Simple sentence parser

parser.hs

word    :: Parser String
word    = do{ c  <- letter
            ; do{ cs <- word
                ; return (c:cs)
                }
              <|> return [c]
            }

sentence    :: Parser [String]
sentence    = do{ words <- sepBy1 word separator
                ; oneOf ".?!"
                ; return words
                }

separator   :: Parser ()
separator   = skipMany1 (space <|> char ',')


-- P.parse sentence "unknown" "a b c."
-- => Right ["a","b","c"] 


-- P.parse sentence "unknown" "a b c ."
-- => unexpected "."  
-- => expecting space, "," or letter