在F#中构建具有Applicative功能的记录

Question

假设有一个type r = {A : int; B : string; C : int; D : string}和一些值:

let aOptional : int option = ...
let bOptional : string option = ...
let cOptional : int option = ...
let dOptional : string option = ...

如何r optional从他们的eleganlty(没有嵌套的案件等)构建？

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顺便说一下,这是如何在haskell中完成的Control.Applicative:

data R = R { a :: Integer, b :: String, c :: Integer, d :: String}

R <$> aOptional <*> bOptional <*> cOptional <*> dOptional :: Maybe R

寻找fsharp中的等价物.

Answer 1

我知道的唯一方法(使用applicatives)是通过创建一个函数来构造记录:

let r a b c d = {A = a; B = b; C = c; D = d}

然后你可以这样做:

> r </map/> aOptional <*> bOptional <*> cOptional <*> dOptional ;;

val it : R option

您可以定义map与<*>自己,但如果你想有一个通用的实现尝试用代码F#+或者如果你想使用FsControl直接,你可以写代码是这样的:

#r "FsControl.Core.dll"

open FsControl.Operators

let (</) = (|>)
let (/>) f x y = f y x

// Sample code
type R = {A : int; B : string; C : int; D : string}
let r a b c d = {A = a; B = b; C = c; D = d}

let aOptional = Some 0
let bOptional = Some ""
let cOptional = Some 1
let dOptional = Some "some string"

r </map/> aOptional <*> bOptional <*> cOptional <*> dOptional
// val it : R option = Some {A = 0; B = ""; C = 1; D = "some string";}

更新:Nuget包现已上市.

Answer 2

直截了当的方式是:

match aOptional, bOptional, cOptional, dOptional with
| Some a, Some b, Some c, Some d -> Some {A=a; B=b; C=c; D=d}
| _ -> None

或者,有一个monad:

let rOptional = 
  maybe {
    let! a = aOptional
    let! b = bOptional
    let! c = cOptional
    let! d = dOptional
    return {A=a; B=b; C=c; D=d}
  }