Type operators in GHC

Simon Peyton-Jones simonpj at microsoft.com
Sat Sep 15 01:26:43 CEST 2012

Fair point.  So you are saying it'd be ok to say

  data T (.->)  = MkT (Int .-> Int)

where (.+) is a type variable?   Leaving ordinary (+) available for type constructors.

If we are inverting the convention I wonder whether we might invert it completely and use ":" as the "I'm different" herald as we do for *constructor* operators in terms.  Thus

  data T (:->)  = MkT (Int :-> Int)

That seems symmetrical, and perhaps nicer than having a new notation.

         In terms                                      In types
a        Term variable                             Type variable
A        Data constructor                         Type constructor
+        Term variable operator               Type constructor operator
:+      Data constructor operator           Type variable operator

Any other opinions?


From: conal.elliott at gmail.com [mailto:conal.elliott at gmail.com] On Behalf Of Conal Elliott
Sent: 06 September 2012 23:59
To: Simon Peyton-Jones
Cc: GHC users
Subject: Re: Type operators in GHC

Oh dear. I'm very sorry to have missed this discussion back in January. I'd be awfully sad to lose pretty infix notation for type variables of kind * -> * -> *. I use them extensively in my libraries and projects, and pretty notation matters.

I'd be okay switching to some convention other than lack of leading ':' for signaling that a symbol is a type variable rather than constructor, e.g., the *presence* of a leading character such as '.'.

Given the increasing use of arrow-ish techniques and of type-level programming, I would not classify the up-to-7.4 behavior as a "foolish consistency", especially going forward.

-- Conal

On Wed, Jan 18, 2012 at 6:27 AM, Simon Peyton-Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
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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