[Haskell-beginners] How to extract from Maybe & Either in GHCi

Ertugrul Soeylemez es at ertes.de
Thu Apr 14 13:12:15 CEST 2011

Christopher Done <chrisdone at googlemail.com> wrote:

> FWIW there is also Data.Either which has the either function.

Only few people seem to realize that 'maybe' and 'either' exist and how
useful they are.  They are defined in the Prelude, so no imports are

  either this that . maybe defValue foo $ someMaybeEitherValue

They also help with monadic code:

  getStuff :: IO (Maybe String)

  getStuff >>= maybe (throwIO (MyError "Stuff not available")) putStrLn


nightmare = unsafePerformIO (getWrongWife >>= sex)

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