Ruby-like squish in Haskell

squish is an onomatopoeic string function provided by Ruby on Rails that works on strings.

Given a string, squish condenses all the consecutive spaces into one.¹ (Imagine that you take a string at both ends and compress it with both hands, spaces will be merged and you will hear the typical squish sound).

I went I little overboard and wrote three variations of this function:

• squishAll that will squish together all repeated chars: squishAll "aaabbbccc" -> "abc"
• squishOn that will squish together any specified char: squishOn 'a' "aaabbbccc" -> "abbbccc"
• squish for spaces only: squish "foo bar baz" -> "foo bar baz"

import Data.List
import Data.Foldable

squishAll :: Eq a => [a] -> [a]
squishAll = (map head) . group

squishOn :: Eq a => a -> [a] -> [a]
squishOn item = Data.List.concat . (map firstIfEqualElseAll) . group 
  where firstIfEqualElseAll (x:xs) = if x == item then [x] else (x:xs)

squish = squishOn ' '

main = print $ [squishAll "aaabbbccc", squishOn 'a' "aaabbbccc", squish "foo   bar   baz"]

¹ The Ruby function that this is modelled on would also remove any spaces at the start and at the end. I've chosen not to implement this.