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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
Digressing a bit, I wish it were possible to write the pattern above as <o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I agree! See: https://ghc.haskell.org/trac/ghc/wiki/ViewPatternsAlternative<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Maybe we should work that up into a new GHC proposal.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-steering-committee [mailto:ghc-steering-committee-bounces@haskell.org]
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 23 March 2017 10:01<br>
<b>To:</b> Roman Leshchinskiy <rleshchinskiy@gmail.com><br>
<b>Cc:</b> ghc-steering-committee@haskell.org; Joachim Breitner <mail@joachim-breitner.de><br>
<b>Subject:</b> Re: [ghc-steering-committee] Constructor synonyms, signatures on pattern synonym constructors<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
On 21 March 2017 at 23:26, Roman Leshchinskiy <<a href="mailto:rleshchinskiy@gmail.com" target="_blank">rleshchinskiy@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hi,<br>
<br>
I'm in a slightly difficult situation here since I quite dislike the<br>
current pattern synonyms extension. I'll try nonetheless.<br>
<br>
<span class="gmail-">On Tue, Mar 21, 2017 at 1:43 PM, Joachim Breitner</span><br>
<span class="gmail-"><<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:</span><br>
<span class="gmail-">> Hi,</span><br>
<span class="gmail-">></span><br>
<span class="gmail-">> Am Dienstag, den 21.03.2017, 00:55 -0500 schrieb Christopher Allen:</span><br>
<span class="gmail-">>> Proposal #41 is constructor synonyms. This would allow users to</span><br>
>> [...]<br>
<span class="gmail-">></span><br>
<span class="gmail-">> it is worth pointing out that under this proposal,</span><br>
<span class="gmail-">></span><br>
<span class="gmail-">> constructor FireMissles :: IO ()</span><br>
<span class="gmail-">> constructor FireMissles = fireMissles</span><br>
<span class="gmail-">></span><br>
<span class="gmail-">> (without a pattern) is also valid. In other words: The capitalization</span><br>
<span class="gmail-">> of an identifier will no longer be of significance.</span><br>
<span class="gmail-">></span><br>
<span class="gmail-">> This makes me dislike the proposal.</span><br>
<br>
I completely agree. In fact, this proposal is basically about allowing<br>
value variable names to start with an upper case letter so if we want<br>
to go that way then I'm not sure why the keyword 'constructor' is<br>
necessary at all. I would much prefer not to go that way, though, so<br>
I'm against #41.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Yes, I think I agree with this too.<o:p></o:p></p>
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<o:p> </o:p></p>
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The point of pattern synonyms is to let you define something that to the importer of a module looks exactly like a data constructor, while allowing the author of the module to change its implementation. It fills in a missing opportunity for abstraction in
the language.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
But constructor synonyms go much further than this, allowing you to define something that looks like a data constructor but doesn't behave anything like one. It drops the requirement that the constructor be expressible as a pattern. If you want to do this,
define a function!<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Why not solve the original problem by allowing a separate type signature on the constructor in the pattern synonym? There's already an example of this in the proposal:<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
pattern Zero :: (Eq a, Num a) => a<o:p></o:p></p>
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pattern Zero <- ((== 0) -> True) where<o:p></o:p></p>
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Zero :: Num a => a<o:p></o:p></p>
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Zero = 0<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
(the proposal has the signature on Zero slightly wrong, I think). This seems like it would address the problem but without also overly generalising what a conid can represent, and avoids the issues with import/export.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Digressing a bit, I wish it were possible to write the pattern above as <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
pattern Zero :: (Eq a, Num a) => a<o:p></o:p></p>
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pattern Zero <- x | x == 0 where<o:p></o:p></p>
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Zero :: Num a => a<o:p></o:p></p>
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Zero = 0<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
because I think ViewPatterns were a mistake. But that's a proposal for another day :)<o:p></o:p></p>
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<o:p> </o:p></p>
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Cheers<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Simon<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span class="gmail-">>> Proposal #42 would add type signatures for bidirectional pattern</span><br>
<span class="gmail-">>> synonym constructor functions.</span><br>
>> [...]<br>
<br>
This seems sensible provided there is a strong relation between the<br>
types of the constructor and the pattern.<br>
<br>
<span class="gmail-">> The proposal does not address if the signatures of the constructor and</span><br>
<span class="gmail-">> the signature of the pattern have to be in any way related. Possible</span><br>
<span class="gmail-">> design choices are:</span><br>
<span class="gmail-">> * May not differ in anything but the constraints.</span><br>
<span class="gmail-">> * Must have the same return type.</span><br>
<span class="gmail-">> * Must have the same outer type constructor in their return type.</span><br>
<span class="gmail-">> * No relation.</span><br>
<span class="gmail-">> This ought to be clarified.</span><br>
<br>
This is a very good point. I'd be in favour of this proposal under the<br>
"May not differ in anything but the constraints" policy and against it<br>
under any of the other three. A looser relationship between the<br>
constructor function and the pattern makes code significantly harder<br>
to read and the proposal doesn't include any justification for such a<br>
looser relationship so I would go with the strongest requirement<br>
possible.<br>
<br>
Thanks,<br>
<br>
Roman<o:p></o:p></p>
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_______________________________________________<br>
ghc-steering-committee mailing list<br>
<a href="mailto:ghc-steering-committee@haskell.org">ghc-steering-committee@haskell.org</a><br>
<a href="https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee" target="_blank">https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee</a><o:p></o:p></p>
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<o:p> </o:p></p>
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