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<p class="MsoNormal"><span style="font-size:12.0pt">Friends<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">George Karachalas, Tom Schrijvers, Dimitrios Vytiniotis, and I are working on finally cracking the problem of accurately reporting pattern-match overlap and redundancy warnings, in the presence of GADTs.
You know the problem; consider<o:p></o:p></span></p>
<p class="Code">vzip :: Vect n a -> Vect n b -> Vect n (a,b)<o:p></o:p></p>
<p class="Code">vzip VN VN = VN<o:p></o:p></p>
<p class="Code">vzip (VC x xs) (VC y ys) = VC (x,y) (vzip xs ys)<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code">data Vect :: Nat -> * -> * where<o:p></o:p></p>
<p class="Code"> VN :: Vect Zero a<o:p></o:p></p>
<p class="Code"> VC :: a -> Vect n a -> Vect (Succ n) a<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Are there any missing equations in vzip? No! But GHC complains anyway. We have lots of Trac tickets about this; e.g.
<a href="https://ghc.haskell.org/trac/ghc/ticket/3927">https://ghc.haskell.org/trac/ghc/ticket/3927</a>.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">We now have a draft paper (wait a week) and a prototype implementation, that fixes the problem. But we need your help. We’d like to try our prototype on real code, not just toy examples.<o:p></o:p></span></p>
<p class="MsoNormalCxSpLast"><b><span style="font-size:12.0pt;color:red">So, could you send George a pointer to any packages you have, or know of, that<o:p></o:p></span></b></p>
<p class="MsoListParagraphCxSpFirst" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-size:12.0pt;color:red">use GADTS (or other fancy type features) and
</span></b><span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoListParagraphCxSpLast" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-size:12.0pt;color:red">would benefit from accurate pattern-match overlap/redundancy warnings?</span></b><span style="font-size:12.0pt;color:red">
</span><span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Specifically:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">Where you have had to add a catch-all<o:p></o:p></span></p>
<p class="Code">f _ _ = error “impossible”<o:p></o:p></p>
<p class="MsoListParagraph"><span style="font-size:12.0pt">to silence GHC from saying “missing patterns”<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">Or where you have added
</span><span class="CodeChar"><span style="font-size:10.0pt">{-# OPTIONS_GHC –fno-warn-missing-patterns #-}</span></span><span style="font-size:12.0pt"> to silence the warnings.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">Or something else like that.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">George’s email is in cc. <o:p>
</o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Time is short – the ICFP deadline is on 27 Feb. So sooner is better than later for us. But later is better than never.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Thank you!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
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