pattern.hs

lucky :: (Integral a) => a -> String
lucky 7 = "LUCKY NUMBER SEVEN!"
lucky x = "Sorry, you're out of luck, pal!"

sayMe :: (Integral a) => a -> String
sayMe 1 = "One!"
sayMe 2 = "Two!"
sayMe 3 = "Three!"
sayMe 4 = "Four!"
sayMe 5 = "Five!"
sayMe x = "Not between 1 and 5"

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

-- order is important in sequence of patterns

addVectors :: (Num a) => (a, a) -> (a, a) -> (a, a)  
addVectors a b = (fst a + fst b, snd a + snd b)  

addVectorsPM :: (Num a) => (a, a) -> (a, a) -> (a, a)  
addVectorsPM (x1, y1) (x2, y2) = (x1 + x2, y1 + y2) 

first :: (a, b, c) -> a  
first (x, _, _) = x  
  
second :: (a, b, c) -> b  
second (_, y, _) = y  
  
third :: (a, b, c) -> c  
third (_, _, z) = z  

xs = [(1,3), (4,3), (2,4), (5,3), (5,6), (3,1)]  
suma = [a+b | (a,b) <- xs]  

-- pattern match agains list

head' :: [a] -> a  
head' [] = error "Can't call head on an empty list, dummy!"  
head' (x:_) = x  -- notice parenthesis


tell :: (Show a) => [a] -> String  
tell [] = "The list is empty"  
tell (x:[]) = "The list has one element: " ++ show x  
tell (x:y:[]) = "The list has two elements: " ++ show x ++ " and " ++ show y  
tell (x:y:_) = "This list is long. The first two elements are: " ++ show x ++ " and " ++ show y  

length' :: (Num b) => [a] -> b  
length' [] = 0  
length' (_:xs) = 1 + length' xs  

sum' :: (Num a) => [a] -> a  
sum' [] = 0  
sum' (x:xs) = x + sum' xs  

-- as patterns
-- whole list

capital :: String -> String  
capital "" = "Empty string, whoops!"  
capital all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x] 

-- can't use ++ in pattern matches