在F#中，如何将元数据附加到已区分的联合值上? [英] In F#, How can I attach metadata to discriminated union values?

问题描述

我想创建一个类似于枚举的东西，它的值是F#记录类型，而不是int.例如，如果我有工会:

I want to create something that's kind of like an enum with an F# record type for a value instead of an int. For example, if I've got the union:

type BologneseIngredients = | Spaghetti
                            | Tomatoes
                            | MincedBeef
                            | GrandmasSecretIngredient

我知道意大利面条总是30厘米长，西红柿总是红色.我所能做的就是拥有获取元数据"功能:

I know that spaghetti is always 30cm long and tomatoes are always red. What I could do is have a 'get metadata' function:

let getMetadata = function
                    | Spaghetti -> { length: 30.0<cm> }
                    | Tomatoes -> { colour: Color.Red }
                    | _ -> { }

但是我真的很想将联合的定义和数据保持在一起.有什么好方法吗?

but I'd really like to keep the definition of the union and the data together. Is there a nice way to do this?

推荐答案

我的建议:

module Recipes =

    type BologneseIngredients = | Spaghetti
                                | Tomatoes
                                | MincedBeef
                                | GrandmasSecretIngredient

    let length (ind : BologneseIngredients) : float<cm> option =
         match ind with
         | Sphaghetti -> Some 30.0<cm>
         | _ -> None

    // .. or a bit more "metadata"ish
    type Metadata = 
        | Length of float<cm>
        | Color of System.Drawing.Color

    let metadata = 
       function
       | Sphaghetti -> [ Length 30.0<cm ]
       | Tomatoes   -> [ Color System.Drawing.Color.Red ]
       | ...

    let metaLength meta =
       meta |> List.tryPick (function | Length l -> Some l | _ -> None)

    let getLength = metadata >> metaLength

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