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.

The GHC Users Guide has a Scoped Type Variables section.

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.

Avoiding Scoped Type Variables

Although Scoped Type Variables are often a simple solution, they are not available in all compilers. Often there is a solution that is Haskell 98. First, there is

asTypeOf :: a -> a -> a
asTypeOf a b = a
.

It is used like x `asTypeOf` y and has the same value like x, but type inference asserts that x and y have the same type.

Sometimes it helps to divide a big function into smaller ones and give each of the small functions a signature. This also helps reading the program.

If this does not help, too, then use a helper function. E.g. if you want to determine the size of an object a pointer points to, then you might define a function like

sizeOfPtr :: Ptr a -> Int
sizeOfPtr = sizeOf . (undefined :: Ptr a -> a)

or

sizeOfPtr :: Ptr a -> a -> Int
sizeOfPtr _ a = sizeOf a

sizeOf :: Ptr a -> Int
sizeOf ptr = sizeOfPtr ptr undefined
.

See also

• Haskell-Cafe on sizeOf on a type
• Haskell-Cafe on What is a rigid type variable?
• Haskell-Cafe on asserting the type of a binding in a "do" expression