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100 lines
4.2 KiB
Markdown
100 lines
4.2 KiB
Markdown
# DSL operators
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## Detailed listing
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Operators are listed on the [DSL built-in functions page](reference-dsl-builtin-functions.md).
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## Operator precedence
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Operators are listed in order of decreasing precedence, from highest to lowest.
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| Operators | Associativity |
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|-------------------------------|---------------|
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| `()` `{}` `[]` | left to right |
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| `**` | right to left |
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| `???` | left to right |
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| `??` | left to right |
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| `!` `~` unary`+` unary`-` | right to left |
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| `.` | left to right |
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| `*` `/` `//` `%` | left to right |
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| binary`+` binary`-` | left to right |
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| `<<` `>>` `>>>` | left to right |
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| `&` | left to right |
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| `^` | left to right |
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| `|` | left to right |
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| `<` `<=` `>` `>=` | left to right |
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| `==` `!=` `=~` `!=~` `<=>` | left to right |
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| `&&` | left to right |
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| `^^` | left to right |
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| `||` | left to right |
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| `? :` | right to left |
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| `=` | N/A for Miller (there is no $a=$b=$c) |
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See also the [section on parsing and operator precedence in the REPL](repl.md#parsing-and-operator-precedence) for information on how to examine operator precedence interactively.
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## Operator and function semantics
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* Functions are often pass-throughs straight to the system-standard Go libraries.
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* The [`min`](reference-dsl-builtin-functions.md#min) and [`max`](reference-dsl-builtin-functions.md#max) functions are different from other multi-argument functions, which return null if any of their inputs are null: for [`min`](reference-dsl-builtin-functions.md#min) and [`max`](reference-dsl-builtin-functions.md#max), by contrast, if one argument is absent-null, the other is returned. Empty-null loses min or max against numeric or boolean; empty-null is less than any other string.
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* Symmetrically with respect to the bitwise OR, AND, and XOR operators
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[`|`](reference-dsl-builtin-functions.md#bitwise-or),
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[`&`](reference-dsl-builtin-functions.md#bitwise-and), and
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[`^`](reference-dsl-builtin-functions.md#bitwise-xor), Miller has logical operators
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[`||`](reference-dsl-builtin-functions.md#logical-or),
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[`&&`](reference-dsl-builtin-functions.md#logical-and), and
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[`^^`](reference-dsl-builtin-functions.md#logical-xor).
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* The exponentiation operator [`**`](reference-dsl-builtin-functions.md#exponentiation) is familiar from many languages, except that an integer raised to an int power is int, not float.
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* The regex-match and regex-not-match operators [`=~`](reference-dsl-builtin-functions.md#regmatch) and [`!=~`](reference-dsl-builtin-functions.md#regnotmatch) are similar to those in Ruby and Perl.
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## The double-purpose dot operator
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The main use for the `.` operator is for string concatenation: `"abc" . "def"` is `"abc.def"`.
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However, in Miller 6, it has an optional use for map traversal. Example:
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GENMD-RUN-COMMAND
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cat data/server-log.json
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GENMD-EOF
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GENMD-RUN-COMMAND
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mlr --json --from data/server-log.json put -q '
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print $req["headers"]["host"];
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print $req.headers.host;
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'
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GENMD-EOF
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This also works on the left-hand sides of assignment statements:
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GENMD-RUN-COMMAND
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mlr --json --from data/server-log.json put '
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$req.headers.host = "UPDATED";
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'
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GENMD-EOF
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A few caveats:
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* This is why `.` has higher precedence than `+` in the table above -- in Miller 5 and below, where `.` was only used for concatenation, it had the same precedence as `+`. So you can now do this:
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GENMD-RUN-COMMAND
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mlr --json --from data/server-log.json put -q '
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print $req.id + $res.status_code
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'
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GENMD-EOF
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* However (awkwardly), if you want to use `.` for map-traversal as well as string-concatenation in the same statement, you'll need to insert parentheses, as the default associativity is left-to-right:
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GENMD-RUN-COMMAND
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mlr --json --from data/server-log.json put -q '
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print $req.method . " -- " . $req.path
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'
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GENMD-EOF
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GENMD-RUN-COMMAND
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mlr --json --from data/server-log.json put -q '
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print ($req.method) . " -- " . ($req.path)
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'
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GENMD-EOF
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