casting - F#:转换 seq <'A> to seq<' B> 的最快方法

Question

我将 Marten 用作事件存储，特别是用于获取事件流。

type AccountCreation = {
    Owner: string
    AccountId: Guid
    CreatedAt: DateTimeOffset
    StartingBalance: decimal
}

type AccountEvents =
    | AccountCreated of AccountCreation
    | AccountCredited of Transaction
    | AccountDebited of Transaction

let settings = {
    Host = "localhost"
    DatabaseName = "postgres"
    UserName = "root"
    Password = "root"
    EventTypes = eventTypes
}
use store = createDocumentStore settings
use session = store.LightweightSession()

let khalidId = Guid.NewGuid()
let billId = Guid.NewGuid()

let khalid = AccountEvents.AccountCreated({
    Owner = "Khalid Abuhakmeh"
    AccountId = khalidId
    StartingBalance = 1000m
    CreatedAt = DateTimeOffset.UtcNow
})

let bill = {
    Owner = "Bill Boga"
    AccountId = billId
    StartingBalance = 0m
    CreatedAt = DateTimeOffset.UtcNow
}

session.Events.Append(khalidId, khalid) |> ignore
session.Events.Append(billId, bill) |> ignore

session.SaveChanges()

let stream = session.Events.FetchStream()

stream正在IReadOnlyList<IEvent>和 IEvent定义为:

public interface IEvent
{
    Guid Id { get; set; }
    int Version { get; set; }
    long Sequence { get; set; }
    object Data { get; }
    Guid StreamId { get; set; }
    string StreamKey { get; set; }
    DateTimeOffset Timestamp { get; set; }
    string TenantId { get; set; }
    void Apply<TAggregate>(TAggregate state, IAggregator<TAggregate> aggregator) where TAggregate : class, new();
}

我想转换每个 IEvent至 AccountEvents ，如果 Data 的基础类型属性是 AccountEvents (如果不是，则该项目不会在结果序列中产生)。

在 C# 中，我会简单地使用关键字 as来实现这一目标，但在 F# 中，我不确定什么是最快的 F#-ish 方式(就性能而言)。

我最终得到了以下代码:

let seqCastOption<'T> sequence =
    sequence
    |> Seq.map(fun x ->
        match box x with
        | :? 'T as value -> Some value
        | _ -> None)

let fetchStream<'T> (session: IDocumentSession) (id: Guid) =
    let stream = session.Events.FetchStream(id)
    stream
    |> Seq.map(fun x -> x.Data)
    |> seqCastOption<'T>
    |> Seq.filter (fun x -> x.IsSome)
    |> Seq.map(fun x -> x.Value)

但这似乎相当“昂贵”，我想知道转换 .Data 的步骤是否到 Option<AccountEvents> + 过滤那些IsSome可以一次完成。

Answer 1

Seq.choose rmunn 的回答中提到的函数对于了解这种情况非常有用，但对于这种确切情况，我建议使用内置的 .NET 方法 Enumerable.OfType<'T> ，这正是您想要的，并且可能已经过优化:

open System.Linq

let fetchStream<'T> (session: IDocumentSession) (id: Guid) =
    let stream = session.Events.FetchStream(id)
    stream
    |> Seq.map(fun x -> x.Data)
    |> Enumerable.OfType<'T>