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| -rw-r--r-- | docs/users_guide/8.0.2-notes.rst | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/docs/users_guide/8.0.2-notes.rst b/docs/users_guide/8.0.2-notes.rst index fa7aa8db3d..237c3b95ef 100644 --- a/docs/users_guide/8.0.2-notes.rst +++ b/docs/users_guide/8.0.2-notes.rst @@ -68,8 +68,25 @@ Language foo :: m () - Some programs using :ghc-flag:`-XDefaultSignatures` that incorrectly - type-checked in GHC 8.0.1 are now rejected by GHC 8.0.2. See - :ghc-ticket:`12784` for details. + type-checked in GHC 8.0.1 are now rejected by GHC 8.0.2. Here is a + characteristic example: :: + + class Monad m => MonadSupply m where + fresh :: m Integer + default fresh :: (MonadTrans t, MonadSupply m) => t m Integer + fresh = lift fresh + + instance MonadSupply m => MonadSupply (IdentityT m) + + Note that the ``m`` in the default type signature is being used in + a completely different way than the ``m`` in the non-default signature! + We can fix this (in a backwards-compatible way) like so: :: + + class Monad m => MonadSupply m where + fresh :: m Integer + default fresh :: (MonadTrans t, MonadSupply m', m ~ t m') => m Integer + -- Same 'm Integer' after the '=>' + fresh = lift fresh - Some programs which combine default type class method implementations and overlapping instances may now fail to type-check. Here is an example: :: |