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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