F# 列表模式匹配限制还是只是写得不好？

Question

给定一些CreditScoreInput:

type CreditScoreInput = { id: string; score: string; years: int }

let input = [
    { id = "CUSTOMER001"; score = "Medium"; years = 1 }
    { id = "CUSTOMER001"; score = "Medium"; years = 1 }
    { id = "CUSTOMER002"; score = "Medium"; years = 10 }
    { id = "CUSTOMER003"; score = "Bad"; years = 0 }
    { id = "CUSTOMER003"; score = "Bad"; years = 0 }
    { id = "CUSTOMER003"; score = "Bad"; years = 0 }
    { id = "CUSTOMER004"; score = "Good"; years = 0 }
    { id = "CUSTOMER005"; score = "Good"; years = 10 }
]

我的函数validateDuplicates正在寻找重复项:

let validate list =
    match list with
    | [] -> failwith "No customers supplied!"
    | _ -> list

let validateDuplicates (group:string * list<CreditScoreInput>) =
    match group with
    | (id, g) when g.Length = 1 -> printf $"No duplicates for {id} is OK.\n"
    | (id, [input1; input2]) -> printf $"Two duplicates for {id} is OK.\n"
    | (id, g) when g.Length > 2 -> printf $"More than two duplicates for {id} is NOT OK.\n"

    true

input
|> validate
|> List.groupBy (fun i -> i.id)
|> List.forall (fun i -> i |> validateDuplicates)
|> ignore

在 validateDuplicates 内部，我注意到 group 下有一点弯曲，导致出现警告:

Incomplete pattern matches on this expression. For example, the value (_,[_;_;_]) may indicate a case not covered by the pattern(s). However, a pattern rule with a when clause might successfully match this value.

有没有办法可以让编译器很好地发挥作用以避免出现此警告？

更新

我不确定是否应该在这里这样做，但以下是我根据出色的指导进行的更改:

let validateDuplicates (group:string * list<CreditScoreInput>) =
    match group with
    | (id, [_]) -> printf $"No duplicates for {id} is OK.\n"
    | (id, [_; _]) -> printf $"Two duplicates for {id} is OK.\n"
    | (id, g) -> printf $"More than two duplicates for {id} is NOT OK.\n"

input
|> validate
|> List.groupBy (fun i -> i.id)
|> List.iter (fun i -> i |> validateDuplicates)

Answer 1

只需删除最后一个 when 子句，因为您知道它永远是 true:

    match group with
    | (id, g) when g.Length = 1 -> printf $"No duplicates for {id} is OK.\n"
    | (id, [input1; input2]) -> printf $"Two duplicates for {id} is OK.\n"
    | (id, g) -> printf $"More than two duplicates for {id} is NOT OK.\n"

证明:

• g.Length 永远不能为负数或 0
• 如果g.Length为1，那么它将匹配第一种情况
• 如果g.Length为2，那么它将匹配第二种情况
• 因此，如果控制达到第三种情况，g.Length 将始终 > 2。

---

以下是我建议您编写此代码的方法:

let validateDuplicates (id, g : List<_>) =
    match g.Length with
    | 0 -> failwith "Unexpected"
    | 1 -> printf $"No duplicates for {id} is OK.\n"
    | 2 -> printf $"Two duplicates for {id} is OK.\n"
    | _ -> printf $"More than two duplicates for {id} is NOT OK.\n"

input
|> validate
|> List.groupBy (fun i -> i.id)
|> List.iter validateDuplicates

我所做的更改是:

• 使用 List.iter 而不是通过管道将 List.forall 传递到 ignore 中。
• 在调用 validateDuplicates 时消除不需要的 lambda。
• 使用模式匹配解构 validateDuplicates 的输入
• 直接匹配 g.Length，而不是使用 when 子句。
• 防御性编程:显式检查空列表以明确您的意图。

您可能还想考虑通过 F# 的 Result 使您的验证函数变得纯粹(即无副作用)。类型。