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<p class="MsoNormal"><span style="font-size:12.0pt">I’m a strong supporter. I think the proposal itself (which will have longevity) could be improved significantly, and have commented to that effect.<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-steering-committee <ghc-steering-committee-bounces@haskell.org>
<b>On Behalf Of </b>Iavor Diatchki<br>
<b>Sent:</b> 26 July 2018 09:09<br>
<b>To:</b> ghc-steering-committee@haskell.org<br>
<b>Subject:</b> [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)<o:p></o:p></span></p>
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Hello,<o:p></o:p></p>
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let's also start the discussion on feature request 126. The idea here is that we allow the @ notation for explicit type applications to also be used on constructors in patterns. Using @ with a constructor in a pattern has the same meaning as it does it an
expression: the provided type is used to instantiate the corresponding type parameter of the constructor. If the type contains variables, those are treated in the same way as in #128, where "unbound" variables name the matching types. Here are some examples:<o:p></o:p></p>
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f1 (Just @Int x) = x -- This has type `Maybe Int -> Int`<o:p></o:p></p>
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f2 (Just @[a] x) = x == "c" -- `a` is an alias for `Char`<o:p></o:p></p>
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f3 (SomeException @e ex) = ... -- `e` is a name for the existentially hidden exception type<o:p></o:p></p>
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Overall I think that is a simple and natural extension to the way @ already works, and I propose that we accept it.<o:p></o:p></p>
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Thoughts?<o:p></o:p></p>
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-Iavor<o:p></o:p></p>
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