Primary constructor of types from signature files (.fsi) are mangled

[ncave]

Given the following project:

//--- test.fsi ---
module M
type Test =
    new : bool -> Test      // <-- here is what's causing it
    member Status: bool

//--- test.fs ---
module M
type Test(flag:bool) =
    member x.Status = flag

//--- project.fsx ---
#load "test.fsi"
#load "test.fs"

[<EntryPoint>]
let main argv =
    let t = M.Test(true)
    printfn "Status: %A" t.Status
    0

Here the constructor call will be generated as:
const t = _test.Test[".ctor"](true);

which causes a runtime error. It should be:
const t = new _test.Test(true);

The runtime error does not happen if the constructor definition is not used.

[alfonsogarciacaro]

I haven't tested Fable that much with .fsi files, probably this needs some work.

Sometimes it seems F# specs are endless... :/

[ncave]

I hear you, unfortunately we can't call the constructor without that constructor definition. The only option in that case is to not use signature files at all, which is a steep limitation for compiling existing code bases.

I was hoping that it's something simple that you can just spring out of your magic hat, like always ;)

[alfonsogarciacaro]

Ok, I've investigated about this and it's not simple ;)

The problem happens because when using the .fsi, meth.IsImplicitConstructor fails (I guess this is because the method declaration in the .fsi file is taken, where there're no implicit constructors), so this line doesn't evaluate to true.

This is a problem because JS only accepts one single constructor and the other must be compiled as static methods. I have tried a hack by checking that the method is the first of all constructors in the type and this seemed to work but then I had problems with the structs we added recently in #568 because they can have constructors but no implicit constructor (which must be generated automatically).

I haven't found a way to match the constructor reference to the actual implementation either :/

Can you please open an issue in FCS asking that .IsImplicitConstructor must evaluate to true also when using signature files?

[ncave]

Thank you for investigating, I think the simplest work-around for now is to add an explicit constructor to call the implicit one:

module A
type Test(flag:bool) =
    new(flag:bool, _) = Test(flag) // <-- added an explicit constructor here
    member x.Status = flag

Not the prettiest solution but it works.

[alfonsogarciacaro]

I would still ask in FCS repo about this. It should be possible to go from the signature declaration of an entity to the implementation declaration, but I couldn't find an easy way to do this.

[alfonsogarciacaro]

Another solution would be just to filter out .fsi files when running the F# type checker.

[ncave]

Idk, sometimes .fsi files are needed so maybe not a good idea to leave them out completely.

I kind of like your hack above (to check if it's the first constructor, and do the rest as static). The 'no implicit constructor' for value types is kind of the same as the one in #573 which is not supported yet, so maybe it won't be a problem.