Patterns in variable bindings.

[01mf02]

This PR adds support for patterns in variable bindings, e.g. . as {a: [$x, {("b", "c"): $y, $z}]} | $x, $y, $z.
This also works in foreach and reduce.

Compared to jq, this also allows indexing with integers, e.g. {(0): $x, (1): $y}, which is equivalent to [$x, $y]. This can be useful if you wish to destructure arrays, but not necessarily the starting items.

However, jaq does not support the (undocumented) syntax {$x: ...}; for example, the filter {x: [1]} as {$x: [$y]} | $x, $y this yields [1] and 1 in jq.

[wader]

Impressive work! and i had no idea jq supported combining key/value pattern like{$x: [$y]}. Index based patter for arrays seems nice addition, wonder how much work it would be to get into jq. I've also wished sometimes for some kind of rest-syntax like [$x, ...$xs] and {$k, ...$obj_with_all_other_keys}

[01mf02]

Regarding {$x: pattern}, I have not implemented this because of a few reasons:

• {$x: pattern} is not the same as {($x): pattern}. I can't think of any other place in jq where ($x) cannot be substituted by $x, so this is some really surprising behaviour. If you only know the {(f): pattern} syntax and are confronted with {$x: pattern}, you may be up for a pretty big surprise.
• {$x: pattern} discriminates keys that are not "identifier-like"; that is, this only works for fields like "x", but not for "here-be-dragons". (The same could be said of course about .x and {$x}, but these can be transformed quite simply into equivalent constructs that work with non-identifier-like keys, such as .["x"] and {"x": $x}. There is no easy transformation for {$x: pattern}, however. This can be really cumbersome if you have used {$x: pattern}, perhaps somewhere inside another pattern, then your input format changes from {"x": ...} to {"here-be-dragons: ...}. Have fun rewriting your pattern, especially if you use it inside of reduce or foreach.)

For these reasons, I would also strongly recommend against using this syntax.