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DSL operators

Detailed listing

Operators are listed on the DSL built-in functions page.

Operator precedence

Operators are listed in order of decreasing precedence, highest first.

Operators Associativity
() {} [] left to right
** right to left
! ~ unary+ unary- & right to left
binary* / // % left to right
. left to right
binary+ binary- left to right
<< >> >>> left to right
& left to right
^ left to right
| left to right
< <= > >= left to right
== != =~ !=~ <=> left to right
??? left to right
?? left to right
&& left to right
^^ left to right
|| left to right
? : right to left
= N/A for Miller (there is no $a=$b=$c)

Operator and function semantics

  • Functions are often pass-throughs straight to the system-standard Go libraries.

  • The min and max functions are different from other multi-argument functions which return null if any of their inputs are null: for min and 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.

  • Symmetrically with respect to the bitwise OR, XOR, and AND operators |, &, and ^, Miller has logical operators ||, &&, and ^^.

  • The exponentiation operator ** is familiar from many languages, except that an integer raised to an int power is int, not float.

  • The regex-match and regex-not-match operators =~ and !=~ are similar to those in Ruby and Perl.

The double-purpose dot operator

The main use for the . operator is for string concatenation: "abc" . "def" is "abc.def".

However, in Miller 6 it has optional use for map traversal. Example:

cat data/server-log.json
{
  "hostname": "localhost",
  "pid": 12345,
  "req": {
    "id": 6789,
    "method": "GET",
    "path": "api/check",
    "host": "foo.bar",
    "headers": {
      "host": "bar.baz",
      "user-agent": "browser"
    }
  },
  "res": {
    "status_code": 200,
    "header": {
      "content-type": "text",
      "content-encoding": "plain"
    }
  }
}
mlr --json --from data/server-log.json put -q '
  print $req["headers"]["host"];
  print $req.headers.host;
'
bar.baz
bar.baz

This also works on the left-hand sides of assignment statements:

mlr --json --from data/server-log.json put '
  $req.headers.host = "UPDATED";
'
{
  "hostname": "localhost",
  "pid": 12345,
  "req": {
    "id": 6789,
    "method": "GET",
    "path": "api/check",
    "host": "foo.bar",
    "headers": {
      "host": "UPDATED",
      "user-agent": "browser"
    }
  },
  "res": {
    "status_code": 200,
    "header": {
      "content-type": "text",
      "content-encoding": "plain"
    }
  }
}

A few caveats:

  • This is why . has higher precedece 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:
mlr --json --from data/server-log.json put -q '
  print $req.id + $res.status_code
'
6989
  • 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:
mlr --json --from data/server-log.json put -q '
  print $req.method . " -- " . $req.path
'
(error)
mlr --json --from data/server-log.json put -q '
  print ($req.method) . " -- " . ($req.path)
'
GET -- api/check
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