Unable to use the Literal attribute in an F# Signature file (.fsi)

Question

I'm unable to define a literal in an F# signature file.

I have a file, File.fs, as below:

module Thingo =
  [<Literal>]
  let hello = "world"

I need to create a signature file for it, so initially tried this (File.fsi):

module Thingo =
  [<Literal>]
  val hello : string

This caused the following error in File.fsi: A declaration may only be the [<Literal>] attribute if a constant value is also given, e.g. 'val x : int = 1'

I gave that a go:

module Thingo =
  [<Literal>]
  val hello : string = "world"

The line after the val hello declaration received the error: Incomplete structured construct at or before this point in signature file. Expected incomplete structured construct at or before this point or other token.

So it doesn't like me assigning values in a signature file (or I don't know how to assign the value, very much a possibility).

The docs say that I do need to assign the value in the signature somehow:

If the Literal attribute is used, it must appear in both the signature and the implementation, and the same literal value must be used for both.

(from https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/signatures)

I also auto-generated a signature file (with the --sig build argument) and it spat out:

module EventSchema = begin
    [<LiteralAttribute ()>]
        val id : string
end

I tried this exact syntax, but it didn't work either.

Thanks for reading! Any suggestions will be greatly appreciated.

Answer 1

The correct answer should have been: it's a bug. It was reported here: https://github.com/dotnet/fsharp/issues/7897.

Note that, as a workaround, you can switch light syntax off, then it will work in current versions.

Note also that the compiler will error when the constant values don't match with the one in the signature file.