pattern matching

如果模式匹配多個值，必須用括號括起來(if you want to bind to several variables (even if one of them is just _ and doesn't actually bind at all), we have to surround them in parentheses. )

head' :: [a] -> a
head' [] = error "can head an empty list, you Dummy"
-- x:xs need to be put in parentheses
head' (x:xs) = x

模式匹配中，List直接用[x, y], [x, _]來進行匹配，不需要括號, kind of syntax sugar

tt :: (Show a) => [a] => String
tt [] = "empty list"
-- same as: tt [x] = ...
tt (x:[]) = "one element" ++ (show x)
-- same as tt [x, y] = ..
tt (x:y:[]) = "two elements" ++ (show x) ++ ", " ++ (show y)
-- can't use tt [x, y, _], may be more than 3
tt (x:y:_) = "3 or more than 3 elements"

patterns可以在匹配的同時保留整體

>>let tall all@(x:xs) = (show all) ++ ", first = " ++ show x
>> tall "554"
"\"554\""

guards

guards可以用來處理多個條件，類似于c的switch

max' :: (Ord a) => a -> a -> a  
max' a b   
    | a > b     = addOne a
    -- otherwise 處理其他
    | otherwise = b 
    where addOne x = (x+1)
              decOne x = (x-1)
-- 也可以寫成一行
-- max' a b | a > b = a | otherwise = b  

可以在guards后面加where定義方法，不過where需要對齊(Notice that all the names are aligned at a single column. If we don't align them nice and proper, Haskell gets confused because then it doesn't know they're all part of the same block.)
where跟正常方法一樣，可以用pattern matching, 也可以嵌套定義where

let表達式

let <bindings> in <expr>, 多行bindings需要對齊，單行可以;隔開

>>(let a = 100; b = 200; c = 300 in a*b*c, let foo="Hey "; bar = "there!" in foo ++ bar) 
(6000000,"Hey there!")  

case表達式

head' :: [a] -> a  
head' [] = error "No head for empty lists!"  
head' (x:_) = x  

-- 跟上面效果一致，方法pattern match只是syntax sugar
head' xs = case xs of [] -> "No head for empty lists!"
                      (x:_) -> x
-- where 里邊也可以用第一種形式
describeList :: [a] -> String
describeList xs = "The list is " ++ what xs
    where what [] = "empty."
          what [x] = "a singleton list."
          what xs = "a longer list."

常見錯誤

-- real world haskell
-- file: ch03/BogusPattern.hs

data Fruit = Apple | Orange
apple = "apple"
orange = "orange" 

-- apple/orange只是模式匹配的變量，不是global var=> apple/orange
whichFruit :: String -> Fruit
whichFruit f = case f of
    apple -> Apple
    orange -> Orange

-- file: ch03/BogusPattern.hs
equational apple = Apple
equational orange = Orange

-- file: ch03/BogusPattern.hs
betterFruit f = case f of
"apple" -> Apple
"orange" -> Orange

pattern match不能多次用同一個變量表示匹配變量相同(A name can appear only once in a set of pattern bindings)

bad_nodesAreSame (Node a _ _) (Node a _ _) = Just a