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
The typechecker in 6.6.1 gets confused by the ($) and loses track of the 'freeness' of s (the thread variable) . The same code should work fine in 6.8.1, or alternatively in 6.6.1 without the ($). Cheers pepe On 09/12/2007, at 12:11, 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")
Compiled with GHC 6.6.1 with extensions activated.
Nicu
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On Dec 9, 2007 2:39 PM, pepe
The typechecker in 6.6.1 gets confused by the ($) and loses track of the 'freeness' of s (the thread variable) . The same code should work fine in 6.8.1, or alternatively in 6.6.1 without the ($).
True. However, note that the release notes of 6.8.1 encourage not to rely in this "new feature" because it can change in the feature.
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
participants (4)
-
Alfonso Acosta -
Nicu Ionita -
pepe -
Stefan O'Rear