Type operators in GHC

Simon Peyton-Jones simonpj at microsoft.com
Wed Jan 18 15:27:21 CET 2012

Dear GHC users

As part of beefing up the kind system, we plan to implement the "Type operators" proposal for Haskell Prime

GHC has had type operators for some kind, so you can say
	data a :+: b = Left a | Right b
but you can only do that for operators which start with ":".

As part of the above wiki page you can see the proposal to broaden this to ALL operators, allowing
	data a + b = Left a | Right b

Although this technically inconsistent the value page (as the wiki page discussed), I think the payoff is huge. (And "A foolish consistency is the hobgoblin of little minds", Emerson)

This email is (a) to highlight the plan, and (b) to ask about flags.  Our preferred approach is to *change* what -XTypeOperators does, to allow type operators that do not start with :.  But that will mean that *some* (strange) programs will stop working. The only example I have seen in tc192 of GHC's test suite
	{-# LANGUAGE TypeOperators #-}
	comp :: Arrow (~>) => (b~>c, c~>d)~>(b~>d)
      comp = arr (uncurry (>>>))	

Written more conventionally, the signature would look like
	comp :: Arrow arr => arr (arr b c, arr c d) (arr b d)
      comp = arr (uncurry (>>>))
or, in infix notation
	{-# LANGUAGE TypeOperators #-}
	comp :: Arrow arr => (b `arr` c, c `arr` d) `arr` (b `arr` d)
      comp = arr (uncurry (>>>))

But tc192 as it stands would become ILLEGAL, because (~>) would be a type *constructor* rather than (as now) a type *variable*.  Of course it's easily fixed, as above, but still a breakage is a breakage.

It would be possible to have two flags, so as to get
  - Haskell 98 behaviour
  - Current TypeOperator behaviuor
  - New TypeOperator behaviour
but it turns out to be Quite Tiresome to do so, and I would much rather not.  Can you live with that? 


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