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Haskell Study Plan
Note : Parentheses ( ), Brackets [ ], and Braces { }
Basic code area
-- Single line comment
{- Multiple line comment -}
-- check type signature
:type
-- a -> String
show 1 -- "1"
show True
-- Int -> Char ,import Data.Char
intToDigit 1 -- '1'
-- if-else and list comprehension
[if x==0 then x else 1 | x <- [1,2,3,4,5,6]]
-- map
map :: (a -> b) -> [a] -> [b]
-- filter
filter :: (a -> Bool) -> [a] -> [a]
-- foldr,foldl
-- zip
zip :: [a] -> [b] -> [(a, b)]
zip [1,2,3] [4,5,6] -- result:[(1,4),(2,5),(3,6)]
-- lambda calculas
(\x y -> x+y)
-- ++ : combine string or list
[1,2,3]++[4,5,6] -- result: [1,2,3,4,5,6]
"sss"++"111" -- "sss111"
-- : combine list
1:[2,3] -- [1,2,3]
-- guard expression, (x:xs) the parentheses are necessary.
func :: [Int] -> Int
func (x:xs)
| x==1 = 0
| otherwise = x
-- isUpper, isLower, toUpper, toLower
toUpper 'a' -- 'A'
toLower 'A' -- 'a'
isLower 'a' -- True
isUpper 'a' -- False
-- char to ascii, (import Data.Char)
ord :: Char -> Int
ord 'a' -- result: 97
-- pattern matching
case sort [a,b,c] of [x,y,z] -> (x + y ) > z
-- sort ,(import Data.List)
sort [2,1,3] -- [1,2,3]
-- reverse ,(import Data.List)
reverse [1,2,3] -- [3,2,1]
-- 喵的，还是用中文吧。。。。。。。
-- read类似js的eval，只不过Haskell是强类型，此处需要指定返回类型
-- if..then..else,else部分不可以省略
[if x>0 then x else 0 | x <- (read "[1,2,3,4]"::[Int])]
read "5" :: Int
read "[1,2,3,4]"::[Int]
-- :type 可缩写为 :t,查看一个运算符的类型，需要加括号
:type ($)
-- guard，where
func w h
| x <= 10.0 = "!"
| x <= 20.0 = "!!"
| x <= 30.0 = "!!!"
| otherwise = "- -"
where x = w / h ^ 2 --局部变量
(s, n, f) = (18.0, 20.0, 30.0) -- where里面的模式匹配
--let..in let..in和guard..where类似，但let中是声明，in里是计算
func r h =
let a = 2 * pi
b = 1
in a + 2 * b
-- 右折叠
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr f acc [] = acc
foldr f acc (x:xs) = f x (foldr f acc xs)
-- foldr f acc [a, b, c, d] = f a (f b (f c (f d acc)))
-- foldr f acc [a, b, c, d] = a `f` (b `f` (c `f` (d `f` acc)))
-- 左折叠
foldl :: (a -> b -> a) -> a -> [b] -> a
foldl f acc [] = acc
foldl f acc (x:xs) = foldl f (f acc x) xs
-- foldl f acc [a, b, c, d] = f (f (f (f acc a) b) c) d
-- foldl f acc [a, b, c, d] = (((acc `f` a) `f` b) `f` c) `f` d
-- 删除第一个等于a元素
delete :: Eq a => a -> [a] -> [a]
removeSmallest :: [Int] -> [Int]
removeSmallest xs = delete (head $ sort xs) xs
-- 等同于 removeSmallest xs = delete (minimum xs) xs
-- minimum 获取list里最小的元素
minimum :: [a] -> a
minimum xs
-- 序列删除符号 \\，注意第二个参数是序列
:t (\\)
(\\) :: Eq a => [a] -> [a] -> [a]
[1,2,3,4,5] \\ [maximum [1,2,3,4,5]]
--printf "%.2f" 格式化字符串 import Text.Printf
--fromInteger n --Int 转Integer
import Text.Printf
series :: [Double]
series = map (1/) [1, 4 ..]
seriesSum :: Integer -> String
seriesSum n = printf "%.2f" $ sum $ take (fromInteger n) series