Haskell 快速入门

Variables

Variables: Local Definition

使用 let ... in 语法，或者 where 语法。where 后置变量定义，let ... in 则是前置。

示例：

solve :: Int -> Int -> [Int] -> String
solve n m doors = do
  case indices of
    [] -> "YES"
    _ -> case end - start + 1 of
      x | x <= m -> "YES"
      _ -> "NO"
  where -- 这里就是 local definition 定义的局部变量
    indices = elemIndices 1 doors
    end = last indices
    start = head indices

Pattern Matching

定义函数的时候，可以根据参数的不同，触发不同分支。

greet :: String -> String -> String
greet "Finland" name = "Hei, " ++ name
greet "Italy"   name = "Ciao, " ++ name
greet "England" name = "How do you do, " ++ name
greet _         name = "Hello, " ++ name

Guards

Condition Guards

和 Pattern Matching 类似，用 Condition 代替具体的值进行 Matching.

示例：

describe :: Int -> String
describe n
  | n == 2      = "Two"
  | even n      = "Even"
  | n == 3      = "Three"
  | n > 100     = "Big!!"
  | otherwise   = "The number " ++ show n

当然，Guards 和 Pattern Matching 也可以一起用。

guessAge :: String -> Int -> String
guessAge "Griselda" age
    | age < 47 = "Too low!"
    | age > 47 = "Too high!"
    | otherwise = "Correct!"
guessAge "Hansel" age
    | age < 12 = "Too low!"
    | age > 12 = "Too high!"
    | otherwise = "Correct!"
guessAge name age = "Wrong name!"

`case ... of`

我觉得 case ... of 语法也算 Pattern Matching 的一部分

Recursion

Helper Function: arguments of the helper function are variables you update in your loop; Tail Recursion Optimization

Haskell programs often use the apostrophe to name helper functions and alternative versions of functions.

Haskell 常用数据结构

`List / [a]`

head :: [a] -> a            -- returns the first element
last :: [a] -> a            -- returns the last element
tail :: [a] -> [a]          -- returns everything except the first element
init :: [a] -> [a]          -- returns everything except the last element
take :: Int -> [a] -> [a]   -- returns the n first elements
drop :: Int -> [a] -> [a]   -- returns everything except the n first elements
(++) :: [a] -> [a] -> [a]   -- lists are catenated with the ++ operator
(!!) :: [a] -> Int -> a     -- lists are indexed with the !! operator
reverse :: [a] -> [a]       -- reverse a list
null :: [a] -> Bool         -- is this list empty?
length :: [a] -> Int        -- the length of a list

实用函数

show :: Any -> String: 将任何东西变成可以输出的字符串