[Haskell] A few newbie questions about tracing/debugging and
order of execution
robdockins at fastmail.fm
Wed Dec 28 10:32:07 EST 2005
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
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:
in trace traceString
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.
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
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