通过模式匹配比较F#区分的Union实例 [英] Comparing F# discriminated union instances via pattern matching

问题描述

首先，对标题不佳表示歉意-我对F#的理解不足，无法更好地描述问题.

Firstly, apologies for the poor title - I don't understand enough F# to describe the problem better.

考虑以下简单的DU:

Consider this simple DU:

type Money =
    | USD of decimal
    | GBP of decimal
    | EUR of decimal
    static member (+) (first: Money, second: Money) =
        match first, second with 
        | USD(x), USD(y) -> USD(x + y)
        | GBP(x), GBP(y) -> GBP(x + y)
        | EUR(x), EUR(y) -> EUR(x + y)
        | _ -> failwith "Different currencies"

我用不同的货币表示货币，并重载(+)运算符，以便我可以安全地进行Money + Money.但是，如果我有很多货币，那么match语句将变得很乏味.有什么表达方式:

I'm representing money in different currencies, and overloading the (+) operator so that I can safely do Money + Money. However, if I have many currencies then the match statement will become tedious to write. Is there any way of expressing something like:

match first, second with 
| _(x), _(y) -> _(x + y)

还是有不同的方法来获得相同的结果?由于

Or is there a different way to achieve the same result? I've considered and discarded units of measure due to the limitations described here.

推荐答案

这对您有用吗?

type Kind = | USD | GBP | EUR

type Money = 
    | Money of Kind * decimal 
    static member (+) (first: Money, second: Money) = 
        match first, second with  
        | Money(k1,x), Money(k2,y) when k1=k2 -> Money(k1, x + y) 
        | _ -> failwith "Different currencies" 

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