Phone exercise from "Haskell Programming from first Principles", Chapter 11

Phone.hs

module Phone where

import Data.Char
import Data.Foldable (maximumBy)
import Data.List (group, map, sort)
import Data.Map (fromListWith, toList)
import qualified Data.Map.Strict as Map
import Data.Ord (comparing)

-- validButtons = "1234567890*#"
type Digit = Char

-- Valid presses: 1 and up
type Presses = Int

data DaPhone =
  Phone [(Digit, String)]
  deriving (Show)

phone :: DaPhone
phone =
  Phone
    [ ('1', "")
    , ('2', "abc")
    , ('3', "def")
    , ('4', "ghi")
    , ('5', "jkl")
    , ('6', "mno")
    , ('7', "pqrs")
    , ('8', "tuv")
    , ('9', "wxyz")
    , ('*', "^")
    , ('0', "+ _")
    , ('#', ".,")
    ]

type Code = (Digit, Presses)

type CodeBook = Map.Map Char (Digit, Int)

codeBook :: DaPhone -> CodeBook
codeBook (Phone xs) = Map.fromList $ concatMap codes xs
  where
    codes (d, cs) = zipWith (\i c -> (c, (d, succ i))) [0 .. (length cs)] cs

reverseTaps :: CodeBook -> Char -> [Code]
reverseTaps codeBook c
  | isUpper c = ('*', 1) : code
  | otherwise = code
  where
    code =
      case Map.lookup c codeBook of
        Just code' -> [code']
        Nothing -> []

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

fingerTaps :: [(Digit, Presses)] -> Presses
fingerTaps = foldr (\(_, n) acc -> acc + n) 0

mostPopular :: Ord a => [a] -> (Int, a)
mostPopular xs = maximum [(length x, head x) | x <- groups]
  where
    groups = (group . sort) xs

mostPopularLetter :: String -> (Int, Char)
mostPopularLetter = mostPopular . filter isLetter

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

coolestWord :: [String] -> String
coolestWord = snd . mostPopular . words . (map toLower) . unlines

convo :: [String]
convo =
  [ "Wanna play 20 questions"
  , "DA"
  , "U 1st haha"
  , "Lol ok. Have u ever tasted alcohol"
  , "Lol ya"
  , "Wow ur cool haha. Ur turn"
  , "Ok. Do u think I am pretty Lol"
  , "Lol ya"
  , "Just making sure rofl ur turn"
  ]

These guys need a better conversation if lol is the coolest thing coming out 😆