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