Tracing:

Debug.Trace:
http://eax.me/haskell-debug-trace/
http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/Debug-Trace.html
traceShow

tracer:
http://hackage.haskell.org/package/tracer
https://github.com/knz/hs-tracer

HAT:
http://hackage.haskell.org/package/hat (http://olafchitil.github.com/hat)

http://hackage.haskell.org/package/loch-th (https://github.com/liskin/loch-th)
http://hackage.haskell.org/package/file-location (https://github.com/gregwebs/FileLocation.hs)
http://hackage.haskell.org/package/trace-function-call
http://hackage.haskell.org/package/trace-call


Debug.HTrace:
http://hackage.haskell.org/package/htrace (https://github.com/jkff/htrace)

module Debug.HTrace (htrace) where

import Data.List (foldl')
import Data.IORef
import System.IO.Unsafe

level = unsafePerformIO $ newIORef 0

-- | Trace "str" on a separate line, and increase indentation of subsequent
--   traces while x is being evaluated. 
htrace str x = unsafePerformIO $ do
  lvl <- readIORef level
  putStrLn (replicate (2*lvl) ' ' ++ str)
  writeIORef level (lvl+1)
  let !vx = x
  writeIORef level lvl
  return vx