6.4.4. Type operators¶
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.
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
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.