This is confusing me, and I'm wondering if it's a compiler bug. I have this:
foo = (++) `on` show
I'm expecting the type to be:
foo :: Show a => a -> a -> String
But GHCI is inferring:
foo :: () -> () -> String
No bug; it's the monomorphism restriction combining with extended defaulting to infer a monomorphic type. If you want to keep the monomorphism restriction and throw a type error, disable ExtendedDefaultRules.