Coins.hs

-- Given an infinite number of quarters (25 cents),  dimes (10 cents),
-- nickels (5 cents) and pennies (1 cent), write code to calculate the
-- number of ways of representing n cents.

module Coins where

comb cents = [(q, d, n, p) | q <- rng 25 cents,
                             d <- rng 10 (cents - 25*q),
                             n <- rng  5 (cents - 25*q - 10*d),
                             p <- rng  1 (cents - 25*q - 10*d - 5*n),
                             25*q + 10*d + 5*n + p == cents]
  where rng k max = [0..(max `div` k)]

result = length . comb