Re: Type operators in GHC

+1. Making ":" the signal for type variables would break even more code, f.e. fclabels. "~" almost means "variable", so I'd like that as a prefix. Sjoerd On Sep 15, 2012, at 2:09 AM, Cale Gibbard wrote:

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 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.elliott@gmail.com [mailto:conal.elliott@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 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.htm...

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-- Sjoerd Visscher https://github.com/sjoerdvisscher/blog