Module for debugging Haskell programs. To use, take the functions that you are interested in debugging, e.g.:

module QuickSort(quicksort) where
import Data.List

quicksort :: Ord a => [a] -> [a]
quicksort [] = []
quicksort (x:xs) = quicksort lt ++ [x] ++ quicksort gt
    where (lt, gt) = partition (<= x) xs

Turn on the TemplateHaskell and ViewPatterns extensions, import Debug, indent your code and place it under a call to debug, e.g.:

{-# LANGUAGE TemplateHaskell, ViewPatterns #-}
module QuickSort(quicksort) where
import Data.List
import Debug

debug [d|
   quicksort :: Ord a => [a] -> [a]
   quicksort [] = []
   quicksort (x:xs) = quicksort lt ++ [x] ++ quicksort gt
       where (lt, gt) = partition (<= x) xs
   |]

We can now run our debugger with:

$ ghci QuickSort.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling QuickSort        ( QuickSort.hs, interpreted )
Ok, 1 module loaded.
*QuickSort> quicksort "haskell"
"aehklls"
*QuickSort> debugView

The final call to debugView starts a web browser to view the recorded information. Alternatively call debugSave to write the web page to a known location.

For more ways to view the result (e.g. producing JSON) or record traces (without using TemplateHaskell) see Debug.Record.