<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Mar 15, 2019, at 1:04 PM, Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com" class="">iavor.diatchki@gmail.com</a>> wrote:</div><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><br class=""></div><div dir="ltr" class=""><div class="">This is a bit orthogonal, but what would you expect to see in this case:</div><div class=""><br class=""></div><div class="">type T b = Bool</div><div class="">bar :: (a ~ Int) => a -> T b -> a<br class=""></div><div class=""><br class=""></div><div class="">:t bar</div></div></div></div></blockquote><div><br class=""></div><div>Without -fprint-explicit-foralls: (a ~ Int) => a -> T b -> a</div><div>With -fprint-explicit-foralls: forall a b. (a ~ Int) => a -> T b -> a</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><div class="">:t bar @Bool</div></div></div></div></blockquote><div><br class=""></div><div>Without -fprint-explicit-foralls: (Bool ~ Int) => Bool -> T b -> Bool</div><div>With -fprint-explicit-foralls: forall b. (Bool ~ Int) => Bool -> T b -> Bool</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">I guess the answer to these depends on if `b` is quantified or not.</div></div></div></div></blockquote><div><br class=""></div><div>b *is* quantified -- the quantification happens on the type as the user writes it.</div><div><br class=""></div><div>Richard</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""><br class=""></div></div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Mar 15, 2019 at 1:26 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" class="">simonpj@microsoft.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<div class="gmail-m_6376247681929278253WordSection1"><p class="MsoNormal" style="margin-left:36pt"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">This looks particularly confusing without the explicit `forall`, although it seems questionable either way.<u class=""></u><u class=""></u></span></p><p class="MsoNormal"><span class=""><u class=""></u> <u class=""></u></span></p><p class="MsoNormal"><span class="">Why? GHC often suppresses foralls when printing types; and had -fprint-explicit-foralls to stop doing that.<u class=""></u><u class=""></u></span></p><p class="MsoNormal"><span class=""><u class=""></u> <u class=""></u></span></p><p class="MsoNormal"><span class="">Simon<u class=""></u><u class=""></u></span></p><p class="MsoNormal"><span class=""><u class=""></u> <u class=""></u></span></p>
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<div style="border-right:none;border-bottom:none;border-left:none;border-top:1pt solid rgb(225,225,225);padding:3pt 0cm 0cm" class=""><p class="MsoNormal"><b class=""><span lang="EN-US" class="">From:</span></b><span lang="EN-US" class=""> ghc-steering-committee <<a href="mailto:ghc-steering-committee-bounces@haskell.org" target="_blank" class="">ghc-steering-committee-bounces@haskell.org</a>>
<b class="">On Behalf Of </b>Richard Eisenberg<br class="">
<b class="">Sent:</b> 14 March 2019 23:46<br class="">
<b class="">To:</b> Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com" target="_blank" class="">iavor.diatchki@gmail.com</a>><br class="">
<b class="">Cc:</b> <a href="mailto:ghc-steering-committee@haskell.org" target="_blank" class="">ghc-steering-committee@haskell.org</a>; Joachim Breitner <<a href="mailto:mail@joachim-breitner.de" target="_blank" class="">mail@joachim-breitner.de</a>><br class="">
<b class="">Subject:</b> Re: [ghc-steering-committee] #179: Tweak the printing of foralls, recommendation: accept<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal">On Mar 14, 2019, at 6:18 PM, Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com" target="_blank" class="">iavor.diatchki@gmail.com</a>> wrote:<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">bar :: forall a b. (a ~ Int) => a -> b -> a<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">Is it the case that `:t bar @Bool` will print<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">bar :: (Bool ~ Int) => Bool -> b -> Bool<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">This looks particularly confusing without the explicit `forall`, although it seems questionable either way.<u class=""></u><u class=""></u></span></p>
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<div class=""><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`.<u class=""></u><u class=""></u></p>
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<div class=""><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.<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal">Richard<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">-Iavor<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal"><span style="font-size:9pt;font-family:Helvetica,sans-serif" class="">On Thu, Mar 14, 2019 at 7:47 AM Joachim Breitner <<a href="mailto:mail@joachim-breitner.de" target="_blank" class="">mail@joachim-breitner.de</a>> wrote:<u class=""></u><u class=""></u></span></p>
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<br class="">
Richard, in collarboration with Simon PJ, proposes some tweaks to how<br class="">
foralls are printed by GHC.<br class="">
<br class="">
In particular, whenever a type variable is printed with an forall, it<br class="">
is printed with {a} unless it is a specified variable. And there are<br class="">
tweaks to what :type does with its argument that makes it more useful,<br class="">
and obviates the need for a separate :type +v.<br class="">
<a href="https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/0000-printing-foralls.rst" target="_blank" class="">https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/0000-printing-foralls.rst</a><br class="">
<br class="">
I like that it actually simplifies the UI (by removing :type +v), so I<br class="">
recommend acceptance.<br class="">
<br class="">
Cheers,<br class="">
Joachim<br class="">
--<span class="gmail-m_6376247681929278253apple-converted-space"> </span><br class="">
Joachim Breitner<br class="">
<span class="gmail-m_6376247681929278253apple-converted-space"> </span><a href="mailto:mail@joachim-breitner.de" target="_blank" class="">mail@joachim-breitner.de</a><br class="">
<span class="gmail-m_6376247681929278253apple-converted-space"> </span><a href="http://www.joachim-breitner.de/" target="_blank" class="">http://www.joachim-breitner.de/</a><br class="">
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