The “weightiest” word in a line

-- The utility reads a set of text lines from stdin, selects the
-- "weightiest" word in each of them and sends it to stdout. The
-- weight of a word is a sum of weights of its letters; the weight of
-- each letter roughly corresponds to its average occurence rate. If
-- applied to this text, the utility outputs the following:

-- utility
-- weightiest
-- weights
-- corresponds
-- following

import Data.Char (ord)

weightsList :: [Int]
weightsList =
  [ 3  -- a
  , 20 -- b
  , 12 -- c
  , 9  -- d
  , 1  -- e
  , 16 -- f
  , 18 -- g
  , 8  -- h
  , 5  -- i
  , 24 -- j
  , 22 -- k
  , 11 -- l
  , 14 -- m
  , 6  -- n
  , 4  -- o
  , 19 -- p
  , 25 -- q
  , 10 -- r
  , 7  -- s
  , 2  -- t
  , 13 -- u
  , 21 -- v
  , 15 -- w
  , 23 -- x
  , 17 -- y
  , 26 -- z
  ]

validCharList :: String
validCharList = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-'… \n"

charToWeight :: Char -> Int
charToWeight char
  | charCode > 96 && charCode < 123 = pickCharWeight forLowerCase
  | charCode > 64 && charCode < 91 = pickCharWeight forUpperCase
  | otherwise = 0
  where
    charCode = ord char
    pickCharWeight offset = weightsList !! (charCode - offset)
    forLowerCase = 97
    forUpperCase = 65

wordToWeight :: String -> Int
wordToWeight "" = 0
wordToWeight word = sum $ map charToWeight word

weightierWord :: String -> String -> String
weightierWord "" string = string
weightierWord string "" = string
weightierWord string1 string2
  | wordToWeight string1 > wordToWeight string2 = string1
  | otherwise = string2

weightiestWordInList :: [String] -> String
weightiestWordInList [] = ""
weightiestWordInList list = foldl1 weightierWord list

uniqueAdjacentSpaces :: String -> String
uniqueAdjacentSpaces [] = []
uniqueAdjacentSpaces [char] = [char]
uniqueAdjacentSpaces (char1:char2:chars)
  | char1 == ' ' && char2 == ' ' = uniqueAdjacentSpaces (char2 : chars)
  | otherwise = char1 : uniqueAdjacentSpaces (char2 : chars)

filterChars :: String -> String
filterChars = filter filterFunction
  where
    filterFunction = flip elem validCharList

main :: IO ()
main = do
  input <- getContents
  mapM_
    ((putStrLn . weightiestWordInList) . words)
    (lines $ (uniqueAdjacentSpaces . filterChars) input)