Dean Herington wrote:
deepSeq :: DeepSeq a => a -> b -> b
I should point out that deepSeq with this type is the composition of two simpler operations:
deepSeq = seq . eval where eval :: DeepSeq a => a -> a
eval ties a demand for a value to a demand for all its subvalues, while seq ties a demand for a value to a demand for another value of an unrelated type.
Of course you can define eval x = x `deepSeq` x instead, so it's largely a matter of taste.
instance DeepSeq (IO a) where deepSeq = seq
This is an interesting instance (which is not to say I think it's wrong). It means the original poster's code won't work. He wanted to write
foo <- eval makeFoo `catch` \e -> defaultFoo
but makeFoo has a monadic type, so eval makeFoo === makeFoo.
-- Ben
Michael's exact intent was not clear to me, but he could consider the following.
-- Dean
import Prelude hiding (catch) import DeepSeq import Control.Exception (catch, evaluate)
deepForce :: DeepSeq a => a -> a deepForce x = x `deepSeq` x
deepForceIO :: DeepSeq a => a -> IO a deepForceIO = evaluate . deepForce
makeFooGood, makeFooBad, defaultFoo :: Int makeFooGood = 1 makeFooBad = error "bad makeFoo" defaultFoo = 0
try f = (deepForceIO f `catch` \e -> return defaultFoo) >>= print
main = try makeFooGood >> try makeFooBad _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
