Haskell Print while in recursion?

Question

foo:: Int -> Int -> Int
foo z x = if (z < 100) 
             then z * foo (z+(x*z)) z
             else z

How would you print out(the integer z) an output every time it gets called from itself?Can you have function that returns an IO and Int? Do you need a secondary function?

Answer 1

Building on @is7s's answer, a useful idiom for using Debug.Trace is to do this:

import Debug.Trace

foo:: Int -> Int -> Int
foo z x | trace ("z = " ++ show z) False = undefined
foo z x = if (z < 100) 
    then z * foo (z+(x*z)) z
    else z

Here, we have introduced a definition of foo with the trace in a guard which evaluates to False, so that it will always fall-through to the original definition. In this way, we do not perturb our function, and can turn the tracing on or off by commenting out the line.