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
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