brad clawsie wrote:

i am working on improving a module for getting Yahoo Finance quote data, hopefully getting it to a point that i can put it on hackage

in the quote retrieval function, there are a few places i would like to call out errors. in a trivial case i could return

IO (Maybe String)

with Nothing signifying any error state, or Just expressing the data

but i would like to be able to express some of these error cases in a more structured manner

i know the Either type can be used in such a case(?), but i've had some problem locating a satisfactory example (if this is indeed appropriate)

could one of the vets here provide a simplistic example expressing error cases, preferrably in the IO Monad (in case there are any gotchas there)?

It's fairly common to use the Either type for this. By convention, "Right" means "correct", and by elimination "Left" means an error... foo x = case x of .... return (Right y) .... return (Left "Some error happened") Instead of just returning a text string indicating the error condition, you could devize your own special type for representing the possible errors that can happen. You could also make a kind of a "Result" type that represents both successful *and* failed results, if you prefer. In addition, in the IO monad, I believe you can do fun stuff with exceptions. Specifically, you can *catch* them. However, beware: Haskell has something of a habit of executing stuff at unpredictable times, which means that if you're throwing errors in pure code, it can be hard to register the exception handler(s) at the right time! Probably simpler and clearer to go with Either or something OTOH, if you're throwing errors from within the IO monad itself, timing should be less of an issue. Just my few cents...

On Jul 11, 2007, at 15:57 , brad clawsie wrote:

i know the Either type can be used in such a case(?), but i've had some problem locating a satisfactory example (if this is indeed appropriate)

You might want to look at MonadError (Control.Monad.Error), more specifically ErrorT layered on top of IO. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

On Wed, Jul 11, 2007 at 12:57:53PM -0700, brad clawsie wrote:

but i would like to be able to express some of these error cases in a more structured manner

okay, i'm going to answer my own question for the sake of documenting it for others who might be interested. thanks to andrew and brandon for clues module Main (main) where data ErrorTestType = ErrorA | ErrorB Int | ErrorC String instance Show ErrorTestType where show ErrorA = "Error A" show (ErrorB n) = "Error B:" ++ (show n) show (ErrorC s) = "Error C:" ++ (show s) type ErrorTestT = Either ErrorTestType f :: IO (ErrorTestT String) f = do print "type something:" s <- getLine case length s of 1 -> return (Left ErrorA) 2 -> return (Left (ErrorB (length s))) 3 -> return (Left (ErrorC "error c")) _ -> return (Right s) main = do r <- f case r of (Left e) -> print e (Right a) -> print a return ()