module StringHelp where

--import Data.Char

import Data.Set (Set)
import qualified Data.Set as Set

isVowel :: Char -> Bool
isVowel x = case x of
    'a' -> True
    'e' -> True
    'i' -> True
    'o' -> True
    'u' -> True
    _ -> False

addIndefiniteArticle :: String -> String
addIndefiniteArticle [] = error "tried to add indefinite article to empty string"
addIndefiniteArticle (x : xs) = genarticle ++ (x : xs)
  where
    flipexception
        | w `Set.member` indefiniteExceptions = not
        | otherwise = id
    w = head (words (x : xs))
    genarticle
        | flipexception (isVowel x) = "an "
        | otherwise = "a "

indefiniteExceptions :: Set String
indefiniteExceptions =
    Set.fromList
        [ "one"
        , "usual"
        , "union"
        , "honor"
        , "honest"
        , "honorable"
        , "heir"
        , "hourglass"
        , "eulogy"
        , "united"
        , "used"
        ]