On Dec 28, 2005, at 6:10 AM, Hunter Kelly wrote:
Heya, I decided to play around with Haskell and see what it's like. I used a small problem to explore it. Basically, given two words, find the least number of 1 letter changes that will go from one word to the other (e.g. for "fig" and "dog" either fig -> fog -> dog or fig -> dig -> dog).
I came up with a solution, but I have to say it was quite difficult to get any debugging information, and when I did, the result was fairly surprising!
I see you are using Debug.Trace to generate your debug messages. The 'trace' function is a sort of strange one, because it breaks the usual rules that Haskell follows; it allows you to generate output in the middle of a pure computation. It works by generating output _when it is evaluated_. However, without the IO monad to make everything sequenced and well-behaved, it can be difficult to predict when that will occur. In the particular program you posted, the 'trace' thunk is not evaluated until after the recursive call has completed, which gives the reversed output. Furthermore, when there is no solution, the 'trace' thunk isn't evaluated at all (the magic of laziness!), so you never see that output. To make your traces show up where you expect, you need to make sure that your trace function gets forced earlier and on both success and failure paths. Since your function is written as a big let...in if ... block you can do something like this: ..... = let ... .... in trace traceString (if ... then else ) That way, the trace will be output before the 'if' is evaluated, so you will get output for both branches. I can't tell from a quick inspection if it will return the results in the order you expect, but I think it may. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG