[Haskell] Catching error / making library functions monadic (in failure)

[Mitchell, Neil]

Hi Philip,

You might want to take a look at the Safe library, which does pretty close to what you request.

http://www-users.cs.york.ac.uk/~ndm/safe/

(or cabal install safe)

I would happily accept a patch adding *Fail variants that failed in some appropriate Monad if that is what you want.

Thanks

Neil

 

> -----Original Message-----
> From: haskell-bounces at haskell.org 
> [mailto:haskell-bounces at haskell.org] On Behalf Of Philip K.F. 
> Hölzenspies
> Sent: 08 October 2008 3:38 pm
> To: haskell at haskell.org
> Subject: [Haskell] Catching error / making library functions 
> monadic (in failure)
> 
> Dear Hask'lers,
> 
> I'm working on a graph generator that involves a lot of 
> random selection out of a list of vertices. Basically, there 
> are functions that look a little like
> this:
> 
> select vs = randomRM (0,length vs - 1) >>= return . (vs !!)
> 
> where randomRM is a lot like Random.randomRIO, except that it 
> is not in the IO monad, but rather in any monad, i.e.
> 
> class Monad m => RandomM m where
> 	randomM :: m a
> 	randomRM :: (a,a) -> m a
> 
> instance RandomM IO where
> 	randomM = randomIO
> 	randomRM = randomRIO
> 
> The problem is, obviously, that when the list of vertices 
> (vs) is empty, "select vs" will result in an error. The 
> Prelude defines (!!) as:
> 
> xs     !! n | n < 0 =  error "Prelude.!!: negative index"
> []     !! _         =  error "Prelude.!!: index too large"
> (x:_)  !! 0         =  x
> (_:xs) !! n         =  xs !! (n-1)
> 
> The function 'error' is implemented (at least in GHC) using 
> Control.Exception.throw. Unfortunately, 'catch' does not seem 
> to work. From
> ghci:
> 
> Prelude> catch (return $ [] !! 0) (const $ putStrLn "foo" >> 
> return 42)
> *** Exception: Prelude.(!!): index too large
> 
> Is there any way to catch errors in functions in libraries 
> (like the Prelude)? 
> This is made even more necessary by the fact that the default 
> implementation for 'fail' in a monad is 'error'. I would 
> already be happy if I could make all applications of error 
> change into applications of fail, as defined in my monad. 
> Preferably, though, I would not even need a failure mechanism 
> in my own monad, but rather have an ErrorMonad (or something 
> similar) and just have
> 
> class Monad m => ErrorMonad error m | m -> error where
> 	onError :: m a -> (error -> a) -> a
> 
> For now, I simply reimplement the functions I need that give errors.
> 
> Regards,
> Philip
> _______________________________________________
> Haskell mailing list
> Haskell at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell
> 
> 

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