F# 联合类型Union Cases转换函数编译报错如何解决

错误根源 #

1. 类型不匹配：ApprovedWorkTime.ExistingWorkTime 联合类型分支要求接收 WorkTime 类型的入参，你在第一个匹配分支中传入了一个新构造的仅含Start、End字段的记录，由于你定义的三个记录类型字段结构完全一致，编译器将该记录推断为FixedWorkTime类型，和要求的WorkTime类型不匹配，触发报错。
2. 语法错误：F# 中构造记录的字段分隔符为分号;，你使用了逗号,，逗号在F#中用于构造元组，属于语法错误。

修复代码 #

最简化写法：

let FixWorkTime (workTime: WorkTime) : ApprovedWorkTime =
    match workTime with
    // 直接传入原workTime即可，本身就是合法的WorkTime类型
    | WorkTime.Workshop _ -> ApprovedWorkTime.ExistingWorkTime workTime
    | WorkTime.Mounting mounting -> 
        ApprovedWorkTime.FixedWorkTime { Start = mounting.Start; End = mounting.End }

如果需要显式构造对应类型，可以写为：

let FixWorkTime (workTime: WorkTime) : ApprovedWorkTime =
    match workTime with
    | WorkTime.Workshop workshop -> 
        ApprovedWorkTime.ExistingWorkTime (WorkTime.Workshop workshop)
    | WorkTime.Mounting mounting -> 
        ApprovedWorkTime.FixedWorkTime { Start = mounting.Start; End = mounting.End }

内容的提问来源于stack exchange，提问作者Alexander Zeitler