Phone Ex from Chapter 11 in Haskell book

gistfile1.txt

{-# LANGUAGE ParallelListComp #-}
module Phone where

import Data.Char
import Data.List
import Data.Foldable

type Presses = Int
data Digit = One | Two | Three | Four | Five | Six | Seven | Eight | Nine | Zero | STAR | HASH deriving (Eq, Show)

dict :: [(Digit, String)]
dict = [(One, "")
           , (Two, "abc")
           , (Three, "def")
           , (Four, "ghi")
           , (Five, "jkl")
           , (Six, "mno")
           , (Seven, "pqrs")
           , (Eight, "tuv")
           , (Nine, "wxyz")
           , (Zero, "+_")
           , (HASH, ".,")]

type DaPhone = [(Char, [(Digit, Presses)])]

phone :: DaPhone

phone = lowerCharPresses dict ++ (upperCharPresses . filter (\(x, _) -> x /= Zero && x /= HASH) $ dict ) where
  lowerCharPresses = concatMap (\(k, vs) -> [(v, [(k, i)]) | v <- vs | i <- [1..(length vs)]])
  upperCharPresses = concatMap (\(k, vs) -> [(v, [(STAR, 1), (k, i)]) | v <- map toUpper vs | i <- [1..(length vs)]])

--phone' =

reverseTaps :: DaPhone -> Char -> [(Digit, Presses)]
reverseTaps phone char = head . map snd . filter (\x -> fst x == char) $ phone

cellPhonesDead :: DaPhone -> String -> [(Digit, Presses)]
cellPhonesDead phone = concatMap (reverseTaps phone)

fingerTaps :: [(Digit, Presses)] -> Presses
fingerTaps = sum . map snd

mostPopularLetter :: String -> Char
mostPopularLetter s = fst . maximumBy (\(_, count) (_, count') -> compare count count')
  . nubBy (\(c, _) (c', _) -> c == c')
  . map (\c -> (c, length . filter (== c) $ s)) $ s

coolestLtr :: [String] -> Char
coolestLtr = mostPopularLetter . concat