[Haskell-cafe] Code snippet, a `trace' with line and column numbers

[Donald Bruce Stewart]

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