[Haskell-cafe] AW: ST Monad - what's wrong?

[Nicu Ionita]

Ok, solved, the use of ($) was the problem.

I changed it

> primes :: Set Integer
> primes = runST (getPrimes "primes10h7.txt")

and it compiles (and works too).

Nicu


> -----Ursprüngliche Nachricht-----
> Von: Nicu Ionita [mailto:nionita at lycos.de] 
> Gesendet: Sonntag, 9. Dezember 2007 12:12
> An: 'haskell-cafe at haskell.org'
> Betreff: ST Monad - what's wrong?
> 
> 
> Hi,
> 
> I'm trying to use the ST monad in order to turn an almost 
> pure function into a pure one: reading a precalculated list 
> of primes into a prime set. But the following code brings an error:
> 
> > primes :: Set Integer
> > primes = runST $ getPrimes "primes10h7.txt"
> 
> > getPrimes :: String -> (forall s. ST s (Set Integer)) 
> getPrimes file =
> >     do cont <- unsafeIOToST (readFile file)
> >        let set = fromList $ map read $ lines cont
> >        return set
> 
> And here is the error:
> 
> Couldn't match expected type `forall s. ST s a'
> against inferred type `ST s (Set Integer)'
> In the second argument of `($)', namely
> `getPrimes "primes10h7.txt"'
> In the expression: runST $ (getPrimes "primes10h7.txt")
> In the definition of `primes':
> primes = runST $ (getPrimes "primes10h7.txt")
> 
> Compiled with GHC 6.6.1 with extensions activated.
> 
> Nicu
>