foo = (++) `on` show

I'm expecting the type to be:

    foo :: Show a => a -> a -> String

But GHCI is inferring:

    foo :: () -> () -> String

If I explicitly provide a type annotation, or if I use -XNoMonormorphismRestriction, then it all works fine. I would expect a compile error if the type couldn't be inferred due to the Monomorphism Restriction, but instead it has silently chosen the least useful type.

Peter