[Beam] Fable Beam (alpha) (#4340)

Author: dbrattli

.devcontainer/Dockerfile

@@ -90,6 +90,10 @@ RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s - -y
 RUN echo "# Load rust path" >> .zshrc
 RUN echo 'export PATH="$HOME/.cargo/bin:$PATH"' >> .zshrc
 
+# Install Erlang/OTP (for Beam target)
+# Debian Trixie includes OTP 27+ in default repos
+RUN sudo apt-get update && sudo apt-get install -y erlang
+
 # Instal dart
 # Use custom dsm install script, because we broke it while
 #  trying to fix another issue with dsm

.github/workflows/build.yml

@@ -221,6 +221,26 @@ jobs:
       - name: Fable Tests - Dart
         run: ./build.sh test dart
 
+  # Separate build job for Beam (Erlang)
+  build-beam:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: "10.0.x"
+
+      - name: Setup Erlang/OTP
+        uses: erlef/setup-beam@v1
+        with:
+          otp-version: "27"
+
+      - name: Fable Tests - Beam
+        run: ./build.sh test beam
+
   # Separate job to run F# analyzers
   analyzers:
     runs-on: ubuntu-latest

src/Fable.AST/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* [Beam] Add `Language.Beam` case and Erlang AST types (`Beam.AST.fs`) (by @dbrattli)
+
 ## 5.0.0-beta.3 - 2025-11-25
 
 * [All] Fix #4221 Fable.Type union case arg names (by @ncave)

src/Fable.AST/Plugins.fs

@@ -16,6 +16,7 @@ type Language =
     | Php
     | Dart
     | Rust
+    | Beam
 
     override this.ToString() =
         match this with
@@ -25,6 +26,7 @@ type Language =
         | Php -> "PHP"
         | Dart -> "Dart"
         | Rust -> "Rust"
+        | Beam -> "Beam"
 
 type CompilerOptions =
     {

src/Fable.Build/Fable.Build.fsproj

@@ -15,6 +15,7 @@
     <Compile Include="FableLibrary/TypeScript.fs" />
     <Compile Include="FableLibrary/JavaScript.fs" />
     <Compile Include="FableLibrary/Rust.fs" />
+    <Compile Include="FableLibrary/Beam.fs" />
     <Compile Include="WorkerJs.fs" />
     <Compile Include="Standalone.fs" />
     <Compile Include="CompilerJs.fs" />
@@ -24,6 +25,7 @@
     <Compile Include="Test/Rust.fs" />
     <Compile Include="Test/Dart.fs" />
     <Compile Include="Test/TypeScript.fs" />
+    <Compile Include="Test/Beam.fs" />
     <Compile Include="Test/Integration.fs" />
     <Compile Include="Test/CompilerJs.fs" />
     <Compile Include="Quicktest/Core.fs" />
@@ -32,6 +34,7 @@
     <Compile Include="Quicktest/Python.fs" />
     <Compile Include="Quicktest/Rust.fs" />
     <Compile Include="Quicktest/Dart.fs" />
+    <Compile Include="Quicktest/Beam.fs" />
     <Compile Include="FcsRepo.fs" />
     <Compile Include="Publish.fs" />
     <Compile Include="GithubRelease.fs" />

src/Fable.Build/FableLibrary/Beam.fs

@@ -0,0 +1,18 @@
+namespace Build.FableLibrary
+
+open System.IO
+
+type BuildFableLibraryBeam() =
+    inherit
+        BuildFableLibrary(
+            "beam",
+            Path.Combine("src", "fable-library-beam"),
+            Path.Combine("src", "fable-library-beam"),
+            Path.Combine("temp", "fable-library-beam"),
+            Path.Combine("temp", "fable-library-beam")
+        )
+
+    override this.CopyStage() =
+        // Copy hand-written .erl runtime files to the output directory
+        for erlFile in Directory.GetFiles(this.SourceDir, "*.erl") do
+            File.Copy(erlFile, Path.Combine(this.OutDir, Path.GetFileName(erlFile)), overwrite = true)

src/Fable.Build/Main.fs

@@ -29,6 +29,7 @@ Available commands:
             python                  Run for Python
             dart                    Run for Dart
             rust                    Run for Rust
+            beam                    Run for BEAM (Erlang)
 
         Options:
             --skip-fable-library    Skip building fable-library if folder already exists
@@ -40,6 +41,7 @@ Available commands:
             python                  Run the tests for Python
             dart                    Run the tests for Dart
             rust                    Run the tests for Rust
+            beam                    Run the tests for BEAM (Erlang)
             integration             Run the integration test suite
             standalone              Tests the standalone version of Fable
                                     (Fable running on top of Node.js)
@@ -126,6 +128,7 @@ let main argv =
             | "--python" :: _ -> BuildFableLibraryPython().Run()
             | "--dart" :: _ -> BuildFableLibraryDart().Run()
             | "--rust" :: _ -> BuildFableLibraryRust().Run()
+            | "--beam" :: _ -> BuildFableLibraryBeam().Run()
             | _ -> printHelp ()
         | "test" :: args ->
             match args with
@@ -134,6 +137,7 @@ let main argv =
             | "python" :: args -> Test.Python.handle args
             | "dart" :: args -> Test.Dart.handle args
             | "rust" :: args -> Test.Rust.handle args
+            | "beam" :: args -> Test.Beam.handle args
             | "integration" :: args -> Test.Integration.handle args
             | "standalone" :: _ -> Test.Standalone.handle args
             // This test is using quicktest project for now,
@@ -147,6 +151,7 @@ let main argv =
             | "python" :: _ -> Quicktest.Python.handle args
             | "dart" :: _ -> Quicktest.Dart.handle args
             | "rust" :: _ -> Quicktest.Rust.handle args
+            | "beam" :: _ -> Quicktest.Beam.handle args
             | _ -> printHelp ()
         | "standalone" :: args -> Standalone.handle args
         | "compiler-js" :: args -> CompilerJs.handle args

src/Fable.Build/Quicktest/Beam.fs

@@ -0,0 +1,15 @@
+module Build.Quicktest.Beam
+
+open Build.FableLibrary
+open Build.Quicktest.Core
+
+let handle (args: string list) =
+    genericQuicktest
+        {
+            Language = "beam"
+            FableLibBuilder = BuildFableLibraryBeam()
+            ProjectDir = "src/quicktest-beam"
+            Extension = ".erl"
+            RunMode = RunScript
+        }
+        args

src/Fable.Build/Test/Beam.fs

@@ -0,0 +1,103 @@
+module Build.Test.Beam
+
+open Build.FableLibrary
+open System.IO
+open Build.Utils
+open BlackFox.CommandLine
+open SimpleExec
+
+let private buildDir = Path.Resolve("temp", "tests", "Beam")
+let private sourceDir = Path.Resolve("tests", "Beam")
+let private testRunnerSrc = Path.Resolve("tests", "Beam", "erl_test_runner.erl")
+
+let handle (args: string list) =
+    let isWatch = args |> List.contains "--watch"
+    let noDotnet = args |> List.contains "--no-dotnet"
+    let skipFableLibrary = args |> List.contains "--skip-fable-library"
+
+    BuildFableLibraryBeam().Run(skipFableLibrary)
+
+    Directory.clean buildDir
+
+    let fableArgs =
+        CmdLine.concat
+            [
+                CmdLine.empty
+                |> CmdLine.appendRaw sourceDir
+                |> CmdLine.appendPrefix "--outDir" buildDir
+                |> CmdLine.appendPrefix "--lang" "beam"
+                |> CmdLine.appendPrefix "--exclude" "Fable.Core"
+                |> CmdLine.appendRaw "--noCache"
+
+                if isWatch then
+                    CmdLine.empty |> CmdLine.appendRaw "--watch"
+            ]
+
+    if isWatch then
+        Async.Parallel
+            [
+                if not noDotnet then
+                    Command.RunAsync("dotnet", "watch test -c Release", workingDirectory = sourceDir)
+                    |> Async.AwaitTask
+
+                Command.WatchFableAsync(fableArgs, workingDirectory = buildDir)
+                |> Async.AwaitTask
+            ]
+        |> Async.RunSynchronously
+        |> ignore
+    else
+        // Test against .NET
+        Command.Run("dotnet", "test -c Release", workingDirectory = sourceDir)
+
+        // Compile tests to Erlang
+        Command.Fable(fableArgs, workingDirectory = buildDir)
+
+        // Copy test runner to build dir
+        File.Copy(testRunnerSrc, Path.Combine(buildDir, "erl_test_runner.erl"), overwrite = true)
+
+        // Copy native Erlang test modules to build dir
+        let nativeErlDir = Path.Resolve("tests", "Beam", "erl")
+
+        if Directory.Exists(nativeErlDir) then
+            for erlFile in Directory.GetFiles(nativeErlDir, "*.erl") do
+                let fileName = Path.GetFileName(erlFile)
+                File.Copy(erlFile, Path.Combine(buildDir, fileName), overwrite = true)
+
+        // fable-library-beam files are auto-copied to fable_modules/fable-library-beam/ by the compiler
+        let fableModulesLibDir =
+            Path.Combine(buildDir, "fable_modules", "fable-library-beam")
+
+        // Compile fable-library-beam .erl files first
+        let mutable compileErrors = 0
+
+        if Directory.Exists(fableModulesLibDir) then
+            for erlFile in Directory.GetFiles(fableModulesLibDir, "*.erl") do
+                let fileName = Path.GetFileName(erlFile)
+
+                try
+                    Command.Run("erlc", fileName, workingDirectory = fableModulesLibDir)
+                with _ ->
+                    printfn $"WARNING: erlc failed for {fileName} (library), skipping"
+                    compileErrors <- compileErrors + 1
+
+        // Compile test .erl files with library on code path
+        let erlFiles = Directory.GetFiles(buildDir, "*.erl")
+
+        for erlFile in erlFiles do
+            let fileName = Path.GetFileName(erlFile)
+
+            try
+                Command.Run("erlc", $"-pa fable_modules/fable-library-beam {fileName}", workingDirectory = buildDir)
+            with _ ->
+                printfn $"WARNING: erlc failed for {fileName}, skipping"
+                compileErrors <- compileErrors + 1
+
+        if compileErrors > 0 then
+            printfn $"WARNING: {compileErrors} file(s) failed to compile with erlc"
+
+        // Run Erlang test runner with library on code path
+        Command.Run(
+            "erl",
+            "-noshell -pa . -pa fable_modules/fable-library-beam -eval \"erl_test_runner:main([\\\".\\\"])\" -s init stop",
+            workingDirectory = buildDir
+        )

src/Fable.Cli/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+* [Beam] Add Erlang/BEAM target (`--lang beam`). Compiles F# to `.erl` source files. 2010 tests passing. (by @dbrattli)
+
 ## 5.0.0-alpha.24 - 2026-02-13
 
 ### Fixed