diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/9.6.1-notes.rst | 28 | ||||
| -rw-r--r-- | docs/users_guide/using-warnings.rst | 16 |
2 files changed, 43 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. diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst index 2c70006a7c..3624a63384 100644 --- a/docs/users_guide/using-warnings.rst +++ b/docs/users_guide/using-warnings.rst @@ -2319,6 +2319,22 @@ of ``-W(no-)*``. triggered whenever this happens, and can be addressed by enabling the extension. +.. ghc-flag:: -Wloopy-superclass-solve + :shortdesc: warn when creating potentially-loopy superclass constraint evidence + :type: dynamic + :reverse: -Wno-loopy-superclass-solve + + :since: 9.6.1 + + As explained in :ref:`undecidable_instances`, when using + :extension:`UndecidableInstances` it is possible for GHC to construct + non-terminating evidence for certain superclass constraints. + + This behaviour is scheduled to be removed in a future GHC version. + In the meantime, GHC emits this warning to inform users of potential + non-termination. Users can manually add the required constraint to the context + to avoid the problem (thus silencing the warning). + .. ghc-flag:: -Wterm-variable-capture :shortdesc: warn when an implicitly quantified type variable captures a term's name :type: dynamic |