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-exc...