pattern-matching.hs

module PatternMatching where
    -- Return the first character of a String, or the question mark character for an
    -- empty string.
    -- > firstCharacter "hello"
    -- 'h'
    -- > firstCharacter ""
    -- '?'
    -- Hint: String is the same as [Char].
    firstCharacter :: String -> Char
    firstCharacter (c:_) = c
    firstCharacter _ = '?'

    -- Move the first character to the end of a String and add "ay." It's Pig Latin!
    -- > pigLatin "hello"
    -- "ellohay"
    -- Watch out for empty strings!
    pigLatin :: String -> String
    pigLatin (c:cs) = cs ++ [c] ++ "ay"
    pigLatin "" = ""

    -- Return True if both items in the tuple are True.
    -- > bothTrue (True, True)
    -- True
    -- > bothTrue (True, False)
    -- False
    bothTrue :: (Bool, Bool) -> Bool
    bothTrue (a, b) = a && b

    -- Return True for a character containing an odd digit, and False for any other
    -- character.
    -- > isOddDigit '3'
    -- True
    -- > isOddDigit '2'
    -- False
    -- > isOddDigit 'q'
    -- False
    isOddDigit :: Char -> Bool
    isOddDigit '1' = True
    isOddDigit '3' = True
    isOddDigit '5' = True
    isOddDigit '7' = True
    isOddDigit '9' = True
    isOddDigit _ = False

    -- Return a new list where the first item is duplicated.
    -- > duplicated [1, 2, 3]
    -- [1, 1, 2, 3]
    -- Watch out for empty lists!
    -- Hint: try to avoid any repetition in your definition.
    duplicateFirst :: [a] -> [a]
    duplicateFirst l@(x:_) = x:l
    duplicateFirst l = l

    -- Implement if/then/else as a function!
    -- Return the first value if the boolean is true, and the second value if the
    -- boolean is false.
    -- > ifThenElse "was true" "was false" True
    -- "was true"
    -- > ifThenElse "was true" "was false" False
    -- "was false"
    ifThenElse :: a -> a -> Bool -> a
    ifThenElse v1 _ True = v1
    ifThenElse _ v2 False = v2