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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Christiaan,<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">Do please submit a bug report on GHC’s issue tracker, with a way to reproduce it.<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">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><br>
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-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Christiaan Baaij<br>
<b>Sent:</b> 17 June 2021 10:44<br>
<b>To:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Error message degradation for (<= :: Nat -> Nat -> Constraint) in GHC 9.2+<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hi Ghc-Devs,<o:p></o:p></p>
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<o:p> </o:p></p>
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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>
```<o:p></o:p></p>
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{-# 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>
```<o:p></o:p></p>
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<o:p> </o:p></p>
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degraded quite badly in terms of the error message. <o:p></o:p></p>
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Where in GHC 9.0.1 we get:<o:p></o:p></p>
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<o:p> </o:p></p>
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```<o:p></o:p></p>
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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>
| <o:p></o:p></p>
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```<o:p></o:p></p>
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<o:p> </o:p></p>
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with GHC 9.2.0.20210422 we get:<o:p></o:p></p>
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<o:p> </o:p></p>
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```<o:p></o:p></p>
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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>
```<o:p></o:p></p>
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<o:p> </o:p></p>
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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.
<o:p></o:p></p>
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<o:p> </o:p></p>
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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">
https://gitlab.haskell.org/ghc/ghc/-/commit/eea96042f1e8682605ae68db10f2bcdd7dab923e</a><o:p></o:p></p>
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<o:p> </o:p></p>
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Is there a way we can get the nicer pre-9.2.0.2021 error message again before the proper 9.2.1 release?<o:p></o:p></p>
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e.g. by doing one of the following:<o:p></o:p></p>
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1. Reinstate `(<=? :: Nat -> Nat -> Bool)` as a builtin type family<o:p></o:p></p>
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2. Somehow add a custom type-error to `Data.Type.Ord.OrdCond`<o:p></o:p></p>
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3. Don't expand type aliases in type errors<o:p></o:p></p>
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<o:p> </o:p></p>
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What do you think? should this be fixed? should this be fixed before the 9.2.1 release?<o:p></o:p></p>
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<o:p> </o:p></p>
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-- Christiaan<o:p></o:p></p>
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<o:p> </o:p></p>
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