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<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">This looks particularly confusing without the explicit `forall`, although it seems questionable either way.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Why? GHC often suppresses foralls when printing types; and had -fprint-explicit-foralls to stop doing that.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><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> 14 March 2019 23:46<br>
<b>To:</b> Iavor Diatchki <iavor.diatchki@gmail.com><br>
<b>Cc:</b> ghc-steering-committee@haskell.org; Joachim Breitner <mail@joachim-breitner.de><br>
<b>Subject:</b> Re: [ghc-steering-committee] #179: Tweak the printing of foralls, recommendation: accept<o:p></o:p></span></p>
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<p class="MsoNormal">On Mar 14, 2019, at 6:18 PM, Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com">iavor.diatchki@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">bar :: forall a b. (a ~ Int) => a -> b -> a<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">Is it the case that `:t bar @Bool` will print<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">bar :: (Bool ~ Int) => Bool -> b -> Bool<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">This looks particularly confusing without the explicit `forall`, although it seems questionable either way.<o:p></o:p></span></p>
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<p class="MsoNormal">Yes, you understand correctly. `:t bar @Bool` will print that unfortunate type. I agree this is questionable. But I don't know how to do better. The current state of affairs is awkward in a different way, in that `:t bar @Bool` errors (that's
OK) and `:t bar @Int` prints `forall {b}. Int -> b -> Int`, even though a further visible type application is possible. Perhaps worse, `:t bar` today prints `forall {b}. Int -> b -> Int` even though the next variable to be instantiated is `a`. This is why
we have `:type +v`.<o:p></o:p></p>
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<p class="MsoNormal">The direction in the proposal came about in wondering if it's possible to improve the mechanics of `:t` (without the `+v`). After we thought of doing maximal instantiation of inferred arguments (but no generalization), then we realized
we could drop `:type +v` as redundant. I agree it's something of an uneasy compromise here, but I think any choice is going to have unfortunate behavior in some scenarios.<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">-Iavor<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">On Thu, Mar 14, 2019 at 7:47 AM Joachim Breitner <<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">Dear Committee,<br>
<br>
Richard, in collarboration with Simon PJ, proposes some tweaks to how<br>
foralls are printed by GHC.<br>
<br>
In particular, whenever a type variable is printed with an forall, it<br>
is printed with {a} unless it is a specified variable. And there are<br>
tweaks to what :type does with its argument that makes it more useful,<br>
and obviates the need for a separate :type +v.<br>
<a href="https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/0000-printing-foralls.rst" target="_blank">https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/0000-printing-foralls.rst</a><br>
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I like that it actually simplifies the UI (by removing :type +v), so I<br>
recommend acceptance.<br>
<br>
Cheers,<br>
Joachim<br>
--<span class="apple-converted-space"> </span><br>
Joachim Breitner<br>
<span class="apple-converted-space"> </span><a href="mailto:mail@joachim-breitner.de" target="_blank">mail@joachim-breitner.de</a><br>
<span class="apple-converted-space"> </span><a href="http://www.joachim-breitner.de/" target="_blank">http://www.joachim-breitner.de/</a><br>
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