Scoped type variables

Scoped Type Variables are an extension to Haskell's type system that allow free type variables to be re-used in the scope of a function. They are also described in the GHC documentation.

As an example, consider the following functions:

{-# LANGUAGE ScopedTypeVariables #-}

...

mkpair1 :: forall a b. a -> b -> (a,b)
mkpair1 aa bb = (ida aa, bb)
    where
      ida :: a -> a -- This refers to a in the function's type signature
      ida = id

mkpair2 :: forall a b. a -> b -> (a,b)
mkpair2 aa bb = (ida aa, bb)
    where
      ida :: b -> b -- Illegal, because refers to b in type signature
      ida = id

mkpair3 :: a -> b -> (a,b)
mkpair3 aa bb = (ida aa, bb)
    where
      ida :: b -> b -- Legal, because b is now a free variable
      ida = id

Scoped type variables make it possible to specify the particular type of a function in situations where it is not otherwise possible, which can in turn help avoid problems with the Monomorphism_restriction.

This feature should be better documented in the Wiki, but this is a start.