Haskell 基础语法

临时变量

可以用 let ... in ... 和 where ... 语法定义临时变量，where 可以后置定义，而 let in 则是前置

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
    indices = elemIndices 1 doors
    end = last indices
    start = head indices

模式匹配

Enumeration Matching

最基础的 Pattern Matching 就是，可以把变量的所有可能取值都枚举出来。用 _ 匹配所有其他情况

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

Condition Guards

在变量后面加 | <conditions> 可以用条件控制 pattern matching，即对变量进行条件约束。用 otherwise 表示所有其他条件

和 case of 语法的区别在于：condition guards 需要的是 condition 而不是具体的值．

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

当然也可以和 Enumeration Matching 进行结合，枚举一部分，另一部分用 Guard 控制

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` 语法

case ... of 也是按值匹配，而非按条件．

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case xxx of
  value1 -> return_value1
  value2 -> return_value2

不过配合 Guards 也可以做到按条件匹配

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case xxx of
  x | someguards -> return_value_1
  _ -> return_value_2