GHCi ignores type signature

Question

Prelude> let myprint = putStrLn . show
Prelude> :t myprint
myprint :: () -> IO ()

OK, nothing too unusual here. Just GHCi type defaulting rule

Answer 1

We can do the following, with monomorphism restriction on:

>let myprint :: Show x => x -> IO (); myprint = putStrLn . show
>:t myprint
myprint :: Show x => x -> IO ()

This is not the same as let myprint = putStrLn . show :: Show x => x -> IO (). In the former case we have a binding with a type signature, in the latter case we a have a let binding with a type annotation inside the right hand side. Monomorphism checks top-level type signatures, but not local annotations.