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Diffstat (limited to 'docs/users_guide/using-warnings.rst')
| -rw-r--r-- | docs/users_guide/using-warnings.rst | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst index a9995268ea..3c09d4c141 100644 --- a/docs/users_guide/using-warnings.rst +++ b/docs/users_guide/using-warnings.rst @@ -1235,6 +1235,34 @@ of ``-W(no-)*``. second pattern overlaps it. More often than not, redundant patterns is a programmer mistake/error, so this option is enabled by default. + If the programmer is dead set of keeping a redundant clause, + for example to prevent bitrot, they can make use of a guard + scrutinising ``GHC.Exts.considerAccessible`` to prevent the + checker from flagging the parent clause as redundant: :: + + g :: String -> Int + g [] = 0 + g (_:xs) = 1 + g "2" | considerAccessible = 2 -- No warning! + + Note that ``considerAccessible`` should come as the last statement of + the guard in order not to impact the results of the checker. E.g., if + you write :: + + h :: Bool -> Int + h x = case (x, x) of + (True, True) -> 1 + (False, False) -> 2 + (True, False) | considerAccessible, False <- x -> 3 + + The pattern-match checker takes you by your word, will conclude + that ``False <- x`` might fail and warn that the pattern-match + is inexhaustive. Put ``considerAccessible`` last to avoid such + confusions. + + Note that due to technical limitations, ``considerAccessible`` will not + suppress :ghc-flag:`-Winaccessible-code` warnings. + .. ghc-flag:: -Winaccessible-code :shortdesc: warn about inaccessible code :type: dynamic |