from_entries of [] is null, should be {}

[rvedotrc]

jq version 1.3

rachel@dudley stanley$ echo '[]' | jq -M 'from_entries'
null
rachel@dudley stanley$

For consistency, I expected {}.

The man page doesn't say anything about this inconsistency, so it sounds like a bug.

[nicowilliams]

I too think it's a bug. from_entries is implemented like this:

def from_entries: map({(.key): .value}) | add;

and add is implemented like this:

def add: reduce .[] as $x (null; . + $x);

The problem is the null in add as the initial reduce value; changing it to '{}' fixes the problem. I think we need a version of add with a default initial value to pass to reduce, then make from_entries use that with {}.

In the meantime you can work around this by defining your own from_entries, or by replacing nulls output by it with {}.

[nicowilliams]

I.e., the workaround should be to add this to your program:

def from_entries: map({(.key): .value}) | reduce .[] as $x ({}; . + $x);

That will probably be the fix too (in builtin.c).

[rvedotrc]

Or more simply, if I understand correctly:

"def from_entries: map({(.key): .value}) | add | .//={};",

[nicowilliams]

Even def from_entries2: from_entries|.//{};.

[nicowilliams]

Oops, I missed this for the 1.4 release.