Question

我在ghci中有这个代码，尝试解码base64代码：

let a=pack "MTExMTEx"
let b=decode a
:t b
b :: Either String ByteString

那么如何从任何一个获取解码字节串？是否有一些像Maybe的那样的函数来自于Just？我找不到它，谢谢。

Answer 1

使用case：

case decode a of
  Left err -> {- what to do if decode gave an error -}
  Right msg -> {- what to do if decode succeeded -}

Alexandre建议的either函数与此基本相同，两个分支仅作为函数;即它相当于写：

either 
  (\err -> {- what to do if decode gave an error -}) 
  (\msg -> {- what to do if decode succeeded -}) 
  (decode a)

Answer 2

您可以使用while (answer == "yes") { ... } if (customers >= 5) cout<<"\nWell that was a good day! I had " <<customers <<" customer<s> today. Tomorrow is another day ..." << endl; else cout<<"\nWell that was a poor day! I had " <<customers <<" customer<s> today. Tomorrow is another day ..." << endl;中的either功能。

它的签名是：

Data.Either

这意味着它需要两个函数作为输入：第一个应用，如果它是either :: (a -> c) -> (b -> c) -> Either a b -> c，第二个应用，如果它是Left。第三个参数是您的Right数据类型。请注意，两个函数的返回值的类型必须相同。

Answer 3

您正在寻找具有类型签名的ids= []; selChk(val) { this.ids.push(val); console.log(this.ids); } fromRight

Data.Either

第一个值是默认值（如果您有fromRight :: b -> Either a b -> b而不是Left，则会获得什么。

Right

如何从Either获得价值？

3 个答案: