Haskell version of code in digitprod.py

digitprod.hs

{-
   Here's a Haskell equivalent of digitprod.py.
   Haskell is a non-strict (aka lazy) pure functional programming language,
   and that makes it differ from the code in digitprod.py:
    - Haskell uses recursion rather than looping, but has higher-order
      functions that let you avoid repeating boilerplate.
    - Functions are "first-class citizens" in Haskell, so you can create
      them and return them and pass them.
    - Haskell lets you write functions in "point free" form. If you look,
      you'll see that dpReduce and digitProduct make no mention of their
      Int arguments, but they both have one. *Use it carefully*. Past a
      certain point it makes code harder to read instead of clearer. Had
      I not fed the original version to pointfree.io and found it better,
      I'd have left it as it was.
    - Despite Guido's dislike of functional programming in Python (or
      perhaps I should say of common functional programming functions,
      since you can get the effect of map, filter, or the combination of
      the two with comprehensions), Python still has itertools and also
      functools, and you can write iterate as a generator expression:
      
      def iterate(f, x0):
          while True:
              yield x0
              x0 = f(x0)
-}

import Data.Char (digitToInt)

dpReduce :: Int -> Int 
dpReduce = length . takeWhile (> 9) . iterate digitProduct 
    where digitProduct = product . map digitToInt . show