Hello, My preference would be to change the behavior of the "TypeOperator" flag. Here is my reasoning: * Having two operators with slightly different meanings would be very confusing, not just for beginners but for everyone. * The two behaviors are not compatible in the sense that they can't co-exist in the same language standard. So something is bound to break later on, and it'd be better to update packages now, before we have grown a bigger code base that uses two incompatible extensions. * As I see it, having type-operator variables is useful in (some) APIs (e.g., the parameters of the arrow classes), while type-operator constructors are useful in defining concrete types/type-functions (e.g., concrete implementations of arrows). I expect that there are a lot more concrete implementations than APIs (this is the whole point of abstraction, after all!) so I think that it is a good trade-off to use the nicer notation for the more common case. -Iavor PS: In the interest of fairness, I am also partial to the new notation because I've been working on support for arithmetic at the type-level, and seem to have developed a bit of an allergy to typing :+:, :*:, :<= etc all the time :-) On Wed, Jan 18, 2012 at 9:36 AM, Brandon Allbery wrote:

On Wed, Jan 18, 2012 at 10:42, Brent Yorgey wrote:

...

I'm not sure at this point which way I would lean on the issue. Having infix type constructors that don't have to start with : is something I would like, too.  But I just thought I would point out that code like tc192 is not as esoteric as you seem to think.

I seem to recall @src outputting a Prelude definition for something (Monoid?) that used (*) as a type variable.

-- brandon s allbery                                      allbery.b@gmail.com wandering unix systems administrator (available)     (412) 475-9364 vm/sms

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Hi, Am Donnerstag, den 19.01.2012, 07:11 -0500 schrieb Matthew Farkas-Dyck:

On 19/01/2012, Joachim Breitner wrote:

...

(I have no good idea, but here is at least one: A dot '.' as the first character indicates a type variable; compared to a ':' this is a non-capitalized character).

So that all symbols that start in dot are variables, and all others are types/constructors?

exactly, that would be an option to get the benefit of the proposal (nicer type constructor operator names) without giving up completely on type variable operators names. Greetings, Joachim -- Joachim "nomeata" Breitner mail@joachim-breitner.de | nomeata@debian.org | GPG: 0x4743206C xmpp: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/

On 19/01/2012, Ian Lynagh wrote:

Do you mean that in

f :: (x, X, (+), (:+))

only x would be a type variable and X, (+), (:+) would be type constructors, but that in

g :: forall y, Y, (*), (:*) . (x, X, (+), (:+), y, Y, (*), (:*))

y, Y, (*), (:*) would be type variables and x, X, (+), (:+) would be whatever is in scope (constructors, unless there is an enclosing forall that binds them)?

Just so.

Perhaps we should be heading towards a case-insensitive syntax for type names.

I've often had that thought myself, for types and terms both. It would make it much nicer to do general numeric computations in Haskell (e.g. fluid mechanics) since one could use upper-case term names, which are often customary. I actually designed a compiled-to-Haskell language for just this reason (for fluid mechanics lab), but it's very crude. That said, for word-names, the case-sensitive system we have is nice and brief.

Thanks Ian

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 mailto:simonpj@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.htm... _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

+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...

_______________________________________________ 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

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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

I also have quite a lot of code (growing daily) that uses (~>) as a type
variable. It's not the only such type variable, because some abstractions
are about combining multiple arrowish things, e.g., more CT variations on
Functor and Foldable that allow valuable flexibility missing in the
standard library. In those cases, I typically use (+>) and (-->) as well.

-- Conal

On Fri, Sep 14, 2012 at 5:09 PM, 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 <
> simonpj@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
> >
>

Hi Simon, Yes, I could live with (.->), (.+), etc more easily than `arr`, `plus` etc. Better yet would be a LANGUAGE pragma I can add to my libraries to get the old behavior back. Better still for me personally would be for other libraries to add a LANGUAGE pragma to get the 7.6.1 behavior. I can live without this option. Using a ":" prefix for type ctor variables would break the other half of my types in these libraries. I use type variables with names like (~>), (+>), (-->) etc in order to express abstractions, and then I typically use those abstractions to define concrete type ctors with names like (:->), (:+>), (:-->), etc. My regrets for raising these issues so late in the game. -- Conal On Fri, Sep 14, 2012 at 4:26 PM, 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...

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users****

** **

Hello, I think that it would be a mistake to have two pragmas with incompatible behaviors: for example, we would not be able to write modules that use Conal's libraries and, say, the type nats I've been working on. If the main issue is the notation for arrows, has anoyone played with what can be done with the current (7.6) system? I just thought of two variations that seem to provide a decent notation for writing arrow-ish programs. The second one, in particular, mirrors the arrow notation at the value level, so perhaps that would be enough? -Iavor {-# LANGUAGE TypeOperators, KindSignatures #-} module Test where import Control.Category -- Variant 1: Post-fix annotation type (a ---> b) c = c a b f :: Category c => (x ---> y) c -> (y ---> z) c -> (x ---> z) c f = undefined -- Variant 2: Arrow notation type a >-- (c :: * -> * -> *) = c a type c --> b = c b infix 2 >-- infix 1 --> g :: Category c => (x >--c--> y) -> (y >--c--> z) -> (x >--c--> z) g = undefined

Iavor: Wow, I really like the >--c--> trick at the type level. Note: we can shorten that somewhat and improve the fixity to associate correctly, matching the associativity of (->), which fortunately associates to the right. (associating to the left can be done with a similar trick, based on the original version of this hack by Chung-Chieh Shan.) {-# LANGUAGE TypeOperators, PolyKinds #-} import Control.Category infixr 0 >~ infixr 0 ~> type (>~) a b = b a type (~>) a b = a b g :: Category c => (x >~c~> y) -> (y >~c~> z) -> x >~c~> z g = undefined Note, this also has the benefit of picking the correct associativity for

~c~>. Unlike naively using a locally bound (~>) and avoids the headaches of picking (-->) and (--->) or something equally hideous when working with two categories.

class (Category c, Category d) => CFunctor f c d | f c -> d, f d -> c where cmap :: (a >~c~> b) -> f a >~d~> f b -Edward On Mon, Sep 17, 2012 at 1:02 PM, Sjoerd Visscher wrote:

Hi,

Note that nobody was suggesting two pragmas with incompatible behaviors, only to have just one symbol reserved to still be able to have type operator variables.

I do like your suggestion, although >--c--> is quite a bit longer than ~>.

Sjoerd

On Sep 17, 2012, at 6:28 PM, Iavor Diatchki wrote:

Hello,

I think that it would be a mistake to have two pragmas with incompatible behaviors: for example, we would not be able to write modules that use Conal's libraries and, say, the type nats I've been working on. If the main issue is the notation for arrows, has anoyone played with what can be done with the current (7.6) system? I just thought of two variations that seem to provide a decent notation for writing arrow-ish programs. The second one, in particular, mirrors the arrow notation at the value level, so perhaps that would be enough?

-Iavor

{-# LANGUAGE TypeOperators, KindSignatures #-} module Test where

import Control.Category

-- Variant 1: Post-fix annotation

type (a ---> b) c = c a b

f :: Category c => (x ---> y) c -> (y ---> z) c -> (x ---> z) c f = undefined

-- Variant 2: Arrow notation

type a >-- (c :: * -> * -> *) = c a type c --> b = c b

infix 2 >-- infix 1 -->

g :: Category c => (x >--c--> y) -> (y >--c--> z) -> (x >--c--> z) g = undefined

_______________________________________________ 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

1) kudos to iavor and edward on the slick notation invention! 2) the key point is that ghc 7.6 does not have support for infix type variable notation, and how to encode infix arrow notations nicely subject that design choice, right? i'm likely just being a tad redundant in this conversation, but it never hurts to sanity check :) cheers -Carter On Mon, Sep 17, 2012 at 6:40 PM, Edward Kmett wrote:

On Mon, Sep 17, 2012 at 1:02 PM, Sjoerd Visscher wrote:

...

Hi,

Note that nobody was suggesting two pragmas with incompatible behaviors, only to have just one symbol reserved to still be able to have type operator variables.

An issue with reserving a symbol for type operator variables is it doesn't help you today.

7.6.1 is already released.

This means that any change in behavior would have to be in 7.6.2 at the earliest. Assuming the bikeshedding could complete and Simon et al. frantically patched the code tomorrow, rushing to release a 7.6.2 before the platform release.

Failing that, you'd have a whole release cycle to wait through, probably a platform, before you could go back to your old behavior, and then your code would have some strange gap of GHC version numbers over which it didn't work.

Everyone would have to pretend 7.6.1 never happened, or and break anyone's code that was already written for 7.6, so instead of one breaking change, we'd now have two.

For instance, I'm already using ~> in 'github.com/ekmett/indexed.git' for natural transformations and I am loving it, and would be sad to lose it to the choice of ~ as a herald, similarly it would make the >~c~> trick more verbose, and ~ is particularly terrible for operators like ~+~.

Other herald choices lead to different issues, '.' is slightly better for the other operators, but makes kind of ugly arrows, plus some day i'd _really_ like to be able to use . as a type constructor for functor composition! It is currently reserved at the type level as an almost accidental consequence of the way forall gets parsed today.

I really like Iavor's entirely-in-language way of addressing the issue, due in part to it providing even better associativity than the existing approach, and honestly, even if GHC HQ was somehow convinced to set aside an ad hoc herald for type variables, I'd probably start using it in my code. (probably sandwiching between something like :- and :> for old GHC compatibility). I really like that I can just call the Category c, and just get >~c~> or something similar as its arrows. This feels more notationally accurate to me.

It also has two major benefits compared to any proposal for adding different heralds:

1.) It is compatible with old code, code written with 7.6.1 and I suppose future code, since (:) is such a remarkably awkward choice of herald for the reasons already documented that it seems an unlikely choice.

2.) I can program with it today.

I just realized if you don't want to worry about collisions with the type naturals from GHC.TypeLits, and didn't care about pre-7.6 compatibility, you could strip the notation down all the way to

cmap :: CFunctor f c d => (x -c> y) -> f x -d> f y

This is even shorter than the conventional

cmap :: CFunctor f (~>) (~~>) => (x ~> y) -> f x ~~> f y

Which turns the "but it is longer" argument against it on its head. ;)

-Edward

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users