Returns the length of its last collection argument or the length of the incoming f.ret
length [ 0 1 2 3 ]
# f.ret ==> 4
{ a: 'A', b: 'B', c: 'C' }
length _
# f.ret ==> 3
It will fail unless "length" receives a (non-attribute) argument that has a length.
Has the "size" alias.