Infix Type Constructors

Brief Explanation

GHC allows type constructors to be infix operators (conops, beginning with :, but not including : itself).

Changes to the syntax may depend on whether CompositionAsDot is adopted, but roughly speaking we add

 qtycon   -> qconid  | ( qconsym )
 qtyconop -> qconsym | ` qconid `

And type gets an extra production:

 type    -> btype qtyconop type

(modulo FixityResolution). Also, there are obvious changes to the grammar for type, data, and newtype declarations.

References

•  Infix type constructors, classes, and type variables in the GHC User's Guide.

Tickets

#78

Add infix type constructors

Pros

• This is a straightforward generalisation, doesn't break any existing code, and improves the consistency of the syntax.

Cons

Observations

• Note that classes can be infix too; this is useful.
• Need to have a way to set the fixity of a type constructor T differently than the data constructor T (or not?).
• Need to allow infix notation in contexts

  f :: (a :>: b) => bla blah
  

• Watch out for code like this ( http://hackage.haskell.org/trac/ghc/ticket/1727)

   infixr 5 `Foo`
   infixr 6 `Bar`
  
   data a `Foo` b = a `FOO` a `Bar` b
   data a `Bar` b = a `BAR` b