DeepSeq.lhs [was: Re: [Haskell] Force evaluation]
heringtonlacey at mindspring.com
Mon Dec 13 22:32:56 EST 2004
At 4:05 PM +0000 12/8/04, Ben Rudiak-Gould wrote:
>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
> foo <- eval makeFoo `catch` \e -> defaultFoo
>but makeFoo has a monadic type, so eval makeFoo === makeFoo.
Michael's exact intent was not clear to me, but he could consider the
import Prelude hiding (catch)
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
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