6.4.4. Type operators






Included in GHC2024, GHC2021

Allow the use and definition of types with operator names.

The language TypeOperators allows you to use infix operators in types.

  • Operator symbols are constructors rather than type variables (as they are in terms).

  • 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
  • Alphanumeric type constructors can now be written infix, using backquote syntax:

    x :: Int `Either` Bool
    x = Left 5
  • 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.

  • There is now potential ambiguity in the traditional syntax for data constructor declarations. For example:

    type a :+: b = Either a b
    data X = Int :+: Bool :+: Char

    This code wants to declare both a type-level :+: and a term-level :+: (which is, generally, allowed). But we cannot tell how to parenthesize the data constructor declaration in X: either way makes sense. We might imagine that a fixity declaration could help us, but it is awkward to apply the fixity declaration to the very definition of a new data constructor. Instead of declaring delicate rules around this issue, GHC simply rejects if the top level of a traditional-syntax data constructor declaration uses two operators without parenthesizing.