On Sun, Dec 09, 2007 at 12:11:42PM +0100, Nicu Ionita wrote:
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")
ST is far too big a hammer to use here. It gives you safety you're not using, at the cost of much more complicated typechecking. I'd just use unsafePerformIO. Stefan