One issue with this proposal is it makes it *completely* impossible to pick a type constructor operator that works with both older GHCs and 7.6.
It is a fairly elegant choice, but in practice it would force me and many others to stop using them completely for the next couple of years, as I wouldn't be able to support any users on older GHCs, or if I did I would have to export completely different operator names, and then the users would have to use conditional compilation to do different things with them. =/ As it is, I can and do at least choose : prefixed names for any type constructor I want to have be compatible with old GHCs. Back when the change was initially proposed I think it was Igloo who suggested that it might be possible to allow the use of symbols as type variables if they were explicitly quantified by a forall. Would that be a viable approach? -Edward On Fri, Sep 14, 2012 at 7:26 PM, Simon Peyton-Jones <simo...@microsoft.com>wrote: > 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?**** > > ** ** > > Simon**** > > ** ** > > *From:* conal.elli...@gmail.com [mailto:conal.elli...@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 < > simo...@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 > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users**** > > ** ** > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
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