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Diffstat (limited to 'docs/users_guide/9.6.1-notes.rst')
| -rw-r--r-- | docs/users_guide/9.6.1-notes.rst | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/docs/users_guide/9.6.1-notes.rst b/docs/users_guide/9.6.1-notes.rst index 34e7723092..36067df41c 100644 --- a/docs/users_guide/9.6.1-notes.rst +++ b/docs/users_guide/9.6.1-notes.rst @@ -3,10 +3,36 @@ Version 9.6.1 ============== - Language ~~~~~~~~ +- GHC is now more conservative when solving constraints that arise from + superclass expansion in terms of other constraints that also arise from + superclass expansion. + + For example: :: + + class C a + class C a => D a b + instance D a a => D a b + + When typechecking the instance, we need to also solve the constraints arising + from the superclasses of ``D``; in this case, we need ``C a``. We could obtain + evidence for this constraint by expanding the superclasses of the context, + as ``D a a`` also has a superclass context of ``C a``. + However, is it unsound to do so in general, as we might be assuming precisely + the predicate we want to prove! This can lead to programs that loop at runtime. + + When such potentially-loopy situations arise, GHC now emits a warning. + In future releases, this behaviour will no longer be supported, and the + typechecker will outright refuse to solve these constraints, emitting a + ``Could not deduce`` error. + + In practice, you should be able to fix these issues by adding the necessary + constraint to the context, e.g. for the above example: :: + + instance (C a, D a a) => D a b + - Record updates for GADTs and other existential datatypes are now fully supported. |