6.4.4. Type operators¶
- TypeOperators¶
- Implies:
- 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 (+) ) 
- 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.