[CSV Type provider] When path to csv file is wrong, type provider shows strange errors.

[Chicker]

Description

I'm using CSV type provider with Schema parameter.

If I accidentally will write the wrong path to my csv file, or the file itself is accidentally will be deleted, then instead of showing error that the file is not found, CSV type provider showing the strange error.

Repro steps

Script which was used.

#r @"./packages/FSharp.Data/lib/net40/FSharp.Data.dll"
open FSharp.Data

[<Literal>]
let incorrectPathToCSV = @"../export_commerce_backoffice_orders.csv"

[<Literal>]
let correctPathToCSV = @"export_commerce_backoffice_orders.csv"

type OrdersCSV = 
  CsvProvider<  incorrectPathToCSV
              , Separators = "$"
              , HasHeaders = false          
              , Schema = "OrderNumber (string),OrderCreated (string),OrderTotal (float),FioShort (string option),PhoneNumber (string option),Email (string option),Comment (string option),FioFull (string option),Region (string option),Town (string option),Address (string option),Postindex (string option),ProductId (int option),ProductTitle (string),ProductQuantity (float),ProductPrice (decimal), NotUsed (string option)"
              , PreferOptionals = true
              >
let csv = OrdersCSV.GetSample()

Expected behavior

Type provider must shows error like this "Input file <...> not found!".

Actual behavior

Type provider shows the strange error like this "The provided schema contains 17 columns, the inference found 1 columns - please check the number of columns and the separator".

In addition:
If the Schema parameter is not presented and path to CSV file also incorrect, then the type provider does not show any error. But the code like this

for r in csv.Rows do
  printfn "%A" r

prints some nonsense (my incorrect path to csv file).

"../export_commerce_backoffice_orders.csv"
val it : unit = ()

Known workarounds

Write right path)

[DmitryBatalov]

Main concern is that it is not possible to say what do you mean in Sample parameter. For example, you have CSV file where you have collected file paths. Please have a look at the example below.

#r @"../../packages/FSharp.Data/lib/net40/FSharp.Data.dll"
open FSharp.Data

/// This is inline sample of CSV content
[<Literal>]
let inlineSample = @"../file_to_be_processed.csv"

/// This is file path to file with CSV sample
[<Literal>]
let fileSample = @"file_with_sample.csv"

/// This is one more case, when you can get url where CSV sample located
[<Literal>]
let urlSample = "http://ichart.finance.yahoo.com/table.csv?s=MSFT"

type OrdersCSV = 
  CsvProvider<  Sample=inlineSample // or fileSample or urlSample
              , Separators = "$"
              , HasHeaders = false          
              , Schema = "FileToBeProcessed (string)"
              , PreferOptionals = true
             >
let csv = OrdersCSV.GetSample()

Here you can see that inline sample could be a file path, though it is not a file path.

My suggestion is to add one more parameter that will instruct somehow that it is a file path to sample. For example, it could be boolean parameter InlineSample.

[ledbutter]

PR opened by @DmitryBatalov for this issue here

[github-actions]

🤖 This is an automated response from Repo Assist.

This is a known ambiguity in how FSharp.Data's CsvProvider resolves the Sample parameter.

Root cause: The provider attempts to determine whether the Sample value is a file path, a URL, or inline CSV content. It tries file resolution first; if the file doesn't exist at the expected path, it falls back to treating the value as inline content. This means:

• A path like "../export_commerce_backoffice_orders.csv" that doesn't exist becomes a one-row, one-column CSV containing the path string as data.
• When Schema is also specified (17 columns), the mismatch between the inferred 1-column layout and the 17-column schema gives the confusing error: "The provided schema contains 17 columns, the inference found 1 columns".

Why this design? CsvProvider supports both file paths and inline samples (e.g. CsvProvider<"Name,Age\nAlice,30">). Distinguishing these cases is non-trivial, especially with relative paths which depend on ResolutionFolder.

Workaround: Make sure the [(Literal)] path is correct relative to your ResolutionFolder. You can also explicitly set ResolutionFolder = __SOURCE_DIRECTORY__ to ensure the path is resolved from the script's directory.

Improvement opportunity: Ideally, when the fallback-to-inline happens, the error message should note "the path was not found; treating as inline content." This would be a welcome good first issue contribution — the relevant code is in src/FSharp.Data.DesignTime/ in the infrastructure shared across type providers.

Generated by Repo Assist

To install this workflow, run gh aw add githubnext/agentics/workflows/repo-assist.md@b87234850bf9664d198f28a02df0f937d0447295. View source at https://github.com/githubnext/agentics/tree/b87234850bf9664d198f28a02df0f937d0447295/workflows/repo-assist.md.