<div dir="ltr">I have reported the issue here: <a href="https://gitlab.haskell.org/ghc/ghc/-/issues/20009">https://gitlab.haskell.org/ghc/ghc/-/issues/20009</a></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 17 Jun 2021 at 18:44, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div style="overflow-wrap: break-word;" lang="EN-GB">
<div class="gmail-m_-4938774256514683699WordSection1">
<p class="MsoNormal"><span>Christiaan,<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<p class="MsoNormal"><span>Do please submit a bug report on GHC’s issue tracker, with a way to reproduce it.<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<p class="MsoNormal"><span>Thanks<u></u><u></u></span></p>
<p class="MsoNormal"><span><br>
Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<div style="border-color:currentcolor currentcolor currentcolor blue;border-style:none none none solid;border-width:medium medium medium 1.5pt;padding:0cm 0cm 0cm 4pt">
<div>
<div style="border-color:rgb(225,225,225) currentcolor currentcolor;border-style:solid none none;border-width:1pt medium medium;padding:3pt 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>>
<b>On Behalf Of </b>Christiaan Baaij<br>
<b>Sent:</b> 17 June 2021 10:44<br>
<b>To:</b> ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> Error message degradation for (<= :: Nat -> Nat -> Constraint) in GHC 9.2+<u></u><u></u></span></p>
</div>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
Hi Ghc-Devs,<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
When upgrading one of our tc plugins <a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2Fghc-typelits-natnormalise&data=04%7C01%7Csimonpj%40microsoft.com%7C42380a30e7f54d6ad06708d931747622%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637595199077626853%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=MDuOq05JaifjtEkq7JrdjmmwgCWEtIyZ%2BYqIFNv7FhY%3D&reserved=0" target="_blank">
https://hackage.haskell.org/package/ghc-typelits-natnormalise</a> to GHC 9.2, one of our tests, repeated here:<br>
<br>
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
{-# LANGUAGE DataKinds, TypeFamilies, TypeOperators #-}<br>
module TestInEq where<br>
<br>
import Data.Proxy<br>
import GHC.TypeLits<br>
<br>
proxyInEq :: (a <= b) => Proxy a -> Proxy b -> ()<br>
proxyInEq _ _ = ()<br>
<br>
proxyInEq1 :: Proxy a -> Proxy (a+1) -> ()<br>
proxyInEq1 = proxyInEq<br>
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
degraded quite badly in terms of the error message. <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
Where in GHC 9.0.1 we get:<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
TestInEq.hs:11:14: error:<br>
• Couldn't match type ‘a <=? (a + 1)’ with ‘'True’<br>
arising from a use of ‘proxyInEq’<br>
• In the expression: proxyInEq<br>
In an equation for ‘proxyInEq1’: proxyInEq1 = proxyInEq<br>
• Relevant bindings include<br>
proxyInEq1 :: Proxy a -> Proxy (a + 1) -> ()<br>
(bound at TestInEq.hs:11:1)<br>
|<br>
11 | proxyInEq1 = proxyInEq<br>
| <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
with GHC 9.2.0.20210422 we get:<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
TestInEq.hs:11:14: error:<br>
• Couldn't match type ‘Data.Type.Ord.OrdCond<br>
(CmpNat a (a + 1)) 'True 'True 'False’<br>
with ‘'True’<br>
arising from a use of ‘proxyInEq’<br>
• In the expression: proxyInEq<br>
In an equation for ‘proxyInEq1’: proxyInEq1 = proxyInEq<br>
• Relevant bindings include<br>
proxyInEq1 :: Proxy a -> Proxy (a + 1) -> ()<br>
(bound at TestInEq.hs:11:1)<br>
|<br>
11 | proxyInEq1 = proxyInEq<br>
|<br>
```<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
Errors messages involving type-level naturals and their operations already weren't the poster-child of comprehensable GHC error messages, but this change has made the situation worse in my opinion.
<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
This change in error message is due to: <a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fcommit%2Feea96042f1e8682605ae68db10f2bcdd7dab923e&data=04%7C01%7Csimonpj%40microsoft.com%7C42380a30e7f54d6ad06708d931747622%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637595199077636846%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=FfAQaEksSYEWNjOzuOmwhPXz6lI%2F5o5LT%2Ftwbh42wFM%3D&reserved=0" target="_blank">
https://gitlab.haskell.org/ghc/ghc/-/commit/eea96042f1e8682605ae68db10f2bcdd7dab923e</a><u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
Is there a way we can get the nicer pre-9.2.0.2021 error message again before the proper 9.2.1 release?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
e.g. by doing one of the following:<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
1. Reinstate `(<=? :: Nat -> Nat -> Bool)` as a builtin type family<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
2. Somehow add a custom type-error to `Data.Type.Ord.OrdCond`<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
3. Don't expand type aliases in type errors<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
What do you think? should this be fixed? should this be fixed before the 9.2.1 release?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
-- Christiaan<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6pt;margin-left:0cm">
<u></u> <u></u></p>
</div>
</div>
</div>
</div>
</div>
</blockquote></div>