Tamas K Papp
I would like to learn a reasonable way (ie how others do it) to debug programs in Haskell. Is it possible to "see" what's going on when a function is evaluated? Eg in
f a b = let c = a+b d = a*b in c+d
evaluating
f 1 2
would output something like
f called with values 1 2 c is now (+) a b => (+) 1 2 => 3 d is now (*) a b => (*) 1 2 => 2
But why should c and d exist at runtime? They're only used once each, so the compiler is free to replace f with \a b -> (a+b)+ a*b and indeed, it's quite likely that f should disappear too. So if your debugger outputs what you want, it's not debugging the programme that you would otherwise have run.
Or maybe I am thinking the wrong way, and functional programming has its own debugging style...
I've said this before, but I think it's worth repeating: in most cases, if you need to debug your programme it's because it's too complicated for you to understand, so the correct thing to do is to simplify it and try again. That's not to say that I don't think you should test programmes... in fact I'm all for testing little bits. You can define f above and try it on a few arguments in ghci or hugs, and you can use QuickCheck and/or HUnit to automate this. But there's no reason for you to care what happens while f is evaluating. Remember there's very little (often no) overhead for defining subsidiary functions, so break complicated stuff into parts you can understand. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2006-07-14)