FutureLearn: Functional Programming in Haskell - Speller

Speller.hs

{-
2019-09-27
  
  Write a Spelling Book Generator

    . Are you familiar with children’s alphabetical spelling books?
      They say things like: a is for apple, b is for baby, and c is for cat

    . Now that you know about Haskell list functions, you can develop a function
      to generate the text for a spelling book, given a list of words.

    . Your task is to develop the Haskell speller function. You are allowed to
      define small helper functions, perhaps to generate a single letter’s phrase
      from a word: f "zoo" -- > "z is for zoo". You are also allowed to use the
      standard Haskell list functions like map and foldr if appropriate.  
-}

-- word
-- helper function to take a string & return a string
--   in the format like, "apple" -> "a is for apple"
word :: [Char] -> [Char]
word [] = []
word xs = (head xs) : " is for " ++ xs

-- appendCommaAndAnd
-- helper function that takes a word & a list of words & returns a word
--   if it's the last word in the list, prepend "and "
--   otherwise, append ", "
appendCommaAndAnd :: [Char] -> [[Char]] -> [Char]
appendCommaAndAnd w [] = "and " ++ w
appendCommaAndAnd w _  = w ++ ", "

-- acaa
-- assign shorter function name to appendCommaAndAnd
acaa = appendCommaAndAnd

-- speller
-- function that takes a list of words and returns a String
-- take a list of words and convert them to the "a is for apple" format
-- separate words by ", "
-- if > 1 words, include "and " before last word
speller :: [[Char]] -> [Char]
speller [] = []
speller [w] = word w
speller ws = concat $ foldr (\w acc -> (acaa (word w) acc) : acc) [] ws

-- explanation:
--  . foldr returns a list of Strings
--  . concat take a list of Strings and returns a concatenated String
--  . :load Speller.hs
--  . then run it via, speller ["almost", "beautiful", "code"]