模式对现有的变量匹配

Question

我有嵌套地图的结构：

[<RequireQualifiedAccess>] 
type NestedMap = 
    | Object of Map<string,NestedMap> 
    | Value of int  

我需要修剪的结构。 代码的目的是维护完整的映射和映射的嵌套结构，找到键值对，修剪没有找到键值对的分支。

下面是测试NestedMap：

let l2' = NestedMap.Object (List.zip ["C"; "S"; "D"] [NestedMap.Value(10); NestedMap.Value(20); NestedMap.Value(30)] |> Map.ofList) 
let l3 = NestedMap.Object (List.zip ["E"; "S"; "F"] [NestedMap.Value(100); NestedMap.Value(200); NestedMap.Value(300)] |> Map.ofList)  
let l2'' = NestedMap.Object (List.zip ["G"; "H"; "I"; "S"] [NestedMap.Value(30); l3; NestedMap.Value(40); NestedMap.Value(50)] |> Map.ofList)  
let l1 = NestedMap.Object (List.zip ["Y"; "A"; "B"] [NestedMap.Value(1); l2'; l2''] |> Map.ofList) 

这是我的代码：

let rec pruneWithKeyValue (keyvalue: string * int) (json: NestedMap) = 
    let condition ck cv = 
     let tgtKey = (fst keyvalue) 
     let tgtVal = (snd keyvalue) 
     match (ck, cv) with 
     | (tgtKey, NestedMap.Value(tgtVal)) -> 
      printfn ">>> Found match : " 
      printfn "  ck = %s " ck 
      printfn "  tgtKey and tgtVal == %s, %i" tgtKey tgtVal 
      true 
     | _ -> false 
    match json with 
    | NestedMap.Object nmap -> 
     if (nmap |> Map.exists (fun k v -> condition k v)) then 
      json 
     else 
      printfn "Expanding w keyvalue: (%s,%i): " (fst keyvalue) (snd keyvalue) 
      let expanded = nmap |> Map.map (fun k v -> pruneWithKeyValue keyvalue v) 
      NestedMap.Object(expanded |> Map.filter (fun k v -> v <> NestedMap.Object (Map.empty))) 
    | _ -> NestedMap.Object (Map.empty) 

let pruned = pruneWithKeyValue ("S",20) l1 
let res = (pruned = l1) 

结果是不是有什么期望：

>>> Found match : 
     ck = Y  
     tgtKey and tgtVal == Y, 1 

val pruneWithKeyValue : string * int -> json:NestedMap -> NestedMap 
val pruned : NestedMap = 
    Object 
    (map 
     [("A", Object (map [("C", Value 10); ("D", Value 30); ("S", Value 20)])); 
     ("B", 
     Object 
      (map 
       [("G", Value 30); 
       ("H", 
       Object 
        (map [("E", Value 100); ("F", Value 300); ("S", Value 200)])); 
       ("I", Value 40); ("S", Value 50)])); ("Y", Value 1)]) 
val remainsTheSame : bool = true 

的代码表示，输出数据结构保持不变（val remainsTheSame : bool = true）。更有趣的是，不知何故keyvalue元组包含键值对的功能是搜索得到了改进：

>>> Found match : 
     ck = Y  
     tgtKey and tgtVal == Y, 1 

这就是问题所在。事实上，如果我硬编码keyvalue元组：

let rec pruneWithKeyValue (keyvalue: string * int) (json: NestedMap) = 
    let condition ck cv = 
     let tgtKey = (fst keyvalue) 
     let tgtVal = (snd keyvalue) 
     match (ck, cv) with 
     | ("S", NestedMap.Value(20)) -> 
      printfn ">>> Found match : " 
      printfn "  ck = %s " ck 
      printfn "  tgtKey and tgtVal == %s, %i" tgtKey tgtVal 
      true 
     | _ -> false 
    match json with 
    | NestedMap.Object nmap -> 
     if (nmap |> Map.exists (fun k v -> condition k v)) then 
      json 
     else 
      printfn "Expanding w keyvalue: (%s,%i): " (fst keyvalue) (snd keyvalue) 
      let expanded = nmap |> Map.map (fun k v -> pruneWithKeyValue keyvalue v) 
      NestedMap.Object(expanded |> Map.filter (fun k v -> v <> NestedMap.Object (Map.empty))) 
    | _ -> NestedMap.Object (Map.empty) 

let pruned = pruneWithKeyValue ("S",20) l1 
let remainsTheSame = (pruned = l1) 

结果（耶）想要的结果：

Expanding w keyvalue: (S,20): 
>>> Found match : 
     ck = S  
     tgtKey and tgtVal == S, 20 
Expanding w keyvalue: (S,20): 
Expanding w keyvalue: (S,20): 

val pruneWithKeyValue : string * int -> json:NestedMap -> NestedMap 
val pruned : NestedMap = 
    Object 
    (map 
     [("A", Object (map [("C", Value 10); ("D", Value 30); ("S", Value 20)]))]) 
val remainsTheSame : bool = false 

这可能是微不足道的，但我不明白的地方，以及如何keyvalue最终被修改后，阻止我通过参数化键值元组获得正确的输出。

Answer 1

您无法对现有变量进行模式匹配，在您的原始代码中tgtKey和tgtVal将是新的绑定，与现有的绑定无关，这些绑定将会被遮蔽。

因此改变你的比赛：

match (ck, cv) with 
    | (tgtKey, NestedMap.Value(tgtVal)) -> 

到：

match (ck, cv) with 
    | (k, NestedMap.Value v) when (k, v) = (tgtKey, tgtVal) -> 

或者只是：

match (ck, cv) with 
    | x when x = (tgtKey, NestedMap.Value(tgtVal)) ->