types - f# Narrow discriminated union in previous pattern matching guard(Control Flow Based Type Analysis)

Question

如果类型在之前的守卫中被缩小，F# 会缩小类型吗？

type FirstOption = bool option
type SecondOption = bool option

let foo first second =
     match first with
     | None -> false
     | Some value when second.IsSome ->
          // second is still Option<bool>
          sencond // x this doesn't fly.
     | Some value -> false

在这些情况下，我一直在使用 Optopn.map/map2/map3，但想知道我是否做错了什么。

对于 TypeScript，他们有基于控制流的分析:https://github.com/microsoft/TypeScript/issues/2388

Answer 1

您可以对两个选项进行模式匹配(很好地匹配两个选项的元组)

转换你的例子:

let foo first second = 
  match (first, second) with
  | (None, _) -> false
  | (Some _, Some s) -> s
  | (Some x, None) -> false

可以简化为:

let foo first second =
   match (first, second) with
   | (Some _, Some s) -> s
   | _ -> false