SQL 查询和系列/类型问题

Question

这是我的代码，用于从我的 SQL 数据库请求 DateTime*float （我使用：打开FSharp.Data.TypeProviders）

    type FxTS = Series<DateTime,float>

    let get (req: RFxTS) =
        query { for d in db.Data_FXBFIX do
                where (d.Fxpair = (req.FxPair.ToString())
                    && (d.DatetimeUTC.Date >= req.Period.startDate)
                    && (d.DatetimeUTC.Date <= req.Period.endDate))
                sortBy d.DatetimeUTC
                select (d.DatetimeUTC, d.Value) }
        |> Series.ofObservations
        |> FxTS

我在 FxTS 下遇到错误：（当我删除 FxTS 时，我可以看到这个函数 returns 确实是一个系列

Type constraint mismatch. The type 
    'Series<DateTime,float>'    
is not compatible with type
    'seq<Collections.Generic.KeyValuePair<DateTime,float>>'

谢谢

Answer 1

Series.ofObservation 适用于元组序列，不适用于 KeyValuePair。在运行 Series.ofObservations 之前将其转换为元组即可。请参阅下面的示例。

open System
open System.Collections.Generic
open Deedle
let pairs = 
  [
    KeyValuePair(DateTime(2018,1,31),1.)
    KeyValuePair(DateTime(2018,2,28),2.)
  ]

pairs
|> Seq.map(fun x -> x.Key, x.Value)
|> Series.ofObservations

Answer 2

我想对@zyzhu 的回答补充的是

|> FxTS

是多余的。基本上，您可以摆脱显式类型注释

let get (req: RFxTS) : FxTS =
        query { for d in db.Data_FXBFIX do
                where (d.Fxpair = (req.FxPair.ToString())
                    && (d.DatetimeUTC.Date >= req.Period.startDate)
                    && (d.DatetimeUTC.Date <= req.Period.endDate))
                sortBy d.DatetimeUTC
                select (d.DatetimeUTC, d.Value) }
        |> Series.ofObservations

只要 FxTS 只是 Series<DateTime,float> 的类型别名，编译器就希望它具有与 Series 相同的构造函数，其中之一确实是 seq<Collections.Generic.KeyValuePair<DateTime,float>>

SQL 查询和系列/类型问题

Issue with SQL queries and Series / Type

f#

deedle