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<p class="MsoPlainText">Some thoughts<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>To me it's clear that we should abandon #41 in favour of #42. Indeed the author did so himself, but then re-opened it saying only "@kosmikus thinks this should be reopened".<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt">I could elaborate, but see the discussion on #41. I think it’s pretty much a consensus.<o:p></o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt"><o:p> </o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>For #42, I’m mildly in favour, as I said on the thread. (But no holes please!)<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>I agree with Roman that that type of the constructor should be the same as that of the pattern, constrains aside. We can always loosen later if there is well-supported evidence that doing so would be good.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Simon<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">| <span lang="EN-US">-----Original Message-----</span></p>
<p class="MsoPlainText">| <span lang="EN-US">From: ghc-steering-committee [mailto:ghc-steering-committee-</span></p>
<p class="MsoPlainText">| <span lang="EN-US">bounces@haskell.org] On Behalf Of Christopher Allen</span></p>
<p class="MsoPlainText">| <span lang="EN-US">Sent: 21 March 2017 05:55</span></p>
<p class="MsoPlainText">| <span lang="EN-US">To: ghc-steering-committee@haskell.org</span></p>
<p class="MsoPlainText">| <span lang="EN-US">Subject: [ghc-steering-committee] Constructor synonyms, signatures on</span></p>
<p class="MsoPlainText">| <span lang="EN-US">pattern synonym constructors</span></p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| I’m shepherding this two intertwined proposals, the Github discussions</p>
<p class="MsoPlainText">| are located at:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| - <a href="https://github.com/ghc-proposals/ghc-proposals/pull/41">
<span style="color:windowtext;text-decoration:none">https://github.com/ghc-proposals/ghc-proposals/pull/41</span></a></p>
<p class="MsoPlainText">| - <a href="https://github.com/ghc-proposals/ghc-proposals/pull/42">
<span style="color:windowtext;text-decoration:none">https://github.com/ghc-proposals/ghc-proposals/pull/42</span></a></p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| The rendered proposals are located at time of review at:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| - <a href="https://github.com/treeowl/ghc-proposals/blob/680d909a07b10b6a37a64adf174ee845783dc2a4/proposals/0000-constructor-synonyms.rst">
<span style="color:windowtext;text-decoration:none">https://github.com/treeowl/ghc-</span></a></p>
<p class="MsoPlainText"><a href="https://github.com/treeowl/ghc-proposals/blob/680d909a07b10b6a37a64adf174ee845783dc2a4/proposals/0000-constructor-synonyms.rst"><span style="color:windowtext;text-decoration:none">| proposals/blob/680d909a07b10b6a37a64adf174ee845783dc2a4/proposals/0000</span></a></p>
<p class="MsoPlainText"><a href="https://github.com/treeowl/ghc-proposals/blob/680d909a07b10b6a37a64adf174ee845783dc2a4/proposals/0000-constructor-synonyms.rst"><span style="color:windowtext;text-decoration:none">| -constructor-synonyms.rst</span></a></p>
<p class="MsoPlainText">| - <a href="https://github.com/treeowl/ghc-proposals/blob/559df2865db8e4427b1f24d3100e59d61cb60e54/proposals/0000-bidir-constr-sigs.rst">
<span style="color:windowtext;text-decoration:none">https://github.com/treeowl/ghc-</span></a></p>
<p class="MsoPlainText"><a href="https://github.com/treeowl/ghc-proposals/blob/559df2865db8e4427b1f24d3100e59d61cb60e54/proposals/0000-bidir-constr-sigs.rst"><span style="color:windowtext;text-decoration:none">| proposals/blob/559df2865db8e4427b1f24d3100e59d61cb60e54/proposals/0000</span></a></p>
<p class="MsoPlainText"><a href="https://github.com/treeowl/ghc-proposals/blob/559df2865db8e4427b1f24d3100e59d61cb60e54/proposals/0000-bidir-constr-sigs.rst"><span style="color:windowtext;text-decoration:none">| -bidir-constr-sigs.rst</span></a></p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| Very briefly,</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| Proposal #41 is constructor synonyms. This would allow users to define</p>
<p class="MsoPlainText">| variables with capitalized names and operators that begin with colons.</p>
<p class="MsoPlainText">| They are meant to complement pattern synonyms. This example from the</p>
<p class="MsoPlainText">| rendered proposal captures the idea and complementarity well I think:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| pattern NF :: a -> NF a</p>
<p class="MsoPlainText">| pattern NF a <- UnsafeNF a</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| constructor NF :: NFData a => a -> NF a</p>
<p class="MsoPlainText">| constructor NF a = a `deepseq` UnsafeNF a</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| pattern Zero :: (Eq a, Num a) => a</p>
<p class="MsoPlainText">| pattern Zero <- ((== 0) -> True)</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| constructor Zero :: Num a => a</p>
<p class="MsoPlainText">| constructor Zero = 0</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| Proposal #42 would add type signatures for bidirectional pattern</p>
<p class="MsoPlainText">| synonym constructor functions. Currently we can write this:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| pattern Zero :: (Num a, Eq a) => a</p>
<p class="MsoPlainText">| pattern Zero <- ((== 0) -> True)</p>
<p class="MsoPlainText">| where</p>
<p class="MsoPlainText">| Zero = 0</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| Per Feuer:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| >The trouble in this case is that the Eq constraint from the pattern</p>
<p class="MsoPlainText">| "infects" the constructor. So if I have a number type I can't test for</p>
<p class="MsoPlainText">| equality, I can't use Zero to construct it.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| It would permit writing things like this instead:</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| pattern Zero :: (Eq a, Num a) => a</p>
<p class="MsoPlainText">| pattern Zero <- ((== 0) -> True) where</p>
<p class="MsoPlainText">| Zero :: Num a => a -- optional</p>
<p class="MsoPlainText">| Zero = 0</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| This would prevent the pattern’s Eq constraint from “infecting” using</p>
<p class="MsoPlainText">| Zero as a constructor, which doesn’t actually need Eq.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| — My thoughts follow —</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| I think Richard’s right about how we’ll need to handle compatibility.</p>
<p class="MsoPlainText">| I agree that the current warnings policy is a bit restrictive but it’s</p>
<p class="MsoPlainText">| not worth getting people riled up over it.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| I agree with Simon's comment on #42. I think we need to take seriously</p>
<p class="MsoPlainText">| whether or not this proposal compromises the use-case of the naïve</p>
<p class="MsoPlainText">| user consuming a library API that uses pattern constructors. Partly</p>
<p class="MsoPlainText">| why I feel this is important is that as much debate as it draws,</p>
<p class="MsoPlainText">| Haskell users consuming a lens-based API are able to cargo cult and</p>
<p class="MsoPlainText">| move on with what they’re doing. I think pattern constructors should</p>
<p class="MsoPlainText">| be held to a higher usability standard than lens due to looking like</p>
<p class="MsoPlainText">| data constructors and being baked into the implementation.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| This seems to clear up a limitation in expressiveness with few</p>
<p class="MsoPlainText">| downsides as long as we avoid using holes and respect the community’s</p>
<p class="MsoPlainText">| desires on backwards compatibility. Avoiding holes should make</p>
<p class="MsoPlainText">| implementation easier and have fewer unpredictable behaviors down the</p>
<p class="MsoPlainText">| road as well.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| — My recommendation for the proposal —</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| My recommendation is that we accept both proposals. My reservation</p>
<p class="MsoPlainText">| would be that it shouldn't have serious downsides for the existing</p>
<p class="MsoPlainText">| users of pattern synonyms, be they authors or consumers.</p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| </p>
<p class="MsoPlainText">| Cheers,</p>
<p class="MsoPlainText">| Chris</p>
<p class="MsoPlainText">| _______________________________________________</p>
<p class="MsoPlainText">| ghc-steering-committee mailing list</p>
<p class="MsoPlainText">| <a href="mailto:ghc-steering-committee@haskell.org"><span style="color:windowtext;text-decoration:none">ghc-steering-committee@haskell.org</span></a></p>
<p class="MsoPlainText">| <a href="https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee">
<span style="color:windowtext;text-decoration:none">https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-</span></a></p>
<p class="MsoPlainText"><a href="https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee"><span style="color:windowtext;text-decoration:none">| committee</span></a></p>
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