There's a fair amount of code out there which uses (~>) as a type variable (we have ~10k lines of heavy arrow code at iPwn). It would be *really* nice if that could be accommodated somehow. But the proposal you just gave at least would allow for a textual substitution, so not quite so bad as having to change everything to prefix notation.
On 14 September 2012 19:26, Simon Peyton-Jones <[email protected]> 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: [email protected] [mailto:[email protected]] 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 <[email protected]> > 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 > [email protected] > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > > > > _______________________________________________ > Glasgow-haskell-users mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list [email protected] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
