[Haskell-cafe] Just 3 >>= (1+)?
Miguel Mitrofanov
miguelimo38 at yandex.ru
Sat May 9 16:54:30 EDT 2009
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
>
> ================
>
> [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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