The task is to write a function findFraction maxDen number to find a short but good rational approximation to a decimal representation of a number — problem statement on TopCoder:

Given a fraction F = A/B, where 0 <= A < B, its quality of approximation with respect to number is calculated as follows:

• Let S be the decimal fraction (infinite or finite) representation of F.
• Let N be the number of digits after the decimal point in number. If number has trailing zeros, all of them are considered to be significant and are counted towards N.
• If S is infinite or the number of digits after the decimal point in S is greater than N, only consider the first N decimals after the decimal point in S. Truncate the rest of the digits without performing any kind of rounding.
• If the number of digits after the decimal point in S is less than N, append trailing zeroes to the right side until there are exactly N digits after the decimal point.
• The quality of approximation is the number of digits in the longest common prefix of S and number. The longest common prefix of two numbers is the longest string which is a prefix of the decimal representations of both numbers with no extra leading zeroes. For example, "3.14" is the longest common prefix of 3.1428 and 3.1415.

[…] You are only allowed to use fractions where the denominator is less than or equal to maxDen. Find an approximation F = A/B of number such that 1 <= B <= maxDen, 0 <= A < B, and the quality of approximation is maximized. Return a String formatted "A/B has X exact digits" (quotes for clarity) where A/B is the approximation you have found and X is its quality. If there are several such approximations, choose the one with the smallest denominator among all of them. If there is still a tie, choose the one among those with the smallest numerator.

import Data.Char
import Data.List
import Data.Maybe

showResult :: (Int, Int, Int) -> String
showResult (a, b, x) = show a ++ "/" ++ show b ++ " has "
++ show x ++ " exact digits"

compareDigits :: [Int] -> [Int] -> Ordering
compareDigits xs [] = EQ
compareDigits (x:xs) (y:ys) = case compare x y of
EQ -> compareDigits xs ys
order -> order

toDigit :: Char -> Int
toDigit c = ord c - ord '0'

preciseDivision :: Int -> Int -> [Int]
preciseDivision a b = div a' b : preciseDivision (mod a' b) b
where a' = 10 * a

estimate :: [Int] -> Double
estimate = sum . zipWith (flip (/)) (iterate (*10) 10) .
map fromIntegral

bestNumerator :: [Int] -> Double -> Int -> Maybe Int
bestNumerator ds p b =
case find atLeast . map (pair ds b) $[pivot .. ] of Just (a, EQ) -> Just a otherwise -> Nothing where pivot = truncate$ p * fromIntegral b
pair ds b a = (a, compareDigits (preciseDivision a b) ds)
atLeast (a, order) = order /= LT

bestResult :: Int -> [Int] -> Maybe (Int, Int, Int)
bestResult n ds = fromMaybe Nothing . find isJust .
map (toResult ds $estimate ds)$ [1..n]
where toResult ds p b = case bestNumerator ds p b of
Just a -> Just (a, b, 1 + length ds)
Nothing -> Nothing

findFraction' :: Int -> [Int] -> (Int, Int, Int)
findFraction' n = fromJust . last . takeWhile isJust .
map (bestResult n) . inits

findFraction :: Int -> String -> String
findFraction n = showResult . findFraction' n .
map toDigit . tail . tail


I'm most worried by bestNumerator and bestResult here. I'm using a find idiom that I invented myself, and I wonder if cleaner, more Haskelly alternatives exist.

Hmmm, not much ideas, only syntax:

showResult :: (Int, Int, Int) -> String
showResult (a, b, x) = concat [show a, "/", show b, " has ", show x, " exact digits"]

preciseDivision :: Int -> Int -> [Int]
preciseDivision a b = let (d,r) = divMod (10*a) b in d : preciseDivision r b

...where toResult ds p b = fmap (\a -> (a, b, 1 + length ds)) \$ bestNumerator ds p b


Ordering is an instance of Monoid.

import Data.Monoid

compareDigits :: [Int] -> [Int] -> Ordering
compareDigits _ [] = EQ
compareDigits (x:xs) (y:ys) = compare x y mappend compareDigits xs ys


toDigit is not exactly necessary; Data.Char has digitToInt, which does about the same thing. The difference is that it supports hex numbers and will fail if the character is not a valid hex digit, whereas your function assumes a decimal digit, is more efficient, but will silently give incorrect results for non-digits.

Whether you replace it or not, though, toDigit is backwards, since the function takes a digit and gives you the numeric value.

In bestNumerator's pair, you shadow the ds and b parameters. That confuses me. In bestResult, toResult also has a shadowing ds.

In general, I think many of your parameter names are too short. I realize that this is common in Haskell, but I still find it abhorrent for things that aren't abstract. For example, n instead of maxDenominator (or maxDen, as in the problem statement, but "den" is an obscure abbreviation).