# haskell_test.hs

length' :: [a] -> a

length' [] = 0

length' (_:x) = 1 + length' x

xs@x:y:ys

xs代表整个list

x代表一部分

方便引用原值，而非分割的

capital :: String -> String

capital "" = "empty"

capital xs@(x:y) = "the first of  " ++ xs ++ " is " ++  x;

# bmi eg

bmiTell weight height
| weight / height ^ 2 <= 18.5 = "you are thin."
| weight / height ^ 2 <= 25 = "you are nomal."
| weight / height ^ 2 <= 30 = "you are fat."
| otherwise = "you are fat to die."

有重复。可改为

bmiTell weight height
| bmi <= thin = "you are thin."
| bmi <= nomal = "you are nomal."
| bmi <= fat = "you are fat."
| otherwise = "you are fat to die."

where bmi = weight / height ^ 2

　　thin = 18.5

　　nomal = 25

　　fat = 30

let (a,b) = (,) in a^ + b

head' :: [a] -> a

head' [] = error "error"

head' (x:_) = x

等价于

head' :: [a] -> a

head' xs = case xs of [] -> error "error"

                               (x:_) -> x