F# Data Type Provider - Create with string variable

Question

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?

Answer 1

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())

Answer 2

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.

Answer 3

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.