Haskell: Why aren't infix type constructors allowed?

Question

In the Haskell 98 report, I found this:

The syntax for Haskell type expressions is given above. Just as data values are built using data constructors,

Answer 1

It's not part of the Haskell standard, but as jamshidh mentions it is still possible in GHC. The caveat is that data constructors (not type constructors) must start with a colon:

{-# LANGUAGE TypeOperators #-}

data a + b = a :+ b

f :: a + b -> a
f (a :+ b) = a

g :: a + b -> b
g (a :+ b) = b

Answer 2

Just to be completely clear: Haskell 98 and Haskell 2000 both allow infix value constructors such as

data Complex r = r :+ r

Here the value constructor (:+) is infix, as in 5 :+ 7.

You only need the TypeOperators extension to have type constructors which are infix. For example,

data x ??! y = Left x | Right y

Here the type constructor (??!) is infix, as in Int ??! Bool.