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Diffstat (limited to 'docs/users_guide/bugs.rst')
| -rw-r--r-- | docs/users_guide/bugs.rst | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/docs/users_guide/bugs.rst b/docs/users_guide/bugs.rst index d6e1970fea..cfcfc0417c 100644 --- a/docs/users_guide/bugs.rst +++ b/docs/users_guide/bugs.rst @@ -170,6 +170,37 @@ This improves efficiency slightly but significantly for most programs, and is bad for only a few. To suppress this bogus "optimisation" use ``-fpedantic-bottoms``. +.. _infelicities-failable-pats: + +Failable patterns +^^^^^^^^^^^^^^^^^ + +Since the `MonadFail Proposal (MFP) <https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail>`__, +do-notation blocks that contain a failable pattern need a `MonadFail <https://hackage.haskell.org/package/base-4.14.1.0/docs/Control-Monad-Fail.html#t:MonadFail>`__ constraint. + +For example + +:: + + mayFail :: (MonadIO m) => m () + mayFail = do + (Just value) <- fetchData + putStrLn value + +Will warn you with + +:: + + • Could not deduce (MonadFail m) + arising from a do statement + with the failable pattern ‘(Just x)’ + from the context: MonadIO m + bound by the type signature for: + mayFail :: forall (m :: * -> *). MonadIO m => m () + +And indeed, since the `Monad <https://hackage.haskell.org/package/base-4.14.1.0/docs/Control-Monad.html#t:Monad>`__ class does not have the ``fail`` method anymore, +we need to explicitly add ``(MonadFail m)`` to the contraints of the function. + .. _infelicities-decls: Declarations and bindings |