I'm a real newbie to Haskell, and I'm having trouble with a particular problem
dealing with higher-order functions.
Exercise 5.9 in Hudak's "School of Expression" asks us to write a function,
"makeChange," s.t. it makes change for a given amount using coins in a coin
supply (represented by a list of decreasing integers). For example,
make Change 99 [5,1] ==> [19,4]
This chapter is about higher order functions, so I'm assuming he wants us to
compute the result using the higher order functions defined in the chapter
(map, foldl, foldr). I devised two solutions:
{-Solution 1-}
makeChange money coinList =
zipWith div (scanl mod money coinList) coinList
{-Solution 2-}
makeChange' money (coin:coins) =
let money' = money `mod` coin
numCoins = money `div` coin
in (numCoins: makeChange' money' coins)
makeChange' 0 _ = []
makeChange' _ [] = []
However, my problem is that neither solution uses the higher-order functions
defined in the chapter. So is it possible to solve this problem using map and
fold?
Furthermore, Hudak makes the case that we should strive to find the
higher-order solutions instead of the recursive ones because the latter leads
to clearer and more concise solutions. Although solution 1 is more concise, I
feel like Solution 2 is clearer to me than Solution 1, but maybe this is just
because I'm new to haskell and higher order functions. It just seems like its
easier to understand the actual algorithm in solution 2 than in solution 1.
Thanks,
Matt Parker
University of North Texas undergrad
http://www.cs.unt.edu
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