6.4.4. Type operators

TypeOperators
Implies:ExplicitNamespaces
Since:6.8.1

Allow the use and definition of types with operator names.

In types, an operator symbol like (+) is normally treated as a type variable, just like a. Thus in Haskell 98 you can say

type T (+) = ((+), (+))
-- Just like: type T a = (a,a)

f :: T Int -> Int
f (x,y)= x

As you can see, using operators in this way is not very useful, and Haskell 98 does not even allow you to write them infix.

The language TypeOperators changes this behaviour:

  • Operator symbols become type constructors rather than type variables.

  • Operator symbols in types can be written infix, both in definitions and uses. For example:

    data a + b = Plus a b
type Foo = Int + Bool

  • There is now some potential ambiguity in import and export lists; for example if you write import M( (+) ) do you mean the function (+) or the type constructor (+)? The default is the former, but with ExplicitNamespaces (which is implied by TypeOperators) GHC allows you to specify the latter by preceding it with the keyword type, thus:

    import M( type (+) )

    See Explicit namespaces in import/export.

  • The fixity of a type operator may be set using the usual fixity declarations but, as in Infix type constructors, classes, and type variables, the function and type constructor share a single fixity.