dependency-injection - 将函数注入(inject)计算表达式

Question

以下代码示例来自 Scott Wlaschin 的网站 F# for fun and profit.

type LoggingBuilder() =
    let log p = printfn "expression is %A" p

    member this.Bind(x, f) = 
        log x
        f x

    member this.Return(x) = 
        x

let logger = new LoggingBuilder()

let loggedWorkflow = 
    logger
        {
        let! x = 42
        let! y = 43
        let! z = x + y
        return z
        }

有没有办法将函数而不是 printfn 注入(inject)到 LoggingBuilder() 中？

Answer 1

您可以只向构建器类型添加一个参数:

type LoggingBuilder(lf: obj -> unit) =
    let log p = lf p

    member this.Bind(x, f) = 
        log x
        f x

    member this.Return(x) = 
        x

let logger = new LoggingBuilder(printfn "expression is %A")

如果您想使输入类型比 obj 更具体，则可以使构建器通用

type LoggingBuilder<'a>(lf: 'a -> unit) =
    ...

let logger = new LoggingBuilder<int>(printfn "Got %i")