[Haskell-beginners] Is this a GHC bug? Problem inferring type

[Peter Hall]

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


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
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