On 9/6/06, Neil Mitchell
Let take for example a bug I spent tracking down in Haskell this weekend. The bug can be summarized as "Program error: pattern match failure: head []". And indeed, thats all you get. A quick grep reveals there are about 60 calls to head in the program. In this instance I have the choice between 1) crying, 2) a debugger, 3) a lot of hard work. [Of course, knowing this situation arises, I had already prepared my getout plan, so it wasn't a massive problem]
I don't know what your getout plan was but when I'm in this situation I do the following (hopefully listing this trick will help the OP): head' :: String -> [a] -> a head' s [] = error $ "head' empty list at " ++ s head' _ xs = head xs Then I do the tedious thing of replacing all the heads in my program with head' and giving each a unique string to print out, such as head' "line 5". You could even take this a step further and let the type system help you find the places to replace head by doing something like: import Prelude hiding (head) import qualified Prelude (head) head :: String -> [a] -> a head s [] = error $ "head empty list at " ++ s head _ xs = Prelude.head Now you should get type errors anywhere you're using head. Or maybe even more extreme you could use template haskell or the c preprocessor to fill in the line number + column. HTH, Jason