Wednesday 2 September 2009

Debugging in Haskell

One thing I was finding a little frustrating is that if I was to do any IO in a function, then it's type signature changes. This is good most of the time, but it's a little painful to have to change all your code to print out a number!

Thankfully, there's a number of solutions.

The GHCi debugger provides a way of inspecting code. :b N sets a break point in the loaded module at a specific line. Taking the anagrams example I set :b 30 on anagrams <-anagramList wordfile.

anagramsOf :: String -> IO ()
anagramsOf word = do
anagrams <- anagramList wordfile
putStrLn (show (Map.lookup (stringToKey word) anagrams))

When the program is run and the breakpoint is hit we get the following:

Stopped at anagrams.hs:31:2-57
_result :: IO () = _
anagrams :: Map String (Set String) = _
word :: String = _
[anagrams.hs:31:2-57] *Main> :list
30 anagrams <- anagramList wordfile
31 putStrLn (show (Map.lookup (stringToKey word) anagrams))

:list is used to list of the free variables in scope (anagrams and word) and these are available to inspect in the debugger. _result is a binding for the result expression. Once you've hit a breakpoint, you can use use :trace to continue to the next breakpoint, recording the history as you go along. :back and :forward allow you to go up and down the list of evaluated expressions and inspect each one.

The Haskell Wiki has a section devoted to debugging which brought me to the Debug.Trace module. This allows you to print some text and return the evaluation of the next expression e.g. Debug.Trace.trace "1+1=" (1 + 1).

Initially, I couldn't understand how the type of Debug.Trace.trace could be String -> a -> a, but then I found System.IO.Unsafe. It comes with a large health warning that it doesn't enforce ordering on IO and it's type unsafe. Evil, yet useful (at least to write the trace functions).