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

[Thomas Davie]

On 21 Feb 2009, at 01:30, Patrick LeBoutillier wrote:

> 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?

I personally wouldn't use two monads at all for this – in fact, I'd  
only use IO in one function:

main = putStr . unlines . results inputs . snd . tests $ inputs

inputs = [1,2]

tests = foldr (\_ (x,l) -> (not x, x:l)) (True,[])

results = zipWith result
result testN True  = "ok "  ++ show testN
result testN False = "nok " ++ show testN

Bob
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