Cannot create Fable plugin

[OrfeasZ]

I'm trying to create a Fable plugin but I'm having some trouble.

As a very basic starting point, I tried creating the following attribute:

namespace Test

open Fable

type TestPluginAttribute() =
    inherit MemberDeclarationPluginAttribute()
    override self.FableMinimumVersion = "4.5.0"
    override self.Transform(compiler, file, decl) = decl
    override self.TransformCall(compiler, memb, expr) = expr

However, when trying to compile my project with Fable, I get the following error:

error FABLE: Cannot reference entity from .dll reference, Fable packages must include F# sources: Fable.MemberDeclarationPluginAttribute

My project file looks like this:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <TargetFramework>net8.0</TargetFramework>
        <RootNamespace>Test</RootNamespace>
        <GenerateDocumentationFile>true</GenerateDocumentationFile>
        <AssemblyName>Test</AssemblyName>
    </PropertyGroup>
    <ItemGroup>
        <Compile Include="Test.fs"/>
    </ItemGroup>
    <ItemGroup>
        <PackageReference Include="Fable.AST" Version="4.3.0"/>
    </ItemGroup>
</Project>

This is included from another F# project, and compiled using the following command (fable version is 4.5.0):

dotnet fable -o out

What am I doing wrong?

[MangelMaxime]

Hello,

When consuming a plugin in local development you need to tell Fable to exclude it from the compilation.

For example, dotnet fable --exclude Fable.CompilerPlugins - o out

Does doing so solve your problem?

[OrfeasZ]

I did try that earlier (ie. --exclude Test), but then my other project fails to compile where I use the attribute:

error FSHARP: The namespace or module 'Test' is not defined. (code 39)
error FSHARP: The type 'TestPlugin' is not defined. (code 39)

[nojaf]

You may need to have Plugin in your assembly name for the filtering to kick in.
I did some experiment with a plugin a while ago and I had --exclude "Nojaf.Fable.React.Plugin"

My full command was:
dotnet fable ./Playground.fsproj -c Debug -e .js -o ./out --noReflection --exclude "Nojaf.Fable.React.Plugin"

[nojaf]

Nah, that might not be it. Do you have

// Tell Fable to scan for plugins in this assembly
[<assembly : ScanForPlugins>]
do ()

in your plugin assembly as well?

[OrfeasZ]

That unfortunately doesn't seem to change anything. Also tried using the .Plugin suffix but that didn't work either.

I've uploaded a minimal repro here: https://github.com/OrfeasZ/fable-plugin-repro

The command I'm using is:

dotnet fable ./FablePluginTest/FablePluginTest.fsproj -o out --exclude FablePluginTest.Plugin

Maybe it has to do with how the plugin project is referenced from the main project?

[MangelMaxime]

So I was able to debug the problem.

If you look at the console output, you should see an error about Fable not being able to compile the Project because of

System.ArgumentException: The index is outside the legal range.
count = 1, array.Length = 0 (Parameter 'count')

This is coming from:

Fable/src/Fable.Cli/ProjectCracker.fs

Lines 495 to 523 in 308acef

	let excludeProjRef
	(opts: CrackerOptions)
	(dllRefs: IDictionary<string, string>)
	(projRef: string)
	=
	let projName = Path.GetFileNameWithoutExtension(projRef)
	let isExcluded =
	opts.Exclude
	|> List.exists (fun e ->
	String.Equals(
	e,
	Path.GetFileNameWithoutExtension(projRef),
	StringComparison.OrdinalIgnoreCase
	)
	)
	if isExcluded then
	try
	opts.BuildDll(dllRefs[projName])
	with e ->
	Log.always ("Couldn't build " + projName + ": " + e.Message)
	None
	else
	let _removed = dllRefs.Remove(projName)
	// if not removed then
	// Log.always("Couldn't remove project reference " + projName + " from dll references")
	Path.normalizeFullPath projRef |> Some

But the actual error exception is occurring from here:

Fable/src/Fable.Cli/ProjectCracker.fs

Lines 123 to 143 in 308acef

	member _.BuildDll(normalizedDllPath: string) =
	if not (builtDlls.Contains(normalizedDllPath)) then
	let projDir =
	normalizedDllPath.Split('/')
	|> Array.rev
	|> Array.skipWhile (fun part -> part <> "bin")
	|> Array.skip 1
	|> Array.rev
	|> String.concat "/"
	Process.runSync
	projDir
	"dotnet"
	[
	"build"
	"-c"
	cliArgs.Configuration
	]
	|> ignore
	builtDlls.Add(normalizedDllPath) |> ignore

The problem is that for some reason, FablePluginTest.Plugin reference is coming from the obj folder instead of the bin folder.

So when Fable tries to |> Array.skipWhile (fun part -> part <> "bin") it never find bin and fails on the next line trying to |> Array.skip 1 an empty array.

Changing bin to obj solve this problem, but then when Fable tries to compile the code I have an error occurring from the plugin application:

Project and references (1 source files) parsed in 2026ms

Loaded FablePluginTest.Plugin.TestPluginAttribute from FablePluginTest.Plugin/obj/Release/net8.0/ref/FablePluginTest.Plugin.dll
Started Fable compilation...
Fable compilation finished in 739ms  

./FablePluginTest/Test.fs(1,1): error EXCEPTION: Exception has been thrown by the target of an invocation.
   at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
   at System.Activator.CreateInstance(Type type, Boolean nonPublic, Boolean wrapExceptions)
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at Fable.CompilerExt.Compiler-ApplyPlugin@249.Invoke(Input input, Attribute att) in /Users/mmangel/Workspaces/Github/fable-compiler/Fable/src/Fable.Transforms/Global/Compiler.fs:line 253
   at Microsoft.FSharp.Collections.SeqModule.Fold[T,TState](FSharpFunc`2 folder, TState state, IEnumerable`1 source) in /home/dev/Projects/fsharp/src/FSharp.Core/seq.fs:line 912
   at Fable.CompilerExt.Compiler.ApplyMemberDeclarationPlugin(Compiler com, File file, MemberDecl decl) in /Users/mmangel/Workspaces/Github/fable-compiler/Fable/src/Fable.Transforms/Global/Compiler.fs:line 286
   at Fable.Transforms.FableTransforms.transformDeclaration(FSharpList`1 transformations, Compiler com, File file, Declaration decl) in /Users/mmangel/Workspaces/Github/fable-compiler/Fable/src/Fable.Transforms/FableTransforms.fs:line 996
   at Fable.Transforms.FableTransforms.newDecls@1087.Invoke(Declaration decl)
   at Microsoft.FSharp.Primitives.Basics.List.mapToFreshConsTail[a,b](FSharpList`1 cons, FSharpFunc`2 f, FSharpList`1 x) in /home/dev/Projects/fsharp/src/FSharp.Core/local.fs:line 236
   at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x) in /home/dev/Projects/fsharp/src/FSharp.Core/local.fs:line 246
   at Microsoft.FSharp.Collections.ListModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 list) in /home/dev/Projects/fsharp/src/FSharp.Core/list.fs:line 97
   at Fable.Transforms.FableTransforms.transformFile(Compiler com, File file) in /Users/mmangel/Workspaces/Github/fable-compiler/Fable/src/Fable.Transforms/FableTransforms.fs:line 1085
   at Fable.Cli.Pipeline.Js.compileFile@241.Invoke(Unit unitVar) in /Users/mmangel/Workspaces/Github/fable-compiler/Fable/src/Fable.Cli/Pipeline.fs:line 242
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke[T,TResult](AsyncActivation`1 ctxt, TResult result1, FSharpFunc`2 part2) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 510
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 112
Compilation failed

I am not sure if this is because of a bug in the plugin implementation or in Fable code itself.

I believe the problem is related the .NET SDK used when running Fable. Indeed, forcing the repository to use .NET 6, then everything is working correctly.

@nojaf @ncave any ideas?

Luckily, the problem seems to only occurs when working with local plugin, so projects in production that use Feliz (for example) and .NET still works.

---

@OrfeasZ If you want to work on your plugin, you can force you repository to use .NET 6 via a global.json.

{
    "sdk": {
        "version": "6.0.100",
        "rollForward": "latestFeature"
    }
}

[OrfeasZ]

@MangelMaxime good catch, thanks a lot! Didn't even notice this error... 😅

Changing the plugin project to target .NET 6 does indeed fix this issue!

[OrfeasZ]

I can also confirm that ScanForPlugins is needed for the plugin to be invoked by Fable.

[nojaf]

Only <TargetFramework> needs to be net6.0.
You can still use a higher SDK.
I'm guessing it needs to be net6.0, because the Fable CLI tool is on that version:

Fable/src/Fable.Cli/Fable.Cli.fsproj

Line 6 in 308acef

<TargetFramework>net6.0</TargetFramework>

[nojaf]

@MangelMaxime where in https://github.com/fable-compiler/fable-compiler.github.io/tree/dev/docs/docs would be a good place to write some docs on what we learned in this thread?

[MangelMaxime]

Currently, we have "Author a Fable library" in the "Your Fable project" section.

So we could add a page "Author a Fable plugin" / "Create a Fable plugin" below that page.

The other solution would be to create a section dedicated to Fable plugins, but creating a section for a single page seems a little wasteful to me.

My feeling currently is that "Your Fable project" name is not the correct name for that section, as "Author a Fable plugin" kind of feel out of place already in that section.

[OrfeasZ]

Only <TargetFramework> needs to be net6.0. You can still use a higher SDK. I'm guessing it needs to be net6.0, because the Fable CLI tool is on that version:

That's exactly what I did!

In regards to docs, I'm happy to contribute some parts after I've played with the plugin capabilities a bit.