Wait, there are two monads in scene here, IO and Maybe.
The right solution is to compose them indeed. One could use the
MaybeT monad transformer defined in the 'All about monads' tutorial
[1], or we could just define the IOmaybe monad:
> import Data.Traversable (mapM)
>
> newtype IOMaybe a = IOM { iom :: IO (Maybe a) }
>
> instance Monad IOMaybe where
> return = IOM . return . Just
> c >>= f = IOM$ do
> mb_v <- iom c
> mapM (iom.f) mb_v >>= return . join
Now we can define:
> t1 = IOM . return . f1
> t2 = IOM . f2
> t3 = IOM . return . f3
> traverse rec1 = t1 rec1 >>= t2 >>= t3
And this scheme lends itself very well to define any kind of traversal.
Note that I used the more general version of mapM defined in
Data.Traversable in the definition of the (>>=) combinator. A more
conventional definition is given the 'All about monads' tutorial.
Cheers
pepe
1- http://www.nomaware.com/monads/html/index.html
On 16/12/2006, at 15:35, Chris Eidhof wrote:
Hey Mark,
How can I concisely compose these functions without having to
write a cascade of case statements such as:
case f1 rec1 of
Nothing -> return Nothing
Just id1 -> do
rec2 <- f2 id2
return $ case rec2 of
Nothing -> return Nothing
Just rec2' -> case f3 rec2' of
....
I understand that if I was just dealing with Maybe I could use the
fact that Maybe is a monad.
Yes, you can write like this:
id2 <- f1 rec1
rec2 <- f2 id2
rec3 <- f3 rec2
return rec3
or, even shorter:
id2 <- f1 rec1
rec2 <- f2 id2
f3 rec2
The cool thing of the Maybe monad is that it combines a result in
such a way that it removes the plumbing of constantly checking for
Nothing. I can definitely recommand you the following tutorials:
http://www.nomaware.com/monads/html/index.html
http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html
Those two tutorials really helped me.
Good luck,
Chris
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