diff options
Diffstat (limited to 'libraries/ghc-prim/GHC/Magic/Dict.hs')
| -rw-r--r-- | libraries/ghc-prim/GHC/Magic/Dict.hs | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/libraries/ghc-prim/GHC/Magic/Dict.hs b/libraries/ghc-prim/GHC/Magic/Dict.hs index 560ab3956f..34886fee3b 100644 --- a/libraries/ghc-prim/GHC/Magic/Dict.hs +++ b/libraries/ghc-prim/GHC/Magic/Dict.hs @@ -5,6 +5,7 @@ {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} +{-# LANGUAGE AllowAmbiguousTypes #-} -- See Note [withDict has an ambiguous type] {-# LANGUAGE Unsafe #-} ----------------------------------------------------------------------------- @@ -40,3 +41,20 @@ import GHC.Types (RuntimeRep, TYPE) -- @Note [withDict]@ in "GHC.Tc.Instance.Class" in GHC. class WithDict cls meth where withDict :: forall {rr :: RuntimeRep} (r :: TYPE rr). meth -> (cls => r) -> r + +{- Note [withDict has an ambiguous type] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The type of `withDict` is ambiguous. Consider + foo :: forall cls meth. WithDict cls meth + => forall rr (r::rr). meth -> (cls => r) -> r + foo m k = withDict m k + +If we instantiate `withDict` with fresh unification variables, including cls0 for cls, +there is nothing that forces the `cls` Wanted from the call to `k` to unify with the +`cls0` Given from the call to `withDict`. You have to give it a class argument: + + foo m k = withDict @cls m k + +That's fine. But it means we need -XAllowAmbiguousTypes for the withDict definition, +at least with deep subsumption. +-}
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