Why doesn't this work?
Michael
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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@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>
|