Is there a Haskell Wiki page on extensible exceptions? The paper is a fine reference, it'd be cool to have a wiki page giving programmer-oriented guidance, saying what comes with GHC what other packages are available and some simple examples of how to use them. Simon | -----Original Message----- | From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of | Pepe Iborra | Sent: 25 April 2009 19:52 | To: haskell@haskell.org | Subject: [Haskell] ANN control-monad-exception-0.1: Explicitly typed exceptions | | The control-monad-exception package [1] provides explicitly typed | exceptions for Haskell. | In other words, this is a perfect example of bundling in a Haskell | library what for other | programming languages is a native feature. | | The type of a computation in the EM monad carries a list of the exceptions that | the computation may throw. A exception is raised with 'throw', which | in addition adds it | to the type, and captured with 'catch', which correspondingly removes | it from the type. | Only safe computations (all exceptions handled) can escape from the monad. | | The encoding used for the exception list is based on a phantom type | variable carrying a | @Throws@ constraint for every exception type. Catching an exception | @e@ satifies the constraint | @Throws e@ thus removing it from the type. It is possible to teach | Throws about exception subtyping | by manually inserting new instances declaring the subtyping relations | between exceptions. I don't | believe there is a better way to handle this, as the existential | wrapper encoding used | for Control.Exception.SomeException does not reveal the subtyping | relations, but ideas are | welcome. | | Example | -------- | GHCi infers the following types | | eval :: (Throws DivideByZero l, Throws SumOverflow l) => Expr -> EM l Double | eval `catch` \ (e::DivideByZero) -> return (-1) :: Throws | SumOverflow l => Expr -> EM l Double | runEM (eval `catch` \ (e::SomeException) -> return (-1)) :: Expr -> Double | | for the code below. | | | > import Control.Monad.Exception | > import Data.Typeable | | > data Expr = Add Expr Expr | Div Expr Expr | Val Double | > eval (Val x) = return x | > eval (Add a1 a2) = do | > v1 <- eval a1 | > v2 <- eval a2 | > let sum = v1 + v2 | > if sum < v1 || sum < v2 then throw SumOverflow else return sum | > eval (Div a1 a2) = do | > v1 <- eval a1 | > v2 <- eval a2 | > if v2 == 0 then throw DivideByZero else return (v1 / v2) | | > data DivideByZero = DivideByZero deriving (Show, Typeable) | > data SumOverflow = SumOverflow deriving (Show, Typeable) | | > instance Exception DivideByZero | > instance Exception SumOverflow | | | Comments and patches are welcome. | Cheers, | Pepe Iborra | | [1] - http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-monad- | exception | _______________________________________________ | Haskell mailing list | Haskell@haskell.org | http://www.haskell.org/mailman/listinfo/haskell