I have no problems in defining those functions: $ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let id2 :: t -> t; id2 = \x -> x Prelude> let a :: a -> a; a = a a Note that I used ';'. This is equivalent to writing let a :: a -> a a = a a If I try the syntax you were trying without ScopedTypeVariables, I get Prelude> let a :: a -> a = a a <interactive>:1:4: Illegal signature in pattern: a -> a Use -XScopedTypeVariables to permit it GHCi thinks that you were trying to define a function by pattern matches and there was a signature inside it, which is a different thing. This is the expected use for such thing: Prelude> :s -XScopedTypeVariables Prelude> let id2 :: forall t. t -> t; id2 (x :: t) = x Cheers, =) -- Felipe.