c4.hs

lucky :: Integral a => a -> String
lucky 7 = "THAT'S SEVEN!"
lucky x = "Not a seven"

factorial :: Integral a => a -> a
factorial 0 = 1
factorial n = n * factorial (n - 1)

addVectors :: Integral a => (a, a) -> (a, a) -> (a, a)
addVectors (x1, y1) (x2, y2) = ((x1 + x2), (y1 + y2))

firstOfThree :: Integral a => (a, a, a) -> a
firstOfThree (a, _, _) = a

secondOfThree :: Integral a => (a, a, a) -> a
secondOfThree (_, a, _) = a

thirdOfThree :: Integral a => (a, a, a) -> a
thirdOfThree (_, _, a) = a

head' :: [a] -> a
head' [] = error "No way"
head' (x:_) = x

tell :: Show a => [a] -> String
tell [] = "Empty"
tell (a:[]) = "One element, " ++ show a
tell (a:b:[]) = "Two elements, " ++ show a ++ ", " ++ show b
tell all@(a:b:_) = "A lot of elements in " ++ show all ++ ": " ++ show a ++ ", " ++ show b

-- sum' :: Num a => [a] -> String
-- sum' xs = "Sum is " ++
--   let calculate xs = case xs of []     -> 0 -- як змусити це працювати?
--                           (x:xs) -> x + sum' xs;
--       result = calculate xs
--   in show result

bmiTell :: RealFloat a => a -> a -> String
bmiTell w h
  | bmi w h < skinny = "Skinny"
  | bmi w h < ok = "Ok"
  | bmi w h < fat = "Lil fat"
  | otherwise = "Fat"
  where bmi w' h' = w' / h' ^ 2
        (skinny, ok, fat) = (18.5, 20, 30)

max' :: Ord a => a -> a -> a
max' a b
  | b > a = b
  | otherwise = a

myC :: Ord a => a -> a -> Ordering
a `myC` b
  | a > b = GT
  | a == b = EQ
  | otherwise = LT

initials :: String -> String -> String -- чому не працює String a => a -> a -> a
initials fn ln = [f] ++ " " ++ [l]
  where (f:_) = fn
        (l:_) = ln