将具有区别联合类型的列表过滤到单个类型

Question

给定一种类型

type C = Circle of int | Rectangle of int * int

和一个集合

let l = [ Circle(1); Circle(2); Rectangle(1,2)]

我只想处理圈子

 let circles = l |> List.filter(fun x-> match x with 
                                        | Circle(l) -> true
                                        | _ -> false)

但我的圈子仍然是C型,所以我做不到

for x in circles do
  printf "circle %d" x.??

我要做

for x in circles do
  match x with 
  | Circle(l) -> printf "circle %d" l
  | _ -> ())

好像错了..

Answer 1

使用List.choose-就像List.filter与List.map集于一身.

let circles =
    l |> List.choose(fun x ->
        match x with 
        | Circle l -> Some l
        | _ -> None)

for x in circles do
  printf "circle %d" x