在 F# 中使用 CsvProvider

Question

我是 F# 的初学者，我正在尝试使用 CsvProvider 并重现此处给出的示例

http://fsharp.github.io/FSharp.Data/library/CsvProvider.html

所以在 F# 交互中，我输入

>type Stocks = CsvProvider<"MSFT.csv">;;

type Stocks = CsvProvider<...>

> let msft = CsvProvider<"MSFT.csv">.GetSample();;

val msft : CsvProvider<...>

> msft;;
val it : CsvProvider<...> =
  FSharp.Data.Runtime.CsvFile`1[System.Tuple`1[System.String]]

    {Headers = Some [|"MSFT.csv"|];
 NumberOfColumns = 1;
 Quote = '"';
 Rows = seq [];
 Separators = ",";}

> let firstRow = msft.Rows |> Seq.head;;
System.ArgumentException: The input sequence was empty.
Parameter name: source
>    at Microsoft.FSharp.Collections.SeqModule.Head[T](IEnumerable`1 source)
   at <StartupCode$FSI_0044>.$FSI_0044.main@()
Stopped due to error

我的理解是 CsvProvider 基于 CSV 文件创建一个类型，这样以后就可以读取该文件或具有相同格式的不同文件/流。 我认为我没有目录问题，因为如果文件位于错误的目录中，该函数将返回错误。 创建 msft 时，F# 显示 NumberOfColumns= 1 但这显然是错误的。

这也不行

> let msft = Stocks.Parse("MSFT.csv");;

val msft : CsvProvider<...>

> msft;;
val it : CsvProvider<...> =
  FSharp.Data.Runtime.CsvFile`1[System.Tuple`1[System.String]]
    {Headers = Some [|"MSFT.csv"|];
     NumberOfColumns = 1;
     Quote = '"';
     Rows = seq [];
     Separators = ",";}

我正在使用这样的 FSharp.Data 库：（它是好版本吗？）

>#r "C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\packages\FSharp.Data.2.2.5\lib\portable-net40+sl5+wp8+win8\FSharp.Data.dll"
--> Referenced 'C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\packages\FSharp.Data.2.2.5\lib\portable-net40+sl5+wp8+win8\FSharp.Data.dll'
> open FSharp.Data;;
> 

请帮忙！！！我已经尝试了几个小时了！谢谢！！！

EDIT：以下是 F# interactive 的完整日志

Microsoft (R) F# Interactive version 14.0.23020.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> #r "C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\packages\FSharp.Data.2.2.5\lib\portable-net40+sl5+wp8+win8\FSharp.Data.dll"
open FSharp.Data;;

--> Referenced 'C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\packages\FSharp.Data.2.2.5\lib\portable-net40+sl5+wp8+win8\FSharp.Data.dll'

> open FSharp.Data;;
> #I "C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I";;

--> Added 'C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I' to library include path

> let msft = CsvProvider<"MSFT.csv">.GetSample();;

val msft : CsvProvider<...>

> msft;;
val it : CsvProvider<...> =
  FSharp.Data.Runtime.CsvFile`1[System.Tuple`1[System.String]]
    {Headers = Some [|"MSFT.csv"|];
     NumberOfColumns = 1;
     Quote = '"';
     Rows = seq [];
     Separators = ",";}
> let msft = CsvProvider<"C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\MSFT.csv">.GetSample();;

  let msft = CsvProvider<"C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\MSFT.csv">.GetSample();;
  ---------------------------------------------------------------------------------------------------^^^^^^^^^

stdin(7,100): error FS0039: The field, constructor or member 'GetSample' is not defined
> let msft = CsvProvider<"MSFT.csv">.GetSample();;

val msft : CsvProvider<...>

> let msft = CsvProvider<"C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\MSFT.csv">.GetSample();;

  let msft = CsvProvider<"C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Algo Stanford I\MSFT.csv">.GetSample();;
  ---------------------------------------------------------------------------------------------------^^^^^^^^^

stdin(9,100): error FS0039: The field, constructor or member 'GetSample' is not defined
> 

Answer 1

我的猜测是 F# Interactive 很难在当前目录中找到 MSFT.csv 文件。您可以使用Literal 指定相对于当前目录的完整路径：

let [<Literal>] Sample = __SOURCE_DIRECTORY__ + "\\MSFT.csv"
type Stocks = CsvProvider<Sample>

然后就可以调用Stocks.GetSample()读取文件了。

这种错误有时会在 F# Interactive 中发生，尤其是当您在不同文件夹中的文件之间切换时（我认为 F# Interactive 会记住上次执行命令的文件夹，但我认为它的行为并不总是像预期的那样）。