Function decorated with `[<NamedParams>]` fails at runtime if all the arguments are optional and none are provided

[Freymaurer]

Description

Hey i am working on a library we want to offer for js as well as f#. So to improve JS syntax we wanted to use [<NamedParams>] on members. I got a very strange error of:

.. when running my Fable.Mocha tests. To add to this:

• If i use [<NamedParams(fromIndex=0)>] or [<NamedParams>] it runs for Example 1.
• Example 2 raises a similiar error no matter if i use (fromIndex) or not. So i would suspect the issue being ?param f# syntax
• Fable Repl replicating Example 2 runs.

Repro code

Fable Repl

link

Example 1:

Library source code

Example 1:

https://github.com/nfdi4plants/ISADotNet/blob/ea4e1cefdceb8b0899f029409ee5ac4b003268f5/src/ISA/ISA/ArcTypes/ArcTable.fs#L166

Example 1:

https://github.com/nfdi4plants/ISADotNet/blob/ea4e1cefdceb8b0899f029409ee5ac4b003268f5/src/ISA/ISA/ArcTypes/ArcAssay.fs#L108

Expected and actual results

I would expect NamedParams to work in this case. Not sure where something is going wrong

Related information

• Fable version: dotnet fable --version: 4.1.4
• Operating system: Windows 11

[MangelMaxime]

Hello @Freymaurer,

If you look at the generated JavaScript, the problem comes from this line export const assay = ArcAssay.create();

When calling ArcAssay.create() without providing any arguments, Fable doesn't generate an empty {}.

Indeed, without provide {} to the create function, it means that the code is trying to destructure undefined which leads to the error you are seeing.

---

I would also like to ask you this question:

What is your goal here for using [<NamedParams>]?

The only usage that I know of [<NamedParams>] and [<ParamObject>] are for bindings right now.

But here, it seems like you are mixing it with some actual code and I don't know if this is an officially supported usage.

In your case, can't you use the default F# method generation, without having the JavaScript modified to use destructuring patterns?

[Freymaurer]

I want to use [<NamedParams>] to avoid passing void if not all params are given.

// Without NamedParams
export const assay = ArcAssay.create();

export const assay2 = ArcAssay.create("Hello", void 0, "MyType");

// With NamedParams
export const assay = ArcAssay.create();

export const assay2 = ArcAssay.create({
    id: "Hello",
    measurementType: "MyType",
});

Both taken from Fable.Repl

[MangelMaxime]

Ok, we will have to wait other maintainer opinion on this one.

NamedParams and ParamObject are a new addition made by @alfonsogarciacaro. I don't know what is position is on this subject.

[Freymaurer]

Update for the REPL. It does in fact not run correctly. I tried printing the created assays to the console and it does not work, unless i remove the ArcAssay.create(). So it also seems to error, without recognizing it errored.

[MangelMaxime]

Yes ArcAssay.create() doesn't work.

This is because, it generates ArcAssay.create() instead of ArcAssay.create({}).

Meaning that there are no object to desconstruct.

If you provide at least 1 argument, then this works because the object will be initialised.

[Freymaurer]

Ok, we will have to wait other maintainer opinion on this one.

NamedParams and ParamObject are a new addition made by @alfonsogarciacaro. I don't know what is position is on this subject.

Any new information on this? 😄 This would be a very nice option for libraries which should be usable on multiple languages.

[MangelMaxime]

@alfonsogarciacaro @ncave

In order to fix this issue, I had make changes in Fable.AST

Does it means we need to ask for a re-release of Feliz.CompilerPlugins ?

[DunetsNM]

addition of Attributes : Attribute seq to MemberRefInfo broke json deserialization in Util.fs :

Json.readWithStringPool<(string * Fable.InlineExpr)

it now throws

error EXCEPTION: Deserialization of interface types is not supported. Type 'Fable.AST.Fable.Attribute'. Path: $[0] | LineNumber: 0 | BytePositionInLine: 2. at System.Text.Json.ThrowHelper.ThrowNotSupportedException(ReadStack& state, Utf8JsonReader& reader, NotSupportedException ex)

I don't have a small repro just yet - will add a separate issue when I have it (seems to be related to using --precompiledLib flag) , but root cause is pretty obvious: Attribute is an interface which also has ConstructorArgs: obj list i.e. no chance it can serialize.

It fails even though in my inline_exprs_0.json file all Attributes collections are empty. Good news it has only one usage: literally to check presence of "Fable.Core.ParamObjectAttribute", so it's easy to fix by replacing Attributes: Attribute seq with AttributeNames: string seq or even HasParamObjectAttribute: bool

Lesson to extract here is to keep coupling between types at minimum