Thanks @LostKobrakai that’s the best answer I’ve seen to this question so far.
So the reason why this works:
iex(2)> one = 1
1
iex(3)> %{a: ^one} = %{a: 1, b: 2}
%{a: 1, b: 2}
But this one doesn’t:
iex(1)> map_with_one = %{a: 1}
%{a: 1}
iex(2)> ^map_with_one = %{a: 1, b: 2}
** (MatchError) no match of right hand side value: %{a: 1, b: 2}
Is because the pin operator can only be used for exact matches but not for partial ones, the reason being intrinsic limitations of the BEAM. Correct?
Yes, but I’d suggest not even trying to think of it like that. There’s (runtime) data assigned to a variable like map_with_one and there’s syntax, which can in some cases look exactly like the syntax used to define the data for map_with_one, but which forms a pattern for a pattern match. One is completely unrelated to the other. This becomes even more apparent when you consider the features of a match pattern, which a map of data doesn’t support, like _ for ignoring values or variable assignments %{a: a} = ….
The pin operator is a way to use data assigned to a variable to be equality matched within a pattern. Nothing more and nothing less. It does not become a pattern by itself. Patterns are compile time structures, which cannot be adjusted at runtime, stored in a variable or otherwise modified after compilation is done. Only patterns can be used for a pattern match on the left side of the match operator (=). When you put a variable on the left side it’s either an assignment, or if you use a pin an equality check to match the pattern.
