I don’t think you can “give” precedence to the pattern match with ( ). I think they will just be ignored
In your example (%{a: x} = %{b: y}) = %{a: "what", b: "happened"} is a pattern match where the rightmost value is %{a: "what", b: "happened"}, so it has to be known and can’t bind variables, and the left side is (%{a: x} = %{b: y}) which is a pattern that the mentioned value has to satisfy. It means that the value need to satisfy both %{a: x} and %{b: y} and it will check against %{b: y} first before doing %{a: x}. That means that if the match fails on both patterns, the execution will stop and error on the b match first, but in order for match to succeed, all the patterns on the left side need to be matched.
So you are not comparing %{a: x} to %{b: y} - these are patterns for the same input. The only way when two patterns on the left side can interrupt each other is when they bind to the same variable, as there is a rule that value can be bind multiple times in one expression only if it binds to the same value. So %{a: x} = %{b: x} = %{a: "what", b: "happened"} will fail
EDIT:
Ok. No longer so sure enough about this statement to give tips on how it actually works. I simply always perceived match operator as folding from right to left, checking patterns in the process but it might be just my mental model
and it will check against
%{b: y}first before doing%{a: x}. That means that if the match fails on both patterns, the execution will stop and error on thebmatch first, but in order for match to succeed, all the patterns on the left side need to be matched.
