# DSL overview

_DSL_ stands for _domain-specific language_: it's language particular to Miller
which you can use to write expressions to specify customer transformations to
your data. (See [Miller programming language](miller-programming-language.md) for an
introduction.)

## Verbs compared to DSL

While `put` and `filter` are [verbs](reference-verbs.md), they're different
from the rest in that they let you use the DSL -- so we often contrast _DSL_
(things you can do in the `put` and `filter` verbs), and _verbs_ (things you
can do using the other verbs besides `put` and `filter`.)

Here's comparison of verbs and `put`/`filter` DSL expressions:

Example:

GENMD-RUN-COMMAND
mlr stats1 -a sum -f x -g a data/small
GENMD-EOF

* Verbs are coded in Go
* They run a bit faster
* They take fewer keystrokes
* There is less to learn
* Their customization is limited to each verb's options

Example:

GENMD-RUN-COMMAND
mlr  put -q '@x_sum[$a] += $x; end{emit @x_sum, "a"}' data/small
GENMD-EOF

* You get to write your own DSL expressions
* They run a bit slower
* They take more keystrokes
* There is more to learn
* They are highly customizable

Please see [Verbs Reference](reference-verbs.md) for information on verbs other than `put` and `filter`.

## Implicit loop over records for main statements

The most important point about the Miller DSL is that it is designed for [streaming operation over records](streaming-and-memory.md).

DSL statements include:

* `func` and `subr` for user-defined functions and subroutines, which we'll look at later in the [separate page about them](reference-dsl-user-defined-functions.md);
* `begin` and `end` blocks, for statements you want to run before the first record, or after the last one;
* everything else, which collectively are called _main statements_.

The feature of _streaming operation over records_ is implemented by the main
statements getting invoked once per record. You don't explicitly loop over
records, as you would in some dataframes contexts; rather, _Miller loops over
records for you_, and it lets you specify what to do on each record: you write
the body of the loop, not the loop itself.

(But you can, if you like, use those per-record statements to grow a list of
records, then loop over them all in an `end` block. This is described in the
page on [operating on all records](operating-on-all-records.md).)

To see this in action, let's take a look at the [data/short.csv](./data/short.csv) file:

GENMD-RUN-COMMAND
cat data/short.csv
GENMD-EOF

There are three records in this file, with `word=apple`, `word=ball`, and
`word=cat`, respectively. Let's print something in a `begin` statement, add a
field in a main statement, and print something else in an `end` statement:

GENMD-RUN-COMMAND
mlr --csv --from data/short.csv put '
  begin {
    print "begin";
  }
  $nr = NR;
  end {
    print "end";
  }
'
GENMD-EOF

The `print` statements for `begin` and `end` went out before the first record
was seen and after the last was seen; the field-creation statement `$nr = NR`
was invoked three times, once for each record. We didn't explicitly loop over
records, since Miller was already looping over records, and invoked our main
statement on each loop iteration.

For almost all simple uses of the Miller programming language, this implicit
looping over records is probably all you will need. (For more involved cases you
can see the pages on [operating on all records](operating-on-all-records.md),
[out-of-stream variables](reference-dsl-variables.md#out-of-stream-variables),
and [two-pass algorithms](two-pass-algorithms.md).)

## Essential use: record-selection and record-updating

The essential usages of `mlr filter` and `mlr put` are for record-selection and
record-updating expressions, respectively. For example, given the following
input data:

GENMD-RUN-COMMAND
cat data/small
GENMD-EOF

you might retain only the records whose `a` field has value `eks`:

GENMD-RUN-COMMAND
mlr filter '$a == "eks"' data/small
GENMD-EOF

or you might add a new field which is a function of existing fields:

GENMD-RUN-COMMAND
mlr put '$ab = $a . "_" . $b ' data/small
GENMD-EOF

## Differences between put and filter

The two verbs `mlr filter` and `mlr put` are essentially the same. The only differences are:

* Expressions sent to `mlr filter` should contain a boolean expression, which is the filtering criterion. (If not, all records pass through.)

* `mlr filter` expressions may not reference the `filter` keyword within them.

## Location of boolean expression for filter

You can define and invoke functions and subroutines to help produce the bare-boolean statement, and record fields may be assigned in the statements before or after the bare-boolean statement. For example:

GENMD-RUN-COMMAND
mlr --c2p --from example.csv filter '
  # Bare-boolean filter expression: only records matching this pass through:
  $quantity >= 70;
  # For records that do pass through, set these:
  if ($rate > 8) {
    $description = "high rate";
  } else {
    $description = "low rate";
  }
'
GENMD-EOF

GENMD-RUN-COMMAND
mlr --c2p --from example.csv filter '
  # Bare-boolean filter expression: only records matching this pass through:
  $shape =~ "^(...)(...)$";
  # For records that do pass through, capture the first "(...)" into $left and
  # the second "(...)" into $right
  $left = "\1";
  $right = "\2";
'
GENMD-EOF


There are more details and more choices, of course, as detailed in the following sections.

