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<div class="WordSection1">
<p class="msipheader7557e74b" style="margin:0in;margin-bottom:.0001pt"><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">OK so I tried out OverloadedStrings and it basically went as bad as I expected. The documentation on `HsOverLit` is very promising: it points to the Note [Overloaded literal
witnesses], which states:<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"><i><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Note [Overloaded literal witnesses]<o:p></o:p></span></i></p>
<p class="MsoNormal"><i><span style="font-size:10.0pt;font-family:"Arial",sans-serif">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<o:p></o:p></span></i></p>
<p class="MsoNormal"><i><span style="font-size:10.0pt;font-family:"Arial",sans-serif">*Before* type checking, the HsExpr in an HsOverLit is the<o:p></o:p></span></i></p>
<p class="MsoNormal"><i><span style="font-size:10.0pt;font-family:"Arial",sans-serif">name of the coercion function, 'fromInteger' or 'fromRational'.<o:p></o:p></span></i></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">So that sounds great, right? It sounds like just before renaming, I should be able to replace `HsLit _ (HsString _ fs)` with `HsOverLit _ (OverLit _ (HsIsString _ fs) unpack`
with my own `unpack` function coming from my own package, and everything would work out. Unfortunately, this is not what happens: if I try getting this through the renamer, I get this error:<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" style="text-autospace:none"><span style="font-size:9.0pt;font-family:"Lucida Console";mso-fareast-language:EN-GB">Failed to load interface for ‘Data.String’<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.0pt;font-family:"Lucida Console";mso-fareast-language:EN-GB">no unit id matching ‘base’ was found<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.0pt;font-family:"Lucida Console";mso-fareast-language:EN-GB">Can't find interface-file declaration for variable fromString<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.0pt;font-family:"Lucida Console";mso-fareast-language:EN-GB"> Probable cause: bug in .hi-boot file, or inconsistent .hi file<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Lucida Console";mso-fareast-language:EN-GB"> Use -ddump-if-trace to get an idea of which file caused the error</span><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"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">So even though I am specifying my own coercion function, it is still looking for `Data.String.fromString` which is not going to work, since I don’t have `base`. So either I am
misunderstanding that Note, or it is simply out of date, but in either case, this isn’t going to be a viable route to going base-less.<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"> 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 style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<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"> Erdi, Gergo
<br>
<b>Sent:</b> Tuesday, July 6, 2021 5:39 PM<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> GHC <ghc-devs@haskell.org><br>
<b>Subject:</b> RE: 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:0in;margin-bottom:.0001pt"><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">Thanks Simon!<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">Of course, you’re right, it’s the renamer, not the typechecker – I didn’t really check, just saw that “it happens during `typecheckModule`.
<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’ll look at the rebindable syntax stuff in detail, but at least for OverloadedStrings, I already know that the problem will be that ultimately they do go through the `String`
type from `base`, and I need to use GHC baselessly. This is a problem for two reasons:<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>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraphCxSpFirst" style="margin-left:0in;mso-add-space:auto;mso-list:l0 level1 lfo6">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">I can’t implement `IsString` for `MyString`, because `IsString` is in `base`<o:p></o:p></span></li><li class="MsoListParagraphCxSpLast" style="margin-left:0in;mso-add-space:auto;mso-list:l0 level1 lfo6">
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">Even if I made my own fake `base` with a fake `IsString` class, there is nothing to put in the codomain of `fromString`: I *<b>only</b>* have `MyString`, not `String`. And renaming `MyString to `String`
in my fake `base` is not going to cut it, since `String` is wired into GHC to be a type synonym for `[Char]` (which `MyString` is not).<o:p></o:p></span></li></ul>
<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 foresee similar problems for OverloadedLists :/<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>
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<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"> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>>
<br>
<b>Sent:</b> Tuesday, July 6, 2021 5:08 PM<br>
<b>To:</b> Erdi, Gergo <<a href="mailto:Gergo.Erdi@sc.com">Gergo.Erdi@sc.com</a>><br>
<b>Cc:</b> GHC <<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> [External] RE: Marking ParsedModule fragments as non-user-originating<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><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>
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<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 <<a href="mailto:ghc-devs-bounces@haskell.org">ghc-devs-bounces@haskell.org</a>>
<b>On Behalf Of </b>Erdi, Gergo via ghc-devs<br>
<b>Sent:</b> 06 July 2021 09:08<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Marking ParsedModule fragments as non-user-originating<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0in;margin-bottom:.0001pt"><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>
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