One issue with this proposal is it makes it <b>completely</b> impossible to pick a type constructor operator that works with both older GHCs and 7.6.<div><br></div><div>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. =/</div>
<div><br></div><div>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.</div><div><br></div><div>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.</div>
<div><br></div><div>Would that be a viable approach?</div><div><br></div><div>-Edward</div><div><div><div><br><div class="gmail_quote">On Fri, Sep 14, 2012 at 7:26 PM, Simon Peyton-Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span> wrote:<br>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">Fair point. So you are saying it’d be ok to say<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"> data T (.->) = MkT (Int .-> Int)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">where (.+) is a type variable? Leaving ordinary (+) available for type constructors.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">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 *<b>constructor</b>*
operators in terms. Thus<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"> data T (:->) = MkT (Int :-> Int)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">That seems symmetrical, and perhaps nicer than having a new notation.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"> <u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"> In terms In types
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">-----------------------------------------------------------------------<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">a Term variable Type variable<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">A Data constructor Type constructor<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">+ Term variable operator Type constructor operator<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">:+ Data constructor operator Type variable operator<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">Any other opinions?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d">Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> <a href="mailto:conal.elliott@gmail.com" target="_blank">conal.elliott@gmail.com</a> [mailto:<a href="mailto:conal.elliott@gmail.com" target="_blank">conal.elliott@gmail.com</a>]
<b>On Behalf Of </b>Conal Elliott<br>
<b>Sent:</b> 06 September 2012 23:59</span></p><div class="im"><br>
<b>To:</b> Simon Peyton-Jones<br>
<b>Cc:</b> GHC users<br>
</div><div class="im"><b>Subject:</b> Re: Type operators in GHC<u></u><u></u></div><p></p>
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<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">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.</p><div><div class="h5"><br>
<br>
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 '.'.<br>
<br>
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.<br>
<br>
-- Conal<br>
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<u></u><u></u></div></div><p></p><div><div class="h5">
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<p class="MsoNormal">On Wed, Jan 18, 2012 at 6:27 AM, Simon Peyton-Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<u></u><u></u></p>
<p class="MsoNormal">Dear GHC users<br>
<br>
As part of beefing up the kind system, we plan to implement the "Type operators" proposal for Haskell Prime<br>
<a href="http://hackage.haskell.org/trac/haskell-prime/wiki/InfixTypeConstructors" target="_blank">http://hackage.haskell.org/trac/haskell-prime/wiki/InfixTypeConstructors</a><br>
<br>
GHC has had type operators for some kind, so you can say<br>
data a :+: b = Left a | Right b<br>
but you can only do that for operators which start with ":".<br>
<br>
As part of the above wiki page you can see the proposal to broaden this to ALL operators, allowing<br>
data a + b = Left a | Right b<br>
<br>
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)<br>
<br>
<br>
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<br>
{-# LANGUAGE TypeOperators #-}<br>
comp :: Arrow (~>) => (b~>c, c~>d)~>(b~>d)<br>
comp = arr (uncurry (>>>))<br>
<br>
Written more conventionally, the signature would look like<br>
comp :: Arrow arr => arr (arr b c, arr c d) (arr b d)<br>
comp = arr (uncurry (>>>))<br>
or, in infix notation<br>
{-# LANGUAGE TypeOperators #-}<br>
comp :: Arrow arr => (b `arr` c, c `arr` d) `arr` (b `arr` d)<br>
comp = arr (uncurry (>>>))<br>
<br>
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.<br>
<br>
It would be possible to have two flags, so as to get<br>
- Haskell 98 behaviour<br>
- Current TypeOperator behaviuor<br>
- New TypeOperator behaviour<br>
but it turns out to be Quite Tiresome to do so, and I would much rather not. Can you live with that?<br>
<br>
<br>
<a href="http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.html" target="_blank">http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.html</a><br>
<br>
<br>
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