Random Hacks: 8 ways to report errors in Haskell

"8 ways to report errors in Haskell" by andrew u frank

Sat, 06 Jun 2009 07:02:54 +0000

Using ghc 6.10 (with base 4.0.0.0) the module Control.Exception becomes easier to use and seem to be quite flexible.

The only limitation I can see is that the exception raised in pure code (this is possible) cannot be caught in pure code; catching exceptions is only possible in the IO monade. I wonder, if this is a serious restriction and how difficult it would be to ovecome. The same example with division by zero becomes (if properly formated):

import qualified Control.Exception as E

import Data.Typeable

data MyException = DivByZero | ThisEx | ThatEx String deriving (Show, Eq, Typeable)

instance E.Exception MyException

myDiv9 :: (Monad m) => Float → Float → m Float

myDiv9 a 0 = E.throw DivByZero

myDiv9 a 99 = E.throw (ThatEx “Fortran style ending”)

myDiv9 a b = return (a / b)

isDivByZero (DivByZero) = True

isDivByZero _ = False

example9 :: Float → Float → IO ()

example9 x y = E.catchJust (\e → if isDivByZero e then Just (e) else Nothing)

(do q <- myDiv9 x y putStrLn (show q) E.throw ThisEx )

(\err → do putStrLn (“my DivByZero ” ++ show (err:: MyException)) return () )

"8 ways to report errors in Haskell" by andrew u frank

Thu, 04 Jun 2009 11:44:07 +0000

moving from ghc 6.8.2 to 6.10 I cannot recompile the examples given here (especially 1 and 4). What has changed in base 4.0.0.0? The error I get is

Ambiguous type variable `e’ in the constraint: `Exception e’ arising from a use of `E.catch’

anybody can help?

"8 ways to report errors in Haskell" by andrew u frank

Sat, 11 Apr 2009 08:15:10 +0000

very useful post. i have then selected version 4 and it works. thanks for shedding light on this issue, which is confusing due to the incompatible but same name functions.

andrew

"8 ways to report errors in Haskell" by tphyahoo

Wed, 09 Jan 2008 10:46:48 +0000

right, uh, with whitespace :)

example4a x y = maybe ( putStrLn “Division by zero” ) ( putStrLn . show ) ( myDiv4 x y)

example4b x y = either putStrLn ( putStrLn . show ) ( myDiv4 x y )

"8 ways to report errors in Haskell" by tphyahoo

Wed, 09 Jan 2008 10:46:11 +0000

uh, sorry, what I meant was

example4a x y = maybe ( putStrLn “Division by zero” ) ( putStrLn . show ) ( myDiv4 x y) example4b x y = either putStrLn ( putStrLn . show ) ( myDiv4 x y )

"8 ways to report errors in Haskell" by tphyahoo

Wed, 09 Jan 2008 10:43:30 +0000

Or (wrt what I just said) you could do what Luis Cabellos suggested above.

Also, examples 4b and 4c could be rewritten without the case statements by using the maybe/either functions.

example4a x y = maybe ( putStrLn “Division by zero” ) ( putStrLn . show )

example4b x y = either putStrLn ( putStrLn . show )

I suspect there’s an idiom for doing this with IO as well but I don’t know it.

"8 ways to report errors in Haskell" by tphyahoo

Wed, 09 Jan 2008 10:35:16 +0000

You need to import Control.Monad.Error for the Either stuff to work in example 4b or you get “No instance for (Monad (Either String))”

I think possibly this used to work, but stopped working at some point due to a reorganization of the haskell standard libs.

Thanks for putting this great post together.

"8 ways to report errors in Haskell" by Luis Cabellos

Tue, 23 Oct 2007 06:27:03 +0000

Great Post!

I found one code error. To test #3 way we need to add this:

instance Monad (Either a) where
  (Right x) >>= k = k x
  (Left err) >>= k = Left err
  return = Right

"8 ways to report errors in Haskell" by xml security geek

Sat, 31 Mar 2007 23:03:46 +0000

Yea, Haskell is nice to play with, but such inconsistencies still make it less appealing for serious development(((

"8 ways to report errors in Haskell" by David Waern

Sun, 11 Mar 2007 13:06:42 +0000

Hi,

Nice post! I’ve been bugged by these issues myself. It would be nice to have standards/conventions for this.

8 ways to report errors in Haskell

Eric Kidd — Sat, 10 Mar 2007 19:05:00 +0000

Haskell is a marvellous language, but there are some things I don’t like about it. My least favorite: Haskell has no fewer than 8 different APIs for reporting errors.

To make a bad situation worse, the choice of API varies between popular libraries. To give a particularly unfortunate example, Network.URI.parseURI and Network.HTTP.simpleHTTP report errors in entirely different ways, turning a “download this URL” program into a page of code, nearly half of which is devoted to dealing with various kinds of errors. (The rest is boilerplate that could be refactored into a nice wrapper.)

Let’s begin with a toy function, the simplest possible program that could actually fail:

myDiv x y = x / y

As every algebra student knows, we can’t divide by zero. Using this function as our example, let’s take a look at all the different ways we can implement error-reporting in Haskell.