Hi Ghc-Devs,
When upgrading one of our tc plugins
https://hackage.haskell.org/package/ghc-typelits-natnormalise 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
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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