diff options
Diffstat (limited to 'compiler/GHC/Tc/Deriv/Generate.hs')
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Generate.hs | 24 |
1 files changed, 9 insertions, 15 deletions
diff --git a/compiler/GHC/Tc/Deriv/Generate.hs b/compiler/GHC/Tc/Deriv/Generate.hs index 3585c9ad70..400d4afbe7 100644 --- a/compiler/GHC/Tc/Deriv/Generate.hs +++ b/compiler/GHC/Tc/Deriv/Generate.hs @@ -1691,19 +1691,14 @@ a polytype. E.g. @(T x -> forall b. b -> b) op -The use of type applications is crucial here. If we had tried using only -explicit type signatures, like so: +The use of type applications is crucial here. We have to instantiate +both type args of (coerce :: Coercible a b => a -> b) to polytypes, +and we can only do that with VTA or Quick Look. Here VTA seems more +appropriate for machine generated code: it's simple and robust. - instance C <rep-ty> => C (T x) where - op :: T x -> forall b. b -> b - op = coerce (op :: <rep-ty> -> forall b. b -> b) - -Then GHC will attempt to deeply skolemize the two type signatures, which will -wreak havoc with the Coercible solver. Therefore, we instead use type -applications, which do not deeply skolemize and thus avoid this issue. -The downside is that we currently require -XImpredicativeTypes to permit this -polymorphic type instantiation, so we have to switch that flag on locally in -GHC.Tc.Deriv.genInst. See #8503 for more discussion. +However, to allow VTA with polytypes we must switch on +-XImpredicativeTypes locally in GHC.Tc.Deriv.genInst. +See #8503 for more discussion. Note [Newtype-deriving trickiness] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -1807,9 +1802,8 @@ break this example (from the T15290d test case): c = coerce @(Int -> forall b. b -> Int) c -That is because the instance signature deeply skolemizes the forall-bound -`b`, which wreaks havoc with the `Coercible` solver. An additional visible type -argument of @(Int -> forall b. b -> Age) is enough to prevent this. +That is because we still need to instantiate the second argument of +coerce with a polytype, and we can only do that with VTA or QuickLook. Be aware that the use of an instance signature doesn't /solve/ this problem; it just makes it less likely to occur. For example, if a class has |