结果

使用 Result<'T,'TFailure> 类型可以编写可组合的容错代码。

语法

// The definition of Result in FSharp.Core
[<StructuralEquality; StructuralComparison>]
[<CompiledName("FSharpResult`2")>]
[<Struct>]
type Result<'T,'TError> =
    | Ok of ResultValue:'T
    | Error of ErrorValue:'TError

备注

请参阅 Result 模块，了解 Result 的内置组合器。 的参数。

请注意，结果类型是结构可区分联合。 结构相等语义在此处适用。

Result 类型通常用于单一错误处理，这在 F# 社区中通常称为面向轨道的编程。 以下详细示例展示了这种方法。

// Define a simple type which has fields that can be validated
type Request =
    { Name: string
      Email: string }

// Define some logic for what defines a valid name.
//
// Generates a Result which is an Ok if the name validates;
// otherwise, it generates a Result which is an Error.
let validateName req =
    match req.Name with
    | null -> Error "No name found."
    | "" -> Error "Name is empty."
    | "bananas" -> Error "Bananas is not a name."
    | _ -> Ok req

// Similarly, define some email validation logic.
let validateEmail req =
    match req.Email with
    | null -> Error "No email found."
    | "" -> Error "Email is empty."
    | s when s.EndsWith("bananas.com") -> Error "No email from bananas.com is allowed."
    | _ -> Ok req

let validateRequest reqResult =
    reqResult
    |> Result.bind validateName
    |> Result.bind validateEmail

let test() =
    // Now, create a Request and pattern match on the result.
    let req1 = { Name = "Phillip"; Email = "phillip@contoso.biz" }
    let res1 = validateRequest (Ok req1)
    match res1 with
    | Ok req -> printfn $"My request was valid! Name: {req.Name} Email {req.Email}"  
    | Error e -> printfn $"Error: {e}"
    // Prints: "My request was valid!  Name: Phillip Email: phillip@consoto.biz"

    let req2 = { Name = "Phillip"; Email = "phillip@bananas.com" }
    let res2 = validateRequest (Ok req2)
    match res2 with
    | Ok req -> printfn $"My request was valid! Name: {req.Name} Email {req.Email}"  
    | Error e -> printfn $"Error: {e}"
    // Prints: "Error: No email from bananas.com is allowed."

test()

如你所见，如果强制所有验证函数都返回 Result，则很容易将各种验证函数链接在一起。 这样，你可以将类似这样的功能分解成小片段，这些小片段可根据需要进行组合。 这还可以增加在一轮验证结束时强制使用模式匹配的价值，从而强制提高程序的正确度。

另请参阅

• 可区分联合
• 模式匹配