Types.
(>>=) :: Monad m => m a -> (a -> m b) -> m b
(1+) :: Num a => a -> a
So, the typechecker deduces that 1) "a" is the same as "m b", and 2)
"a" (and "m b", therefore) must be of class "Num"
Now,
Just 3 :: Num t => Maybe t
and the typechecker learns from that that "m a" must be the same as
"Maybe t", with "t" being of class "Num". This leads to two
observations: 3) "m" is "Maybe", and 4) "a" is of class "Num" - the
same as (2) above
Now, from (1) and (3) it follows that "a" is the same as "Maybe b".
(2) lead than to "Maybe b" being of class "Num" - but GHCi doesn't
have this instance, and complains.
What you've probably meant is something like
Just 3 >>= \x -> return (x + 1)
or, equivalently,
liftM (+1) $ Just 3
On 9 May 2009, at 23:31, michael rice wrote:
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
================
[mich...@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>
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
