.. _infix-tycons: 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.