[Haskell-cafe] Just 3 >>= (1+)?

[michael rice]

Why doesn't this work?

Michael

================

data Maybe a = Nothing | Just a

instance Monad Maybe where
    return         = Just
    fail           = Nothing
    Nothing  >>= f = Nothing
    (Just x) >>= f = f x
    
instance MonadPlus Maybe where
    mzero             = Nothing
    Nothing `mplus` x = x
    x `mplus` _       = x

================

[michael at localhost ~]$ ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> Just 3 >>= (1+)

<interactive>:1:0:
    No instance for (Num (Maybe b))
      arising from a use of `it' at <interactive>:1:0-14
    Possible fix: add an instance declaration for (Num (Maybe b))
    In the first argument of `print', namely `it'
    In a stmt of a 'do' expression: print it
Prelude> 




      
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