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| -rw-r--r-- | docs/users_guide/8.12.1-notes.rst | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/docs/users_guide/8.12.1-notes.rst b/docs/users_guide/8.12.1-notes.rst index eecd7570fc..75adb1fb41 100644 --- a/docs/users_guide/8.12.1-notes.rst +++ b/docs/users_guide/8.12.1-notes.rst @@ -24,6 +24,23 @@ Language would have accepted ``x``, but its type would have involved the mysterious ``Any`` internal type family. Now, GHC rejects, explaining the situation. +* GHC now more faithfully implements the instance-lookup scheme described with + :extension:`QuantifiedConstraints`. Previous bugs meant that programs like this + were accepted:: + + data T (c :: Type -> Constraint) + instance (forall h. c h => Functor h) => Functor (T c) + instance (forall h. c h => Applicative h) => Applicative (T c) + + Note that in the instance declaration for ``Applicative (T c)``, we cannot prove + ``Functor (T c)``, because the quantified constraint shadows the global instance. + There is an easy workaround, though: just include ``Functor (T c)`` as an assumption. :: + + instance (forall h. c h => Applicative h, Functor (T c)) => Applicative (T c) + + There is a chance we will tweak the lookup scheme in the future, to make this + workaround unnecessary. + Compiler ~~~~~~~~ |