John Meacham wrote:
ST doesn't have exceptions which IO does. It would be no good to make ST
pay for the cost of exception handling. GHC handles them behind the
scenes (I think?) but in jhc they are explicit and IO is defined as
follows:
data World__
data IOResult a = FailIO World__ IOError | JustIO World__ a
newtype IO a = IO (World__ -> IOResult a)
Supposing you had
data STResult s a where
FailIO :: World__ IOWorld__ -> IOError -> STResult IOWorld__ a
JustST :: World__ s -> a -> STResult s a
could static analysis then eliminate the test for FailIO in ST code? It
would be pretty cool if the answer was yes, since it would mean that merging
IO and ST would be an optimization instead of a pessimization (the test
could also be omitted in IO code that uses only the ST subset). In jhc I
suppose this would have to happen late in compilation, when you eliminate
unused type parameters.
Actually, won't the test for FailIO always be eliminated by the existing
points-to analysis?
-- Ben
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