haskell笔记(1)

myElem :: Int-> [Int] -> Bool

myElem x [] = False

myElem x (y : ys)

|x==y = True

|otherwise = myElem x ys

重载函数(overloaded)

myElem :: Eq a => a -> [a] -> Bool

myElem x [] = False

myElem x (y : ys)

|x==y = True

|otherwise = myElem x ys等价于

myElem x (y : ys) = (x==y) || myElem x ys

普通函数(单态)

add :: Int -> Int -> Int

add x y = x + y

多态函数(polymorphic)

length :: [a] -> Int –多态类型，多态函数

length [] = 0

length (x : xs) = length xs + 1

–一个图书馆的借书程序

type Person = String –name

type Book = String –name of the book

tyoe Database = [(Book,Person)]

借书

makeLoan :: DataBase -> Person -> Book -> DataBase

makeLoan db person book = (book,person) : db

–db ++ [(book,person)]

还书

returnLoan :: Database -> Person -> Book -> Database

returnLoad db person book = [pair | pair <- db, pair /= (book,person)]