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

Nicu Ionita nionita at lycos.de
Sun Dec 9 06:11:42 EST 2007


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.


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