6.8.1
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:
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.
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