I haven’t wanted to flood this lovely place with all my recent posts on jq, but I thought this short one might be of interest as it hopefully explains some subtleties of the comma as generator, and streaming.

This is cool, writing a condition this way is unfamiliar to me but arguably very natural. A beginner mistake, in say Python, is to write thing == ("a" or "b") when you meant thing == "a" or thing == "b". It must happen by transliterating from English too closely: “it equals ‘a’ or ‘b’”. But in jq you can correctly write thing == ("a", "b") and everything distributes over the commas automatically.

The choice operator in Verse is like this too:

• Beyond Functional Programming: The Verse Programming Language (Simon Peyton Jones) talk
• The Verse Calculus: a Core Calculus for Functional Logic Programming paper

And I think Verse and jq do a similar cross product when both arguments are a generator, like (10, 20, 30) + (8, 9) gives you 6 results.

Very cool! I especially like how when you have multiple generators, jq backtracks and produces all combinations. (Leftmost varies fastest.) Look at this!

# -n = --null-input
# -c = --compact-output
# objects
jq -nc '{a: (1, 2), b: (10, 20)}' -c
{"a":1,"b":10}
{"a":1,"b":20}
{"a":2,"b":10}
{"a":2,"b":20}

Generators inside arrays are flattened …

jq -nc '[(10, 20), range(3)]'
[10, 20, 0, 1, 2]

… but variables bind one value at a time, and you can use that to produce arrays. This also demonstrates that the generator backtracking happens across pipes.

jq -nc '(1, 2) as $a | (10, 20) as $b | [$a, $b]'
[1,10]
[1,20]
[2,10]
[2,20]
# Or use this: jq -c -n '[[0, 1], [3, 4]] | combinations' 

Funnily enough: when I use string concatenation, the output varies leftmost-fastest instead!

jq -nc '("0", "1") + ("0", "1")'
"00"
"10"
"01"
"11"