I have reported the issue here: https://gitlab.haskell.org/ghc/ghc/-/issues/20009
On Thu, 17 Jun 2021 at 18:44, Simon Peyton Jones <simo...@microsoft.com> wrote: > Christiaan, > > > > Do please submit a bug report on GHC’s issue tracker, with a way to > reproduce it. > > > > Thanks > > > Simon > > > > *From:* ghc-devs <ghc-devs-boun...@haskell.org> *On Behalf Of *Christiaan > Baaij > *Sent:* 17 June 2021 10:44 > *To:* ghc-devs <ghc-devs@haskell.org> > *Subject:* Error message degradation for (<= :: Nat -> Nat -> Constraint) > in GHC 9.2+ > > > > Hi Ghc-Devs, > > > > When upgrading one of our tc plugins > https://hackage.haskell.org/package/ghc-typelits-natnormalise > <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> > to GHC 9.2, one of our tests, repeated here: > > ``` > > {-# LANGUAGE DataKinds, TypeFamilies, TypeOperators #-} > module TestInEq where > > import Data.Proxy > import GHC.TypeLits > > proxyInEq :: (a <= b) => Proxy a -> Proxy b -> () > proxyInEq _ _ = () > > proxyInEq1 :: Proxy a -> Proxy (a+1) -> () > proxyInEq1 = proxyInEq > ``` > > > > degraded quite badly in terms of the error message. > > Where in GHC 9.0.1 we get: > > > > ``` > > TestInEq.hs:11:14: error: > • Couldn't match type ‘a <=? (a + 1)’ with ‘'True’ > arising from a use of ‘proxyInEq’ > • In the expression: proxyInEq > In an equation for ‘proxyInEq1’: proxyInEq1 = proxyInEq > • Relevant bindings include > proxyInEq1 :: Proxy a -> Proxy (a + 1) -> () > (bound at TestInEq.hs:11:1) > | > 11 | proxyInEq1 = proxyInEq > | > > ``` > > > > with GHC 9.2.0.20210422 we get: > > > > ``` > > TestInEq.hs:11:14: error: > • Couldn't match type ‘Data.Type.Ord.OrdCond > (CmpNat a (a + 1)) 'True 'True 'False’ > with ‘'True’ > arising from a use of ‘proxyInEq’ > • In the expression: proxyInEq > In an equation for ‘proxyInEq1’: proxyInEq1 = proxyInEq > • Relevant bindings include > proxyInEq1 :: Proxy a -> Proxy (a + 1) -> () > (bound at TestInEq.hs:11:1) > | > 11 | proxyInEq1 = proxyInEq > | > ``` > > > > 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. > > > > This change in error message is due to: > https://gitlab.haskell.org/ghc/ghc/-/commit/eea96042f1e8682605ae68db10f2bcdd7dab923e > <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> > > > > Is there a way we can get the nicer pre-9.2.0.2021 error message again > before the proper 9.2.1 release? > > e.g. by doing one of the following: > > 1. Reinstate `(<=? :: Nat -> Nat -> Bool)` as a builtin type family > > 2. Somehow add a custom type-error to `Data.Type.Ord.OrdCond` > > 3. Don't expand type aliases in type errors > > > > What do you think? should this be fixed? should this be fixed before the > 9.2.1 release? > > > > -- Christiaan > > >
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