Question

这是我的代码，用于从我的SQL DB请求一个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下收到一个错误：（当我删除FxTS时，我可以看到此函数确实返回了Series

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，只需将其转换为元组即可。参见下面的示例。

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

我想补充到@zyzhu的答案是

是多余的。基本上，您可以摆脱显式类型注释

|> FxTS

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只是FxTS的类型别名，编译器期望它具有与Series<DateTime,float>相同的构造函数，并且其中一个确实是Series

SQL查询和系列/类型问题

2 个答案: