问题
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?
回答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())
回答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.
回答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.
来源：https://stackoverflow.com/questions/23598250/f-data-type-provider-create-with-string-variable