this function maps letters onto another letter. Could someone explain to me what does the !! 0 mean here in the function?

findGuess letter guessList 
  | length guessList == 0 = (letter, letter)
  | letter == snd (guessList !! 0) || letter == fst (guessList !! 0) = guessList !! 0
  | otherwise = findGuess letter (tail guessList)

CodePudding user response：

(!!) is the (partial) indexing operator for lists. guessList !! 0 is basically equivalent to head guessList. Neither is recommended, as they only work when guessList is not empty. Prefer pattern matching:

findGuess letter [] = (letter, letter)
findGuess letter ((c1, c2):rest) | letter == c1 || letter == c2 = (c1, c2)
                                 | otherwise = findGuess letter rest
                     

As an aside, findGuess mosty reimplements Data.List.find:

import Data.List

findGuess letter guessList = case find p guessList of
                               Nothing -> (letter, letter)
                               Just x -> x
     where p (c1, c2) = letter == c1 || letter == c2

CodePudding user response：

guessList !! 0 is equal to guessList[0] in Java/C#... So, this function check if letter matches the first or second of head pair tuple of the list, after that recursively continue to check all items of list.