--
-- Pure Haskell line/column numbers in a trace message.
--
-- Copyright 2006 Don Stewart <dons@cse.unsw.edu.au>
--
-- Free for any use.
--

module Location (trace, assert) where

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

type Assert = Bool -> IO () -> IO ()

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

--
-- Examples:
--
-- *Location> trace "trace me now!" assert $ map (+1) [1..10]
-- <interactive>:1:22-27: trace me now!
-- [2,3,4,5,6,7,8,9,10,11]
--