CODEWITHSUNDEEP
haskell
hgiesel
hgiesel

Reputation: 5648

How do you write exception handlers in Haskell

Say I have an exception like ExitFailure. How can I match against the different values of ExitFailure in my handler?

import System.Exit (ExitCode(..))
import Control.Monad.Catch as Exc

value :: Either Exc.SomeException String
value = Exc.throwM . ExitFailure $ 23

valHandler :: Exc.SomeException -> Either Exc.SomeException String
valHandler e = ??

main = print $ Exc.catch value valHandler

I know valHandler e = someValue would match against any kind of exception, but I've never seen an example, where it matches against the kind of exception.

I can't use pattern matching, because I have SomeException at my hands, and I can't use equality checks, because SomeException doesn't have an Eq instance?

Upvotes: 0

Views: 127

Answers (1)

dfeuer
dfeuer

Reputation: 48591

The Exception class offers a method for this:

fromException :: SomeException -> Maybe e

You can use this to recover the exception.

valHandler :: Exc.SomeException -> Either Exc.SomeException String
valHandler e
  | Just (ExitFailure code) <- fromException e
  = Right ...
  | otherwise = Left e

Upvotes: 1

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