[Haskell-beginners] Help with monads (I think...)

[Daniel Fischer]

Am Samstag, 21. Februar 2009 01:30 schrieb Patrick LeBoutillier:
> Hi all,
>
> I'm trying to implement the following simple Perl program in Haskell:
>
>   my $nb_tests = 0 ;
>
>   sub ok {
>           my $bool = shift ;
>           $nb_tests++ ;
>           print STDOUT ($bool ? "ok" : "nok") . " $nb_tests\n" ;
>   }
>
>   ok(0) ;
>   ok(1) ;
>
> The output is:
>
>   nok 1
>   ok 2
>
> I'm pretty much a Haskell newbie, but I know a bit about monads (and
> have been reading "Real World Haskell"), and I think I need to put the
> ok function must live inside some kind of state monad. My problem is
> that I also would like the ok function to perform some IO (as shown
> above, print).
>
> How is a case like this handled? Can my function live in 2 monads?

Yes, it can:
--------------------
module OK where
import Control.Monad.State

ok :: Bool -> StateT Int IO ()
ok b = do
    increment
    nr <- get
    lift $ putStrLn $ (if b then "ok " else "nok ") ++ show nr

increment :: StateT Int IO ()
increment = modify succ

main :: IO ()
main = evalStateT (ok False >> ok True) 0
--------------------

Loading package base ... linking ... done.
[1 of 1] Compiling OK               ( OK.hs, interpreted )
Ok, modules loaded: OK.
*OK> main
Loading package mtl-1.1.0.1 ... linking ... done.
nok 1
ok 2

What you need for this kind of stuff is a monad-transformer, there are 
transformers for most(? many, anyway) monads, recognizable by ending in T.
They wrap one monad (here IO) inside another (State), combining their 
respective abilities.

I'm sure there's lots of useful stuff on monad-transformers in the wikibook, 
too - they should also be treated in RWH, because in real-world apps you tend 
to need them:)

>
>
> Thanks a lot,
>
> Patrick

Cheers,
Daniel