Haskell - Pattern matches overlap?

Question

Why do the pattern matches overlap in this code:

(+) (Roman (_, [])) x = x
(+) x (Roman (_, [])) = x

It would make sense considering x + y is the same as y + x but I don't think haskell takes care of that, or does it?

Answer 1

The problem is that x matches anything.

You can use at-patterns to make them unique:

(+) (Roman (_, [])) x@(Roman(_, _:_) = x
(+) x@(Roman(_, _:_) (Roman (_, [])) = x

Answer 2

x could be anything, so it could be Roman (_,[]).