GitLab

In upgrading to7.10, code of the form

{-# LANGUAGE FlexibleContexts #-}
import Data.Coerce (coerce, Coercible)

data X a

doCoerce :: Coercible a (X a) => a -> X a
doCoerce = coerce

fails to compile in 7.10.1 and 7.10.2 with the error

testCoerce.hs:6:13:
    Could not deduce (a ~ X a)
    from the context (Coercible a (X a))
      bound by the type signature for
                 doCoerce :: Coercible a (X a) => a -> X a
      at testCoerce.hs:6:13-41
      ‘a’ is a rigid type variable bound by
          the type signature for doCoerce :: Coercible a (X a) => a -> X a
          at testCoerce.hs:6:13
    Relevant role signatures: type role X phantom
    In the ambiguity check for the type signature for ‘doCoerce’:
      doCoerce :: forall a. Coercible a (X a) => a -> X a
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature for ‘doCoerce’:
      doCoerce :: Coercible a (X a) => a -> X a

while it works in 7.8.4.

Surprisingly (to me at least), the code works in 7.10.1 and 7.10.2 if I change it to

{-# LANGUAGE FlexibleContexts #-}
import Data.Coerce (coerce, Coercible)

data X a

doCoerce :: Coercible a (X b) => a -> X a
doCoerce = coerce

while it fails to compile in 7.8.4 with the error

testCoerce.hs:6:13:
    Could not coerce from ‘a’ to ‘X b0’
      because ‘a’ and ‘X b0’ are different types.
      arising from the ambiguity check for ‘doCoerce’
    from the context (Coercible a (X b))
      bound by the type signature for
                 doCoerce :: Coercible a (X b) => a -> X a
      at testCoerce.hs:6:13-41
    The type variable ‘b0’ is ambiguous
    In the ambiguity check for:
      forall a b. Coercible a (X b) => a -> X a
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature for ‘doCoerce’:
      doCoerce :: Coercible a (X b) => a -> X a

The coercion pattern may look a bit funny, but it is rather useful when one has newtypes of the form

newtype Y = Y (ForeignPtr Y)

which appear naturally when writing bindings to C libraries, and one wants to get access to the underlying ForeignPtr from Y (here X is ForeignPtr). The relevant Coercible instance here is Coercible Y (ForeignPtr Y), as above.

I would have expected the version with context "Coercible a (X a)" to be accepted, as 7.8.4 does, since it seems to be a specialization of the more general coerce, but maybe I am missing something?