Type operators in GHC

Conal Elliott conal at conal.net
Fri Sep 7 00:59:21 CEST 2012

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>wrote:

> Dear GHC users
> As part of beefing up the kind system, we plan to implement the "Type
> operators" proposal for Haskell Prime
> http://hackage.haskell.org/trac/haskell-prime/wiki/InfixTypeConstructors
> 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?
> http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.html
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