6.4.3. Infix type constructors, classes, and type variables

GHC allows type constructors, classes, and type variables to be operators, and to be written infix, very much like expressions. More specifically:

  • A type constructor or class can be any non-reserved operator. Symbols used in types are always like capitalized identifiers; they are never variables. Note that this is different from the lexical syntax of data constructors, which are required to begin with a :.

  • Data type and type-synonym declarations can be written infix, parenthesised if you want further arguments. E.g.

    data a :*: b = Foo a b
    type a :+: b = Either a b
    class a :=: b where ...
    data (a :**: b) x = Baz a b x
    type (a :++: b) y = Either (a,b) y
  • Types, and class constraints, can be written infix. For example

    x :: Int :*: Bool
    f :: (a :=: b) => a -> b
  • Back-quotes work as for expressions, both for type constructors and type variables; e.g. Int `Either` Bool, or Int `a` Bool. Similarly, parentheses work the same; e.g. (:*:) Int Bool.

  • Fixities may be declared for type constructors, or classes, just as for data constructors. However, one cannot distinguish between the two in a fixity declaration; a fixity declaration sets the fixity for a data constructor and the corresponding type constructor. For example:

    infixl 7 T, :*:

    sets the fixity for both type constructor T and data constructor T, and similarly for :*:. Int `a` Bool.

  • The function arrow -> is infixr with fixity -1.