group by

groups items into a dict of lists

(list xs, function f) -> any

Returns a dict. Each x of xs is collected in a list located at the key given by f(x).

If xs is a list:
If f accepts one argument, x is passed.
If f accepts two arguments, x and its index are passed.

If xs is a dict:
If f accepts one argument, each entry’s value is passed.
If f accepts two arguments, each entry’s value and key are passed.

The indexing function f must return a string representing the key to the list where item x should be placed. The order of items in collecting lists is undefined.

In case f(x) returns nil, the corresponding x is omitted from the result.

Returns nil if xs is nil.

Throws an error if xs is neither a list nor a dict.

Throws an error if f is nil.

Parameters