Question

It occurs a lot when implementing algorithms that you check in every iteration of a while loop if a certain variable is smaller than a certain treshold.

In imperative languages it would look like :

 while ( x < TRESHOLD ) {
      x = usefullStuff();
 }

Where of course usefullStuff is affecting x somehow…

How do you translate this construct into haskell exploiting the functional programming paradigm?

Answer 1

Even though I don’t know what your program does, I have a feeling that you’ll want to structure your logic a little differently. Have a look at section 4 of the paper Why Functional Programming Mattes – for some ideas. It deals with a similar situation involving finding the roots of an equation using Newton’s method. There the responsibilities are partitioned so that the loop logic is decoupled from the generation of successive approximations.

If usefulStuff is a monad – i.e. a function with side effects, you’ll have to use something like this:

whileLoop x
  | x < THRESHOLD = return x
  | otherwise     = do x <- usefulStuff
                       whileLoop x