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Diffstat (limited to 'docs/users_guide/using-warnings.rst')
| -rw-r--r-- | docs/users_guide/using-warnings.rst | 22 |
1 files changed, 12 insertions, 10 deletions
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst index 31060d701d..fa44b1a370 100644 --- a/docs/users_guide/using-warnings.rst +++ b/docs/users_guide/using-warnings.rst @@ -831,20 +831,22 @@ of ``-W(no-)*``. h = \[] -> 2 Just k = f y -.. ghc-flag:: -fmax-pmcheck-iterations=⟨n⟩ - :shortdesc: the iteration limit for the pattern match checker +.. ghc-flag:: -fmax-pmcheck-models=⟨n⟩ + :shortdesc: soft limit on the number of parallel models the pattern match + checker should check a pattern match clause against :type: dynamic :category: - :default: 2000000 + :default: 100 - Sets how many iterations of the pattern-match checker will perform before - giving up. This limit is to catch cases where pattern-match checking might - be excessively costly (due to the exponential complexity of coverage - checking in the general case). It typically shouldn't be necessary to set - this unless GHC informs you that it has exceeded the pattern match checker's - iteration limit (in which case you may want to consider refactoring your - pattern match, for the sake of future readers of your code. + Pattern match checking can be exponential in some cases. This limit makes + sure we scale polynomially in the number of patterns, by forgetting refined + information gained from a partially successful match. For example, when + matching ``x`` against ``Just 4``, we split each incoming matching model + into two sub-models: One where ``x`` is not ``Nothing`` and one where ``x`` + is ``Just y`` but ``y`` is not ``4``. When the number of incoming models + exceeds the limit, we continue checking the next clause with the original, + unrefined model. .. ghc-flag:: -Wincomplete-record-updates :shortdesc: warn when a record update could fail |