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