CODEWITHSUNDEEP
f#type-providersf#-data
evolvedmicrobe
evolvedmicrobe

Reputation: 2722

F# Data Type Provider - Create with string variable

In F# I have the following bit of code

[<Literal>]
let fname=fstFile.FullName.ToString()
type genomeFile = CsvProvider<fname>

Where fstFile is an instance of the FileInfo class. However, this fails with:

This is not a valid constant expression or custom attribute value

And I can't seem to create the type without an actual hard coded string. Does anyone know how to fix this?

Upvotes: 2

Views: 952

Answers (3)

Eric Stokes
Eric Stokes

Reputation: 21

However, if you reference a literal in an already compiled DLL it will work. So, for example if you split your example into two projects,

e.g. create a new project with TypeProviderArgs.fs ==> TypeProviderArgs.dll

module TypeProviderArgs
[<Literal>]
let fname=fstFile.FullName.ToString()

Then reference TypeProviderArgs.dll in your program and it should work

type genomeFile = CsvProvider<TypeProviderArgs.fname>

This is kind of horrible if you have to create a library just for this purpose, but if you are in a larger project that already has libraries it's not so bad.

Upvotes: 2

Tomas Petricek
Tomas Petricek

Reputation: 243051

The parameter to the CSV type provider needs to be a constant, so that the types can be generated at compile-time (without actually evaluating the program. However, you can load a different file with the same schema at runtime.

So, the best way to handle this is to copy one of your actual inputs to some well-known location (e.g. sample.csv in the same directory as your script) and then use the actual path at runtime:

// Generate type based on a statically known sample file
type GenomeFile = CsvProvider<"sample.csv">

// Load the data from the actual input at runtime
let actualData = GenomeFile.Load(fstFile.FullName.ToString())

Upvotes: 3

John Palmer
John Palmer

Reputation: 25516

The problem is that [<Literal>] can only be applied to things known at compile time (e.g. "Hello").

The result of fstFile.FullName.ToString() is only known at run-time.

This sort of approach won't work.

Upvotes: 1

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