On 26 April 2011 00:53, Ryan Ingram <ryani.spam@gmail.com> wrote:

I've been working on Haskell for quite a while and it's not too often that a beginner shows me a new trick--this trick with trace seems really cool and I hadn't seen it before.

f x | trace ("f " ++ show x) False = undefined
f ... -- rest of regular definition

Makes it really easy to add the trace and then later comment it out. One problem I always have with traces the usual way is that adding/removing them is kind of annoying--it can change the indentation of your code, etc. So this trick is really useful!

Yes. It would be much better if trace were a primitive, whose semantics were to ignore the first argument and yield the second argument as a value. But these semantics would be changed by supplying an argument to the runtime (so you could turn tracing on-and-off between runs).

Actually, this still isn't very convenient. Better would be to have another String argument - the trace key. Then the runtime argument would list those trace keys for which tracing should be performed.

This way you never have to edit the code to add and remove tracing (though you may want to remove many trace statements when you are convinced the code is thoroughly debugged).
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Colin Adams
Preston, Lancashire, ENGLAND
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