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<span style="font-family:"Courier New"">foo b@(Bar :: Foo [a]) = quux b</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">The pattern (p :: ty) checks that the thing being matched has type ‘ty’ and THEN matches ‘p’. You expected it to FIRST match ‘p’, and THEN check that the thing being matched has type ‘ty’. But that’s not
the way it works.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">e.g what about this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> rats ([Just (Bar, True)] :: [Maybe (Foo [a], Bool)]) = …<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Here the pattern to be matched is deep, with the Bar part deep inside. Do you still expect it to work?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">This would be hard to change. And I’m not sure it’d be an improvement.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Gabor Greif<br>
<b>Sent:</b> 29 October 2017 17:43<br>
<b>To:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Q: Types in GADT pattern match<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">
Hi Devs!<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">
I encountered a curious restriction with type signatures (tyvar bindings) in GADT pattern matches.<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">
GHC won't let me directly capture the refined type structure of GADT constructors like this:<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">{-# Language GADTs, ScopedTypeVariables #-}</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">data Foo a where</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> Bar :: Foo [a]</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">foo :: Foo a -> ()</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">foo b@(Bar :: Foo [a]) = quux b</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> where quux :: Foo [b] -> ()</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> quux Bar = ()</span><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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<o:p> </o:p></p>
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I get:<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif">test.hs:7:8:
<span style="color:#C33720">error:</span></span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> • Couldn't match type ‘a1’ with ‘[a]’</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> ‘a1’ is a rigid type variable bound by</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> the type signature for:</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> foo :: forall a1. Foo a1 -> ()</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> at test.hs:6:1-18</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> Expected type: Foo a1</span></b><span style="font-size:11.5pt"><o:p></o:p></span></p>
<p style="margin:0cm;margin-bottom:.0001pt"><b><span style="font-size:11.5pt;font-family:"Arial",sans-serif"> Actual type: Foo [a]</span></b><span style="font-size:11.5pt"><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"><o:p> </o:p></p>
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<p class="MsoNormal">To me it appears that the type refinement established by the GADT pattern match is not accounted for.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Of course I can write:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">foo :: Foo a -> ()</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">foo b@Bar | (c :: Foo [a]) <- b = quux c</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> where quux :: Foo [b] -> ()</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> quux Bar = ()</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">but it feels like a complicated way to do it...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">My question is whether this is generally seen as the way to go or whether <span style="font-family:"Courier New"">ScopedTypeVariables</span> coming from a GADT pattern match should be able to capture the refined type. To me the latter seems
more useful.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Just wanted to feel the waters before writing a ticket about this.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers and thanks,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> Gabor<o:p></o:p></p>
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