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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/debugging.rst | 19 |
1 files changed, 13 insertions, 6 deletions
diff --git a/docs/users_guide/debugging.rst b/docs/users_guide/debugging.rst index 0955af1270..8304434703 100644 --- a/docs/users_guide/debugging.rst +++ b/docs/users_guide/debugging.rst @@ -851,14 +851,21 @@ Checking for consistency However forcing larger alignments in general reduces performance. .. ghc-flag:: -fcatch-bottoms - :shortdesc: Insert ``error`` expressions after bottoming expressions; useful - when debugging the compiler. + :shortdesc: Add a default ``error`` alternative to case expressions without + a default alternative. :type: dynamic - Instructs the simplifier to emit ``error`` expressions in the continuation - of empty case analyses (which should bottom and consequently not return). - This is helpful when debugging demand analysis bugs which can sometimes - manifest as segmentation faults. + GHC generates case expressions without a default alternative in some cases: + + - When the demand analysis thinks that the scrutinee does not return (i.e. a + bottoming expression) + + - When the scrutinee is a GADT and its type rules out some constructors, and + others constructors are already handled by the case expression. + + With this flag GHC generates a default alternative with ``error`` in these + cases. This is helpful when debugging demand analysis or type checker bugs + which can sometimes manifest as segmentation faults. .. _checking-determinism: |