Suggested by a question from sethk on #haskell irc channel. Solves an FAQ where people have often resorted to cpp or m4: a `trace' that prints line numbers.

module Location (trace, assert) where

import qualified Control.Exception as C (catch) import System.IO.Unsafe (unsafePerformIO) import GHC.Base (assert) import System.IO

-- An identity function that also prints the current line and column number trace :: (Bool -> IO () -> IO ()) -> a -> a trace assrt f = (unsafePerformIO $ C.catch (assrt False $ return ()) printIt) `seq` f where printIt e = let (x,_) = break (== ' ') $ show e in hPutStrLn stderr (x ++ " trace")

for example:

import Location

main = do let x = trace assert (1+2)

putStrLn . show $ x

Generates: $ ./a.out M.hs:4:18-23: trace 3 This continues a theme I've noticed: catching internal exceptions can yield some interesting results, i.e. with undefined, missing class methods, and here, assertion failures. Hope this little thing is useful. Cheers, Don