<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
p.Code, li.Code, div.Code
{mso-style-name:Code;
margin-top:0cm;
margin-right:0cm;
margin-bottom:0cm;
margin-left:22.7pt;
font-size:10.0pt;
font-family:"Courier New";
mso-fareast-language:EN-US;
font-weight:bold;}
p.msipheader7557e74b, li.msipheader7557e74b, div.msipheader7557e74b
{mso-style-name:msipheader7557e74b;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.EmailStyle21
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-GB" link="#525355" vlink="#0473EA" style="word-wrap:break-word">
<div class="WordSection1">
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">The typechecker now complains that the `ViewPatterns` language extension is not turned on<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I think it’s the renamer:<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="Code">rnPatAndThen mk p@(ViewPat _ expr pat)<o:p></o:p></p>
<p class="Code"> = do { liftCps $ do { vp_flag <- xoptM LangExt.ViewPatterns<o:p></o:p></p>
<p class="Code"> ; checkErr vp_flag (badViewPat p) }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">More generally, don’t you just want OverloadedStrings or OverloadedLists?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">You might want to read Note [Handling overloaded and rebindable constructs] in GHC.Rename.Expr, and<o:p></o:p></p>
<p class="MsoNormal">Note [Rebindable syntax and HsExpansion] in GCH.Hs.Expr. These Notes describe how GHC already does something similar to what you want. Maybe you can use the same mechanism in your plugin.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Erdi, Gergo via ghc-devs<br>
<b>Sent:</b> 06 July 2021 09:08<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Marking ParsedModule fragments as non-user-originating<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0cm"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I’d like to hijack some syntax (like string literals or list patterns) for my own use, and I thought a low-tech way of doing that is to transform the ParsedModule before typechecking.
For example, if I have a function `uncons :: Array a -> Maybe (a, Array a)`, I can rewrite the pattern `[x1, x2, x3]` into the view pattern `(uncons -> Just (x1, (uncons -> Just (x2, (uncons –> Just (x3, (uncons -> Nothing)))))))` and let the normal GHC type
checker take over from here. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">This is working for me so far, except for one problem: the typechecker now complains that the `ViewPatterns` language extension is not turned on. I would like to make the view
patterns coming from my ParsedModule rewriter to be exempt from this check (but of course still require the `ViewPatterns` extension for user-originating code). Is there a way to do that? Or would I be better off checking for user-originating view patterns
myself before the rewrite and then changing the `DynFlags` to always enable view patterns for typechecking?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"> Gergo<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
</div>
</div>
</body>
</html>