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<p class="MsoNormal"><span style="font-size:12.0pt">Ha. We could tighten this up further to require a non-blank immediately after the @ for a type application. Thus<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> f @Int or g @(Tree Int)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">but not<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> f @ Int g @ (Tree Int)<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">That would leave (p1@p2) and (p1 @ p2) free for patterns.<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>Richard Eisenberg<br>
<b>Sent:</b> 17 August 2018 17:18<br>
<b>To:</b> Joachim Breitner <mail@joachim-breitner.de><br>
<b>Cc:</b> ghc-steering-committee@haskell.org<br>
<b>Subject:</b> Re: [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Note that this rule is documented, in brief, in the manual:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">> A visible type application is preceded with an <tt><span style="font-size:10.0pt">@</span></tt> sign. (To disambiguate the syntax, the
<tt><span style="font-size:10.0pt">@</span></tt> must be preceded with a non-identifier letter, usually a space. For example,
<span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">read@Int</span></span><tt><span style="font-size:10.0pt">
</span></tt><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">5</span></span> would not parse.)<o:p></o:p></p>
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<br>
<o:p></o:p></p>
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<p class="MsoNormal">On Aug 17, 2018, at 12:16 PM, Richard Eisenberg <<a href="mailto:rae@cs.brynmawr.edu">rae@cs.brynmawr.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">The rule is this, implemented in the lexer:<br>
<br>
With -XTypeApplications on, the character immediately before an @ determines how it is lexed.<br>
1. If the character is a legal end-of-identifier character (respecting the state of the -XMagicHash flag), then the @ is an as-pattern.<br>
2. Otherwise, then the @ is the beginning of a type application.<br>
<br>
I abbreviate the rule as talking about whitespace sensitivity, because if the character preceding the @ is, say, a +, then we lex +@ as a user-defined operator.<br>
<br>
This rule has been implemented since GHC 8.0 and applies in patterns as well as expressions. I don't think anyone noticed. :) So it works reasonably well. It's a silly rule and I'd be happy to do better, but I don't think it's strictly necessary to aim for
better here.<br>
<br>
It would be reasonable to warn when a user writes an as-pattern that doesn't conform to this rule (with -XNoTypeApplications).<br>
<br>
Richard<br>
<br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">On Aug 17, 2018, at 1:37 AM, Joachim Breitner <<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:<br>
<br>
Hi Richard,<br>
<br>
Am Donnerstag, den 16.08.2018, 22:28 -0400 schrieb Richard Eisenberg:<br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">Considering this future of relaxed ordering requirements on data<br>
constructors isn't compulsory, but I do think it's better if we don't<br>
paint ourselves into a corner around this.<o:p></o:p></p>
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<p class="MsoNormal"><br>
I agree.<br>
<br>
But so what do we do? Already now require that there is no space<br>
between @ and the following token when the user wants to use type<br>
applications in the pattern?<br>
<br>
And would we also require the user to add a space after an at-pattern?<br>
In which case we should probably start a deprecation cycle for the<br>
currently legal<br>
<br>
foo (a @b) = …<br>
<br>
<br>
Cheers,<br>
Joachim<br>
<br>
-- <br>
Joachim Breitner<br>
<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a><br>
<a href="https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7C549984ffda524c614d6008d6045cf7bd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636701194744752293&sdata=i4KtV3B%2B2YUluqn3Rx7th3sm6%2BDYkinoMQE6n%2FNqdZQ%3D&reserved=0">http://www.joachim-breitner.de/</a><br>
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<p class="MsoNormal"><br>
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