Random Hacks: 8 ways to report errors in Haskell

"8 ways to report errors in Haskell" by tphyahoo

Wed, 09 Jan 2008 10:46:48 -0500

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 -0500

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 -0500

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 -0500

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 -0400

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 -0400

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 -0400

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 -0500

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.