Add mul/0 builtin filter folding arrays with * similar to add/0 for +

[syco]

jq/ChangeLog

Lines 826 to 829 in 913b264

	Add a recursive object merge strategy and bind it to *
	This commit adds a jv_object_merge_recursive function, that performs
	recursive object merging, and binds it to multiply when applied to
	two objects.

Hi,
when you add multiple files you can do either jq -s '.[0] + .[1] + .[2]' file0.json file1.json file2.json or jq -s 'add' *.json

can we have a similar syntax for jq -s '.[0] * .[1] * .[2]' file0.json file1.json file2.json ?
Could maybe be: jq -s 'multiply' *.json

Thanks

[itchyny]

There is a pull request for this feature #1643. My concern is what [] | mul should be. Considering null is the identity element of addition (i.e. null + x == x + null == x for any value x regardless of type), [] | add == null is reasonable. However, there is no identity element of multiplication (while {} is the value for objects, 1 is the value for numbers). #1643 suggests using null for the value, but mathematically speaking, it's very weird that the additive identity and the multiplicative identity are the same value.

[syco]

I don't know if I would have considered null as the additive identity. Typed languages throw an error trying to add null and numbers. The only language where null is the additive identity afaik is javascript.

But regardless of that I would have mul to follow the same identical logic as *,
so echo '[{"a": 1}, {"b": 2}]' | jq '.[0] * .[1]' can be written as echo '[{"a": 1}, {"b": 2}]' | jq 'mul'
and echo '[null, {"b": 2}]' | jq 'mul' throws an error same as echo '[null, {"b": 2}]' | jq '.[0] * .[1]'

(I would make null + x throw an error as well, but that's not the point here)

[itchyny]

The point is add is the folding operation of addition. Since null is the additive identity, add is cleanly defined as def add: reduce .[] as $x (null; . + $x);. Since we don't have multiplication identity now, we will end up making [] | mul to throw an error (something like def mul: if length == 0 then error("cannot use mul/0 on empty array") else reduce .[1:][] as $x (.[0]; . * $x) end).