@@ -23,6 +23,39 @@ And `type` gets an extra production:
(modulo [FixityResolution](fixity-resolution)). Also, there are obvious changes to the grammar for `type`, `data`, and `newtype` declarations.
Secondly, I propose to allow varsyms to be used as type *constructors*. For example, currently "+" is a varsym, so at the type level it'd behave like a type *variable*
data T (+) = MkT (Int + Int)
It's not impossible that this might be useful, although the binding site looks clumsy. But it misses a much more useful opportunity. What we *want* is to say
data a + b = Left a | Right b
That is, we want to define the type *constructor*`(+)`. Currently we have to use the clumsy `:+` notation:
data a :+ b = Left a | Right b
Yuk. **So I propose that varsyms can be used as type constructors, and not as type variables.**
You may say that is inconsistent, because at the value level you have to start data constructors with a ":". But the type level is already funny. The whole type-family idea (beginning with type synonyms) defines things that begin with a capital letter, but which (unlike data constructors) are not head normal forms. By the time we have full type-synonym families, they really are \*functions\* as much as any value-level function is.
Some people use Haskell as a laboratory in which to write their cunning type ideas. In mathematics, operators are invariably top-level type constructors (think of the type a+b). Mirroring this in Haskell would make the transcription more elegantly direct.
I can't think of any down-sides, except the slight loss of consistency ("the hobgoblin of tiny minds").
-[ Infix type constructors, classes, and type variables](http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#infix-tycons) in the GHC User's Guide.