diff options
Diffstat (limited to 'docs/users_guide/using-warnings.rst')
| -rw-r--r-- | docs/users_guide/using-warnings.rst | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst index baabbbdc37..6ddeb66e41 100644 --- a/docs/users_guide/using-warnings.rst +++ b/docs/users_guide/using-warnings.rst @@ -57,6 +57,7 @@ To reverse ``-Werror``, which makes all warnings into errors, use ``-Wwarn``. * :ghc-flag:`-Wambiguous-fields` * :ghc-flag:`-Wunicode-bidirectional-format-characters` * :ghc-flag:`-Wforall-identifier` + * :ghc-flag:`-Wgadt-mono-local-binds` The following flags are simple ways to select standard "packages" of warnings: @@ -2203,6 +2204,8 @@ of ``-W(no-)*``. :type: dynamic :category: + :since: 9.0.2 + Explicit unicode bidirectional formatting characters can cause source code to be rendered misleadingly in many viewers. We warn if any such character is present in the source. @@ -2212,8 +2215,31 @@ of ``-W(no-)*``. category of the `Unicode Bidirectional Character Type Listing <https://www.unicode.org/reports/tr9/#Bidirectional_Character_Types>`_ - :since: 9.0.2 +.. ghc-flag:: -Wgadt-mono-local-binds + :shortdesc: warn when pattern matching on a GADT without MonoLocalBinds + :type: dynamic + :reverse: -Wno-gadt-mono-local-binds + + :since: 9.4.1 + + This warning is triggered on pattern matching involving GADTs, + if :extension:`MonoLocalBinds` is disabled. + Type inference can be fragile in this case. + + See the `OutsideIn(X) <https://www.microsoft.com/en-us/research/publication/outsideinx-modular-type-inference-with-local-assumptions/>`__ + paper (section 4.2) and :ref:`mono-local-binds` for more details. + + To resolve this warning, you can enable :extension:`MonoLocalBinds` + or an extension implying it (:extension:`GADTs` or + :extension:`TypeFamilies`). + + The warning is also triggered when matching on GADT-like + pattern synonyms (i.e. pattern synonyms containing equalities in provided + constraints). + In previous versions of GHC (9.2 and below), it was an error + to pattern match on a GADT if neither :extension:`GADTs` + nor :extension:`TypeFamilies` were enabled. If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice. It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's |