Haskel Function Pattern Match Syntax Examples

examples.hs

import Data.List -- sortBy

myLength :: [a] -> Int
myLength []     = 0
myLength (x:xs) = 1 + (myLength xs)

meanOfList [] = 0
meanOfList xs = (fromIntegral (listSum xs)) / (fromIntegral (myLength xs))
    where listSum []     = 0
          listSum (e:es) = e + (listSum es)

palindrome [] = []
palindrome xs = xs ++ (toReverse xs)
    where toReverse (e:es) = (toReverse es) ++ [e]
          toReverse []     = []

isPalindrome []        = True
isPalindrome [e]       = False
isPalindrome (e1:[e2]) = (e1 == e2)
isPalindrome (e1:e2)   = (getLast e2) == e1 && (isPalindrome (getPreList e2))
    where getLast [e]       = e
          getLast (x:xs)    = getLast xs
          getPreList [e]    = []
          getPreList (e:es) = e:(getPreList es)


lend3 amount balance
     | amount <= 0            = Nothing
     | amount > reserve * 0.5 = Nothing
     | otherwise              = Just newBalance
    where reserve    = 100
          newBalance = balance - amount

sortBySubListLength xs = sortBy ording xs
    where ording l1 l2
            | (myLength l1) > (myLength l2)  = GT
            | (myLength l1) == (myLength l2) = EQ
            | otherwise                      = LT

intersperse1 sep []     = ""
intersperse1 sep ([x])  = x
intersperse1 sep (x:xs) = x ++ sep ++ (intersperse1 sep xs)

data Tree a = Node a (Tree a) (Tree a)
            | Leaf
            deriving (Show)

treeHight Leaf         = 0
treeHight (Node _ l r) =  1 + (maxNum (treeHight l) (treeHight r))
    where maxNum n1 n2 
            | n1 > n2   = n1
            | otherwise = n2

data Direction = Lef | Righ | Straight deriving (Show)

calculateDir (x1, y1) (x2, y2) (x3, y3) =
    if angle1 > angle2
    then Righ
    else if angle1 == angle2
    then Straight
    else Lef
    where angle1 = (y2 - y1) / (x2 - x1)
          angle2 = (y3 - y2) / (x3 - x2)