Type operators in GHC

Edward Kmett ekmett at gmail.com
Thu Jan 19 04:36:27 CET 2012

I can live with it and I probably have as many packages as anyone that will
be broken by it. =/

Things like


will need a pretty invasive rewrite, but the simplicity is worth it, and it
makes for much better operators at the type level.

I can pre-emptively change the packages so as to be compatible with both,
so I have an upgrade path, which makes it all good.


On Wed, Jan 18, 2012 at 9:27 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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