Challenge: find the top three most frequently used words in a string. Only imports from the base package are allowed (source: codewars)

I'm looking for feedback on (1) readability, and (2) performance

  • Was folding the string into "frequency map" a good choice?
  • I have several small functions that I compose together. Are these abstraction layers helping or hurting readability?
  • How are my functions names?
  • Does the base package have any libraries that would make this code significantly easier to write?
  • Could I benefit from using more pointfree style?
import qualified Data.Map as Map
import Data.List
import Data.Function
import Data.Char
import Data.Maybe

top3 :: [Char] -> [[Char]]
top3 str =
    wordFrequencyMap = foldr (Map.alter increment) Map.empty (normalizedWords str)
    sortedWordFrequencies = sortBy reverseBySecond (Map.toList wordFrequencyMap)
  in map fst (take 3 sortedWordFrequencies)
    normalizedWords = filter containsAtLeastOneAlphaNumericChar . lowerCaseWords . map isLegalChar
    containsAtLeastOneAlphaNumericChar w = find isAlphaNum w /= Nothing
    lowerCaseWords = map (map toLower) . words
    isLegalChar c = if isAlphaNum c || c =='\'' then c else ' '
    reverseBySecond x y = if snd x < snd y then GT else LT
    increment Nothing = Just 1
    increment (Just x) = Just (x + 1)

3 Answers 3


The frequency map is a good way to solve this. A nice trick to generate the frequency map is by creating a list of singleton maps and merging them all together:

wordFrequencyMap = Map.unionsWith (+) $ map (\x -> singleton x 1) $ normalizedWords str

I think this way is a bit more declarative which is what we aim for in Haskell.

You could simplify:

    containsAtLeastOneAlphaNumericChar w = find isAlphaNum w /= Nothing


    containsAtLeastOneAlphaNumericChar = any isAlphaNum

At that point you might find it clearer to inline the definition:

    normalizedWords = filter (any isAlphaNum) . lowerCaseWords . map isLegalChar

To sort in descending order, use Down from Data.Ord, this will reverse the ordering of whatever is passed into it, e.g. Down 1 > Down 2 is True.

    sortedWordFrequencies = sortOn (Down . snd) (Map.toList wordFrequencyMap)

I'd write

increment = (<|> (Just 1)) . fmap (+1)

You can probably also write

reverseBySecond = flip $ comparing snd

if you don't mind getting EQ for equal pairs.

  • \$\begingroup\$ I like the use of <|>, I've never used it. I'll add it to my toolkit. \$\endgroup\$
    – cgoates
    Commented Sep 11, 2021 at 0:41

i had already answered this from within my http://lynx.browser.org/ unbloated hypertext browser within superfast http://www.washington.edu/alpine/ mail commander but http://stackexchange.com/ does not behave like written from humans for humans and rather unfortunately insists on a robot captcha unsolvable on a braille typewriter.

i will recreate my first intuitive answer which goes more lambda calculus and less hoare calculus than your code to try

top3=map head.take 3.reverse.sortBy(comparing length).group.sort.words

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