True(t, mv1.IsMap())
+	assert.Equal(t, int64(2), mv1.GetMap().FieldCount)
+
+	mv2, eof, err := MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.False(t, eof)
+	assert.True(t, mv2.IsMap())
+	assert.Equal(t, int64(2), mv2.GetMap().FieldCount)
+
+	_, eof, err = MlrvalDecodeFromYAML(decoder)
+	assert.NoError(t, err)
+	assert.True(t, eof)
+}
+
+func TestYAMLKeyStringNonStringKeys(t *testing.T) {
+	// map[interface{}]interface{} with int key
+	m := map[interface{}]interface{}{
+		1:     "one",
+		"two": 2,
+	}
+	mv, err := mlrvalFromYAMLMap(m)
+	assert.NoError(t, err)
+	assert.True(t, mv.IsMap())
+	rec := mv.GetMap()
+	v := rec.Get("1")
+	assert.True(t, v.IsString())
+	s, _ := v.GetStringValue()
+	assert.Equal(t, "one", s)
+	v = rec.Get("two")
+	assert.True(t, v.IsInt())
+}
+
+func TestMlrmapToYAMLNativeNested(t *testing.T) {
+	inner := NewMlrmapAsRecord()
+	inner.PutReference("p", FromInt(1))
+	inner.PutReference("q", FromInt(2))
+	rec := NewMlrmapAsRecord()
+	rec.PutReference("a", FromString("x"))
+	rec.PutReference("nested", FromMap(inner))
+
+	native, err := MlrmapToYAMLNative(rec)
+	assert.NoError(t, err)
+	out, err := yaml.Marshal(native)
+	assert.NoError(t, err)
+	assert.True(t, strings.Contains(string(out), "nested:"))
+	assert.True(t, strings.Contains(string(out), "p: 1"))
+}
diff --git a/pkg/output/record_writer_factory.go b/pkg/output/record_writer_factory.go
index bb6aba5fa..c7bb9ad02 100644
--- a/pkg/output/record_writer_factory.go
+++ b/pkg/output/record_writer_factory.go
@@ -18,6 +18,8 @@ func Create(writerOptions *cli.TWriterOptions) (IRecordWriter, error) {
 		return NewRecordWriterJSON(writerOptions)
 	case "jsonl":
 		return NewRecordWriterJSONLines(writerOptions)
+	case "yaml":
+		return NewRecordWriterYAML(writerOptions)
 	case "md":
 		return NewRecordWriterMarkdown(writerOptions)
 	case "markdown":
diff --git a/pkg/output/record_writer_yaml.go b/pkg/output/record_writer_yaml.go
new file mode 100644
index 000000000..4cf8b4d53
--- /dev/null
+++ b/pkg/output/record_writer_yaml.go
@@ -0,0 +1,91 @@
+package output
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+
+	"github.com/johnkerl/miller/v6/pkg/cli"
+	"github.com/johnkerl/miller/v6/pkg/mlrval"
+	"github.com/johnkerl/miller/v6/pkg/types"
+)
+
+type RecordWriterYAML struct {
+	writerOptions   *cli.TWriterOptions
+	bufferedRecords []interface{} // used when WrapYAMLOutputInOuterList is true
+	wroteAnyRecords bool          // for multi-doc: emit "---\n" before 2nd and later docs
+}
+
+func NewRecordWriterYAML(writerOptions *cli.TWriterOptions) (*RecordWriterYAML, error) {
+	return &RecordWriterYAML{
+		writerOptions:   writerOptions,
+		bufferedRecords: nil,
+		wroteAnyRecords: false,
+	}, nil
+}
+
+func (writer *RecordWriterYAML) Write(
+	outrec *mlrval.Mlrmap,
+	context *types.Context,
+	bufferedOutputStream *bufio.Writer,
+	outputIsStdout bool,
+) error {
+	if writer.writerOptions.WrapYAMLOutputInOuterList {
+		writer.writeWithListWrap(outrec, bufferedOutputStream)
+	} else {
+		writer.writeWithoutListWrap(outrec, bufferedOutputStream)
+	}
+	return nil
+}
+
+func (writer *RecordWriterYAML) writeWithListWrap(
+	outrec *mlrval.Mlrmap,
+	bufferedOutputStream *bufio.Writer,
+) {
+	if outrec != nil {
+		if writer.bufferedRecords == nil {
+			writer.bufferedRecords = make([]interface{}, 0)
+		}
+		native, err := mlrval.MlrmapToYAMLNative(outrec)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		writer.bufferedRecords = append(writer.bufferedRecords, native)
+	} else {
+		// End of stream: emit single YAML document as array
+		out, err := yaml.Marshal(writer.bufferedRecords)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		bufferedOutputStream.Write(out)
+		writer.bufferedRecords = nil
+	}
+}
+
+func (writer *RecordWriterYAML) writeWithoutListWrap(
+	outrec *mlrval.Mlrmap,
+	bufferedOutputStream *bufio.Writer,
+) {
+	if outrec == nil {
+		return
+	}
+	if writer.wroteAnyRecords {
+		bufferedOutputStream.WriteString("---\n")
+	}
+	native, err := mlrval.MlrmapToYAMLNative(outrec)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	out, err := yaml.Marshal(native)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	bufferedOutputStream.Write(out)
+	writer.wroteAnyRecords = true
+}
diff --git a/pkg/terminals/help/entry.go b/pkg/terminals/help/entry.go
index 47d3f6e9d..9d08e9f9c 100644
--- a/pkg/terminals/help/entry.go
+++ b/pkg/terminals/help/entry.go
@@ -401,6 +401,16 @@ JSON Lines (sequence of one-line objects):
   Record 1: "apple":"1", "bat":"2", "cog":"3"
   Record 2: "dish:egg":"7", "dish:flint":"8", "garlic":""
 
+YAML (single document = object or array of objects; multiple documents with ---):
++---------------------+
+| apple: 1            |
+| bat: 2              | Record 1: "apple":"1", "bat":"2", "cog":"3"
+| cog: 3              |
+| ---                 |
+| dish: 7             | Record 2: "dish":"7", "egg":"8"
+| egg: 8              |
++---------------------+
+
 PPRINT: pretty-printed tabular
 +---------------------+
 | apple bat cog       |
diff --git a/pkg/terminals/regtest/regtester.go b/pkg/terminals/regtest/regtester.go
index 5fea3a963..4734bf9ce 100644
--- a/pkg/terminals/regtest/regtester.go
+++ b/pkg/terminals/regtest/regtester.go
@@ -60,6 +60,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"sort"
 	"strings"
 
 	"github.com/johnkerl/miller/v6/pkg/colorizer"
@@ -167,6 +168,7 @@ func (regtester *RegTester) Execute(
 	regtester.resetCounts()
 
 	lib.InternalCodingErrorIf(len(casePaths) == 0)
+	sort.Strings(casePaths)
 
 	if !regtester.plainMode {
 		fmt.Println("REGRESSION TEST:")
@@ -342,6 +344,7 @@ func (regtester *RegTester) hasCaseSubdirectories(
 		fmt.Printf("%s: %v\n", dirName, err)
 		os.Exit(1)
 	}
+	sort.Strings(names)
 
 	for _, name := range names {
 		path := dirName + string(filepath.Separator) + name
diff --git a/test/cases/io-format-conversion-keystroke-savers/c2y/cmd b/test/cases/io-format-conversion-keystroke-savers/c2y/cmd
new file mode 100644
index 000000000..a1c2f69a0
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/c2y/cmd
@@ -0,0 +1 @@
+mlr --c2y cat test/input/abixy.csv
diff --git a/test/cases/io-format-conversion-keystroke-savers/c2y/experr b/test/cases/io-format-conversion-keystroke-savers/c2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/c2y/expout b/test/cases/io-format-conversion-keystroke-savers/c2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/c2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/d2y/cmd b/test/cases/io-format-conversion-keystroke-savers/d2y/cmd
new file mode 100644
index 000000000..d1812dde3
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/d2y/cmd
@@ -0,0 +1 @@
+mlr --d2y cat test/input/abixy.dkvp
diff --git a/test/cases/io-format-conversion-keystroke-savers/d2y/experr b/test/cases/io-format-conversion-keystroke-savers/d2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/d2y/expout b/test/cases/io-format-conversion-keystroke-savers/d2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/d2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/j2y/cmd b/test/cases/io-format-conversion-keystroke-savers/j2y/cmd
new file mode 100644
index 000000000..5065a9881
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/j2y/cmd
@@ -0,0 +1 @@
+mlr --j2y cat test/input/abixy.json
diff --git a/test/cases/io-format-conversion-keystroke-savers/j2y/experr b/test/cases/io-format-conversion-keystroke-savers/j2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/j2y/expout b/test/cases/io-format-conversion-keystroke-savers/j2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/j2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/m2y/cmd b/test/cases/io-format-conversion-keystroke-savers/m2y/cmd
new file mode 100644
index 000000000..d9124b2c6
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/m2y/cmd
@@ -0,0 +1 @@
+mlr --m2y cat test/input/abixy.md
diff --git a/test/cases/io-format-conversion-keystroke-savers/m2y/experr b/test/cases/io-format-conversion-keystroke-savers/m2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/m2y/expout b/test/cases/io-format-conversion-keystroke-savers/m2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/m2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/n2y/cmd b/test/cases/io-format-conversion-keystroke-savers/n2y/cmd
new file mode 100644
index 000000000..c20d36ccb
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/n2y/cmd
@@ -0,0 +1 @@
+mlr --n2y cat test/input/abixy.nidx
diff --git a/test/cases/io-format-conversion-keystroke-savers/n2y/experr b/test/cases/io-format-conversion-keystroke-savers/n2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/n2y/expout b/test/cases/io-format-conversion-keystroke-savers/n2y/expout
new file mode 100644
index 000000000..11099d6b9
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/n2y/expout
@@ -0,0 +1,50 @@
+- "1": pan
+  "2": pan
+  "3": 1
+  "4": 0.3467901443380824
+  "5": 0.7268028627434533
+- "1": eks
+  "2": pan
+  "3": 2
+  "4": 0.7586799647899636
+  "5": 0.5221511083334797
+- "1": wye
+  "2": wye
+  "3": 3
+  "4": 0.20460330576630303
+  "5": 0.33831852551664776
+- "1": eks
+  "2": wye
+  "3": 4
+  "4": 0.38139939387114097
+  "5": 0.13418874328430463
+- "1": wye
+  "2": pan
+  "3": 5
+  "4": 0.5732889198020006
+  "5": 0.8636244699032729
+- "1": zee
+  "2": pan
+  "3": 6
+  "4": 0.5271261600918548
+  "5": 0.49322128674835697
+- "1": eks
+  "2": zee
+  "3": 7
+  "4": 0.6117840605678454
+  "5": 0.1878849191181694
+- "1": zee
+  "2": wye
+  "3": 8
+  "4": 0.5985540091064224
+  "5": 0.976181385699006
+- "1": hat
+  "2": wye
+  "3": 9
+  "4": 0.03144187646093577
+  "5": 0.7495507603507059
+- "1": pan
+  "2": wye
+  "3": 10
+  "4": 0.5026260055412137
+  "5": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/p2y/cmd b/test/cases/io-format-conversion-keystroke-savers/p2y/cmd
new file mode 100644
index 000000000..4a1eb07be
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/p2y/cmd
@@ -0,0 +1 @@
+mlr --p2y cat test/input/abixy.pprint
diff --git a/test/cases/io-format-conversion-keystroke-savers/p2y/experr b/test/cases/io-format-conversion-keystroke-savers/p2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/p2y/expout b/test/cases/io-format-conversion-keystroke-savers/p2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/p2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/t2y/cmd b/test/cases/io-format-conversion-keystroke-savers/t2y/cmd
new file mode 100644
index 000000000..736c608d6
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/t2y/cmd
@@ -0,0 +1 @@
+mlr --t2y cat test/input/abixy.tsv
diff --git a/test/cases/io-format-conversion-keystroke-savers/t2y/experr b/test/cases/io-format-conversion-keystroke-savers/t2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/t2y/expout b/test/cases/io-format-conversion-keystroke-savers/t2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/t2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/x2y/cmd b/test/cases/io-format-conversion-keystroke-savers/x2y/cmd
new file mode 100644
index 000000000..b8ee28797
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/x2y/cmd
@@ -0,0 +1 @@
+mlr --x2y cat test/input/abixy.xtab
diff --git a/test/cases/io-format-conversion-keystroke-savers/x2y/experr b/test/cases/io-format-conversion-keystroke-savers/x2y/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/x2y/expout b/test/cases/io-format-conversion-keystroke-savers/x2y/expout
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/x2y/expout
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2c/cmd b/test/cases/io-format-conversion-keystroke-savers/y2c/cmd
new file mode 100644
index 000000000..b61391cd9
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2c/cmd
@@ -0,0 +1 @@
+mlr --y2c cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2c/experr b/test/cases/io-format-conversion-keystroke-savers/y2c/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2c/expout b/test/cases/io-format-conversion-keystroke-savers/y2c/expout
new file mode 100644
index 000000000..37eac50e4
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2c/expout
@@ -0,0 +1,11 @@
+a,b,i,x,y
+pan,pan,1,0.34679014,0.72680286
+eks,pan,2,0.75867996,0.52215111
+wye,wye,3,0.20460331,0.33831853
+eks,wye,4,0.38139939,0.13418874
+wye,pan,5,0.57328892,0.86362447
+zee,pan,6,0.52712616,0.49322129
+eks,zee,7,0.61178406,0.18788492
+zee,wye,8,0.59855401,0.97618139
+hat,wye,9,0.03144188,0.74955076
+pan,wye,10,0.50262601,0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2c/input b/test/cases/io-format-conversion-keystroke-savers/y2c/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2c/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2d/cmd b/test/cases/io-format-conversion-keystroke-savers/y2d/cmd
new file mode 100644
index 000000000..0acb4c02f
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2d/cmd
@@ -0,0 +1 @@
+mlr --y2d cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2d/experr b/test/cases/io-format-conversion-keystroke-savers/y2d/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2d/expout b/test/cases/io-format-conversion-keystroke-savers/y2d/expout
new file mode 100644
index 000000000..940df3d11
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2d/expout
@@ -0,0 +1,10 @@
+a=pan,b=pan,i=1,x=0.34679014,y=0.72680286
+a=eks,b=pan,i=2,x=0.75867996,y=0.52215111
+a=wye,b=wye,i=3,x=0.20460331,y=0.33831853
+a=eks,b=wye,i=4,x=0.38139939,y=0.13418874
+a=wye,b=pan,i=5,x=0.57328892,y=0.86362447
+a=zee,b=pan,i=6,x=0.52712616,y=0.49322129
+a=eks,b=zee,i=7,x=0.61178406,y=0.18788492
+a=zee,b=wye,i=8,x=0.59855401,y=0.97618139
+a=hat,b=wye,i=9,x=0.03144188,y=0.74955076
+a=pan,b=wye,i=10,x=0.50262601,y=0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2d/input b/test/cases/io-format-conversion-keystroke-savers/y2d/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2d/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2j/cmd b/test/cases/io-format-conversion-keystroke-savers/y2j/cmd
new file mode 100644
index 000000000..09556834a
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2j/cmd
@@ -0,0 +1 @@
+mlr --y2j cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2j/experr b/test/cases/io-format-conversion-keystroke-savers/y2j/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2j/expout b/test/cases/io-format-conversion-keystroke-savers/y2j/expout
new file mode 100644
index 000000000..369f76808
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2j/expout
@@ -0,0 +1,10 @@
+[
+{
+  "a": 1,
+  "b": 2
+},
+{
+  "a": 3,
+  "b": 4
+}
+]
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2j/input b/test/cases/io-format-conversion-keystroke-savers/y2j/input
new file mode 100644
index 000000000..33d41a2a0
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2j/input
@@ -0,0 +1,5 @@
+a: 1
+b: 2
+---
+a: 3
+b: 4
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2m/cmd b/test/cases/io-format-conversion-keystroke-savers/y2m/cmd
new file mode 100644
index 000000000..6ffb0c6fe
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2m/cmd
@@ -0,0 +1 @@
+mlr --y2m cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2m/experr b/test/cases/io-format-conversion-keystroke-savers/y2m/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2m/expout b/test/cases/io-format-conversion-keystroke-savers/y2m/expout
new file mode 100644
index 000000000..3d77a0324
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2m/expout
@@ -0,0 +1,12 @@
+| a | b | i | x | y |
+| --- | --- | --- | --- | --- |
+| pan | pan | 1 | 0.34679014 | 0.72680286 |
+| eks | pan | 2 | 0.75867996 | 0.52215111 |
+| wye | wye | 3 | 0.20460331 | 0.33831853 |
+| eks | wye | 4 | 0.38139939 | 0.13418874 |
+| wye | pan | 5 | 0.57328892 | 0.86362447 |
+| zee | pan | 6 | 0.52712616 | 0.49322129 |
+| eks | zee | 7 | 0.61178406 | 0.18788492 |
+| zee | wye | 8 | 0.59855401 | 0.97618139 |
+| hat | wye | 9 | 0.03144188 | 0.74955076 |
+| pan | wye | 10 | 0.50262601 | 0.95261836 |
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2m/input b/test/cases/io-format-conversion-keystroke-savers/y2m/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2m/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2n/cmd b/test/cases/io-format-conversion-keystroke-savers/y2n/cmd
new file mode 100644
index 000000000..5decfdbd6
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2n/cmd
@@ -0,0 +1 @@
+mlr --y2n cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2n/experr b/test/cases/io-format-conversion-keystroke-savers/y2n/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2n/expout b/test/cases/io-format-conversion-keystroke-savers/y2n/expout
new file mode 100644
index 000000000..17f7e1ee6
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2n/expout
@@ -0,0 +1,10 @@
+pan pan 1 0.34679014 0.72680286
+eks pan 2 0.75867996 0.52215111
+wye wye 3 0.20460331 0.33831853
+eks wye 4 0.38139939 0.13418874
+wye pan 5 0.57328892 0.86362447
+zee pan 6 0.52712616 0.49322129
+eks zee 7 0.61178406 0.18788492
+zee wye 8 0.59855401 0.97618139
+hat wye 9 0.03144188 0.74955076
+pan wye 10 0.50262601 0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2n/input b/test/cases/io-format-conversion-keystroke-savers/y2n/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2n/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2p/cmd b/test/cases/io-format-conversion-keystroke-savers/y2p/cmd
new file mode 100644
index 000000000..be9a17d42
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2p/cmd
@@ -0,0 +1 @@
+mlr --y2p cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2p/experr b/test/cases/io-format-conversion-keystroke-savers/y2p/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2p/expout b/test/cases/io-format-conversion-keystroke-savers/y2p/expout
new file mode 100644
index 000000000..b8ac13481
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2p/expout
@@ -0,0 +1,11 @@
+a   b   i  x          y
+pan pan 1  0.34679014 0.72680286
+eks pan 2  0.75867996 0.52215111
+wye wye 3  0.20460331 0.33831853
+eks wye 4  0.38139939 0.13418874
+wye pan 5  0.57328892 0.86362447
+zee pan 6  0.52712616 0.49322129
+eks zee 7  0.61178406 0.18788492
+zee wye 8  0.59855401 0.97618139
+hat wye 9  0.03144188 0.74955076
+pan wye 10 0.50262601 0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2p/input b/test/cases/io-format-conversion-keystroke-savers/y2p/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2p/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2t/cmd b/test/cases/io-format-conversion-keystroke-savers/y2t/cmd
new file mode 100644
index 000000000..ab2433ef4
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2t/cmd
@@ -0,0 +1 @@
+mlr --y2t cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2t/experr b/test/cases/io-format-conversion-keystroke-savers/y2t/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2t/expout b/test/cases/io-format-conversion-keystroke-savers/y2t/expout
new file mode 100644
index 000000000..03ac8f384
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2t/expout
@@ -0,0 +1,11 @@
+a	b	i	x	y
+pan	pan	1	0.34679014	0.72680286
+eks	pan	2	0.75867996	0.52215111
+wye	wye	3	0.20460331	0.33831853
+eks	wye	4	0.38139939	0.13418874
+wye	pan	5	0.57328892	0.86362447
+zee	pan	6	0.52712616	0.49322129
+eks	zee	7	0.61178406	0.18788492
+zee	wye	8	0.59855401	0.97618139
+hat	wye	9	0.03144188	0.74955076
+pan	wye	10	0.50262601	0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2t/input b/test/cases/io-format-conversion-keystroke-savers/y2t/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2t/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2x/cmd b/test/cases/io-format-conversion-keystroke-savers/y2x/cmd
new file mode 100644
index 000000000..432efdd8b
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2x/cmd
@@ -0,0 +1 @@
+mlr --y2x cat ${CASEDIR}/input
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2x/experr b/test/cases/io-format-conversion-keystroke-savers/y2x/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2x/expout b/test/cases/io-format-conversion-keystroke-savers/y2x/expout
new file mode 100644
index 000000000..9c955fb87
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2x/expout
@@ -0,0 +1,59 @@
+a pan
+b pan
+i 1
+x 0.34679014
+y 0.72680286
+
+a eks
+b pan
+i 2
+x 0.75867996
+y 0.52215111
+
+a wye
+b wye
+i 3
+x 0.20460331
+y 0.33831853
+
+a eks
+b wye
+i 4
+x 0.38139939
+y 0.13418874
+
+a wye
+b pan
+i 5
+x 0.57328892
+y 0.86362447
+
+a zee
+b pan
+i 6
+x 0.52712616
+y 0.49322129
+
+a eks
+b zee
+i 7
+x 0.61178406
+y 0.18788492
+
+a zee
+b wye
+i 8
+x 0.59855401
+y 0.97618139
+
+a hat
+b wye
+i 9
+x 0.03144188
+y 0.74955076
+
+a pan
+b wye
+i 10
+x 0.50262601
+y 0.95261836
diff --git a/test/cases/io-format-conversion-keystroke-savers/y2x/input b/test/cases/io-format-conversion-keystroke-savers/y2x/input
new file mode 100644
index 000000000..0fa69f851
--- /dev/null
+++ b/test/cases/io-format-conversion-keystroke-savers/y2x/input
@@ -0,0 +1,50 @@
+- a: pan
+  b: pan
+  i: 1
+  x: 0.3467901443380824
+  "y": 0.7268028627434533
+- a: eks
+  b: pan
+  i: 2
+  x: 0.7586799647899636
+  "y": 0.5221511083334797
+- a: wye
+  b: wye
+  i: 3
+  x: 0.20460330576630303
+  "y": 0.33831852551664776
+- a: eks
+  b: wye
+  i: 4
+  x: 0.38139939387114097
+  "y": 0.13418874328430463
+- a: wye
+  b: pan
+  i: 5
+  x: 0.5732889198020006
+  "y": 0.8636244699032729
+- a: zee
+  b: pan
+  i: 6
+  x: 0.5271261600918548
+  "y": 0.49322128674835697
+- a: eks
+  b: zee
+  i: 7
+  x: 0.6117840605678454
+  "y": 0.1878849191181694
+- a: zee
+  b: wye
+  i: 8
+  x: 0.5985540091064224
+  "y": 0.976181385699006
+- a: hat
+  b: wye
+  i: 9
+  x: 0.03144187646093577
+  "y": 0.7495507603507059
+- a: pan
+  b: wye
+  i: 10
+  x: 0.5026260055412137
+  "y": 0.9526183602969864
diff --git a/test/cases/io-yaml-io/0001/cmd b/test/cases/io-yaml-io/0001/cmd
new file mode 100644
index 000000000..6fd32a30b
--- /dev/null
+++ b/test/cases/io-yaml-io/0001/cmd
@@ -0,0 +1 @@
+mlr --iyaml --ocsv cat ${CASEDIR}/input
diff --git a/test/cases/io-yaml-io/0001/experr b/test/cases/io-yaml-io/0001/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-yaml-io/0001/expout b/test/cases/io-yaml-io/0001/expout
new file mode 100644
index 000000000..de7e0001c
--- /dev/null
+++ b/test/cases/io-yaml-io/0001/expout
@@ -0,0 +1,3 @@
+a,b,c
+1,2,hello
+10,20,world
diff --git a/test/cases/io-yaml-io/0001/input b/test/cases/io-yaml-io/0001/input
new file mode 100644
index 000000000..5d7c68867
--- /dev/null
+++ b/test/cases/io-yaml-io/0001/input
@@ -0,0 +1,7 @@
+a: 1
+b: 2
+c: hello
+---
+a: 10
+b: 20
+c: world
diff --git a/test/cases/io-yaml-io/0002/cmd b/test/cases/io-yaml-io/0002/cmd
new file mode 100644
index 000000000..31cb8b6be
--- /dev/null
+++ b/test/cases/io-yaml-io/0002/cmd
@@ -0,0 +1 @@
+mlr --iyaml --oyaml --no-yarray cat ${CASEDIR}/input
diff --git a/test/cases/io-yaml-io/0002/experr b/test/cases/io-yaml-io/0002/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/cases/io-yaml-io/0002/expout b/test/cases/io-yaml-io/0002/expout
new file mode 100644
index 000000000..33d41a2a0
--- /dev/null
+++ b/test/cases/io-yaml-io/0002/expout
@@ -0,0 +1,5 @@
+a: 1
+b: 2
+---
+a: 3
+b: 4
diff --git a/test/cases/io-yaml-io/0002/input b/test/cases/io-yaml-io/0002/input
new file mode 100644
index 000000000..33d41a2a0
--- /dev/null
+++ b/test/cases/io-yaml-io/0002/input
@@ -0,0 +1,5 @@
+a: 1
+b: 2
+---
+a: 3
+b: 4