str.hs

upperLetter, downLetter :: Char -> Char

upperLetter letter = 
  let ascii = fromEnum letter in
    if ascii >= 97 && ascii <= 122
      then toEnum (ascii - 32)
      else letter


downLetter letter =
  let ascii = fromEnum letter in
    if ascii >= 65 && ascii <= 90
      then toEnum (ascii + 32)
      else letter


strToUpper, strToDown :: String -> String

strToUpper [] = []
strToUpper (h:t) = upperLetter h:strToUpper t
-- uppercase str = [ upperLetter l | l <- str ]

strToDown [] = []
strToDown (h:t) = downLetter h:strToDown t 
-- downcase str = [ downLetter l | l <- str ]