feat: add Fable/BEAM (Erlang) as a test target

Add `beam` as a fourth Fable runtime (alongside dotnet/js/ts/python) for the
core pilot libraries: Ink, Nib, Parchment, Quill. Nib.Browser stays JS-only;
Hedgehog and Nib.Snapshot are deferred.

- build/Commands/Test.fs: Runtime.Beam (beam/erlang alias) + a runBeam step
  (fable --lang beam -> rebar3 compile -> erl main:main([]) via ERL_LIBS).
- Quill: TargetPlatform.Beam, Compiler.isBeam detection, skipIfBeam, and
  halt(exitCode) exit propagation in universalRunTests.
- Idiomatic FABLE_COMPILER_BEAM branches for platform primitives (cwd, isCI,
  stopwatch, writeRaw, Ink NO_COLOR, Parchment stderr); the JS-only browser
  sink is excluded from beam.
- CI: erlef/setup-beam (OTP + rebar3); .gitignore beam-build/.

Verified against local Fable: Ink 51/51, Nib 68/68, Quill 29 + 1 skip,
Parchment 27/27. Requires an unreleased Fable (3 Beam runtime fixes:
Async.StartChild, DU ordered comparison, function PhysicalEquality); bump the
`fable` tool once released so the tool-based build picks them up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

chore: bump Fable

chore: simplify some comments

Author: dbrattli

.github/workflows/test.yml

@@ -29,6 +29,12 @@ jobs:
           node-version: 24
           cache: npm
 
+      - name: Setup Erlang/OTP
+        uses: erlef/setup-beam@v1
+        with:
+          otp-version: "27"
+          rebar3-version: "3"
+
       - name: Install Playwright browsers
         run: |
           npm install

.gitignore

@@ -42,3 +42,6 @@ docs/src/content/docs/.gitignore
 # Fable TS/Python test output (kept out of obj/)
 ts-build/
 py-build/
+
+# Fable BEAM/Erlang test output (transpiled .erl + rebar3 build)
+beam-build/

build/Commands/Test.fs

@@ -1,5 +1,6 @@
 module EasyBuild.Commands.Test
 
+open System.IO
 open Spectre.Console.Cli
 open SimpleExec
 open BlackFox.CommandLine
@@ -12,6 +13,7 @@ type Runtime =
     | JavaScript
     | TypeScript
     | Python
+    | Beam
     | AllRuntime
 
     static member All =
@@ -26,6 +28,7 @@ type Runtime =
         | JavaScript -> "js"
         | TypeScript -> "ts"
         | Python -> "py"
+        | Beam -> "beam"
 
     static member fromString(value: string) =
         match value.ToLowerInvariant() with
@@ -37,18 +40,19 @@ type Runtime =
         | "ts" -> TypeScript
         | "python"
         | "py" -> Python
+        | "beam"
+        | "erlang" -> Beam
         | _ ->
             failwith
                 $"""Unknown runtime value: '%s{value}'
 
 It should be one of the following:
 - dotnet
-- javascript
-- js
-- typescript
-- ts
-- python
-- py
+- javascript (or js)
+- typescript (or ts)
+- python (or py)
+- beam (or erlang)
+- all
         """
 
 type Project =
@@ -98,6 +102,7 @@ It should be one of the following:
 - nib-snapshot
 - parchment
 - quill
+- hedgehog
         """
 
     member this.Dir =
@@ -126,13 +131,15 @@ It should be one of the following:
                 JavaScript
                 TypeScript
                 Python
+                Beam
             ]
         | Scriptorium_Nib ->
             [
                 DotNet
                 JavaScript
                 TypeScript
                 Python
+                Beam
             ]
         | Scriptorium_Nib_Browser -> [ JavaScript ]
         | Scriptorium_Nib_Snapshot ->
@@ -148,13 +155,15 @@ It should be one of the following:
                 JavaScript
                 TypeScript
                 Python
+                Beam
             ]
         | Scriptorium_Quill ->
             [
                 DotNet
                 JavaScript
                 TypeScript
                 Python
+                Beam
             ]
         | AllProject -> failwith "All is not a real project, it should be captured before"
 
@@ -218,15 +227,46 @@ let private testProject (project: Project) (runtime: Runtime) (isWatch: bool) =
             let dotnetRun (args: string) =
                 Command.Run("dotnet", args, workingDirectory = project.Dir)
 
+            // BEAM has no single-step `--run`: Fable emits `.erl`, which must be compiled
+            // with rebar3 and executed on the Erlang VM.
+            let runBeam () =
+                let beamBuildDir = Path.Combine(project.Dir, "beam-build")
+
+                let erlLibs =
+                    Path.GetFullPath(Path.Combine(beamBuildDir, "_build", "default", "lib"))
+
+                // 1. Transpile F# to Erlang into beam-build/ (also emits fable-library-beam).
+                Command.Run(
+                    "dotnet",
+                    "fable --lang beam -o beam-build",
+                    workingDirectory = project.Dir
+                )
+                // 2. Compile the generated Erlang with rebar3.
+                Command.Run("rebar3", "compile", workingDirectory = beamBuildDir)
+                // 3. Run the suite on the BEAM. ERL_LIBS makes every compiled app's ebin
+                //    (incl. fable-library-beam) loadable. Fable emits the [<EntryPoint>] as
+                //    main:main/1 (argv); it runs the suite and halts with the exit code.
+                Command.Run(
+                    "erl",
+                    "-noshell -eval main:main([]) -s init stop",
+                    workingDirectory = beamBuildDir,
+                    configureEnvironment =
+                        (fun (env: System.Collections.Generic.IDictionary<string, string>) ->
+                            env.["ERL_LIBS"] <- erlLibs
+                        )
+                )
+
             match runtime with
             | DotNet -> dotnetRun dotnet
             | JavaScript -> dotnetRun javascript
             | TypeScript -> dotnetRun typescript
             | Python -> dotnetRun python
+            | Beam -> runBeam ()
             | AllRuntime ->
                 dotnetRun javascript
                 dotnetRun typescript
                 dotnetRun python
+                runBeam ()
                 dotnetRun dotnet
         with :? ExitCodeException ->
             printfn
@@ -252,6 +292,7 @@ Accepted values:
 - nib-snapshot
 - parchment
 - quill
+- hedgehog
 - all
 
 Default: all""")>]
@@ -265,6 +306,7 @@ Accepted values:
 - javascript (or js)
 - typescript (or ts)
 - python (or py)
+- beam (or erlang)
 - all
 
 Default: all""")>]

dotnet-tools.json

@@ -10,7 +10,7 @@
       "rollForward": false
     },
     "fable": {
-      "version": "5.7.0",
+      "version": "5.8.0",
       "commands": [
         "fable"
       ],

pyproject.toml

@@ -3,7 +3,7 @@
 name = "scriptorium-python-tests"
 version = "0"
 requires-python = ">=3.12,<4.0"
-dependencies = ["fable-library==5.7.0"]
+dependencies = ["fable-library==5.8.0"]
 
 # Not a package: install the dependencies only, never build/install this project itself.
 [tool.uv]

src/Scriptorium.Ink/Ink.fs

@@ -20,7 +20,12 @@ let private colorsEnabled: bool =
     Fable.Core.PyInterop.emitPyExpr () "__import__('os').environ.get('NO_COLOR') is None"
 #endif
 
-#if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON)
+#if FABLE_COMPILER_BEAM
+    // os:getenv/1 returns the atom false when NO_COLOR is unset (i.e. colors enabled).
+    Fable.Core.BeamInterop.emitErlExpr () "os:getenv(\"NO_COLOR\") =:= false"
+#endif
+
+#if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
     System.Environment.GetEnvironmentVariable("NO_COLOR") |> isNull
 #endif
 

src/Scriptorium.Parchment/Sinks/JavaScript.fs

@@ -4,9 +4,9 @@ open Fable.Core.JS
 open Fable.Core.JsInterop
 open Scriptorium.Parchment
 
-// This module emits raw JavaScript (console API), which the Python target cannot compile, so it is
-// excluded there. It stays available on .NET for source compatibility (as before).
-#if !FABLE_COMPILER_PYTHON
+// This module emits raw JavaScript (console API), which the Python and BEAM targets cannot compile,
+// so it is excluded there. It stays available on .NET for source compatibility (as before).
+#if !(FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
 /// <summary>
 /// Sinks that rely on JavaScript-specific APIs.
 /// </summary>

src/Scriptorium.Parchment/Sinks/Universal.fs

@@ -24,7 +24,11 @@ module Universal =
         #if FABLE_COMPILER_PYTHON
         Fable.Core.PyInterop.emitPyStatement msg "print($0, file=__import__('sys').stderr)"
         #endif
-        #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON)
+        #if FABLE_COMPILER_BEAM
+        // ~ts treats the binary as data (not a format string); standard_error is stderr.
+        Fable.Core.BeamInterop.emitErlStatement msg "io:format(standard_error, \"~ts~n\", [$0])"
+        #endif
+        #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
         System.Console.Error.WriteLine(msg)
         #endif
 

src/Scriptorium.Quill/DSL.fs

@@ -39,6 +39,14 @@ module Configurers =
         else
             config
 
+    let skipIfBeam (config: TestConfig) : TestConfig =
+        if currentPlatform = Beam then
+            { config with
+                Skip = true
+            }
+        else
+            config
+
     let timeout (ms: int) (config: TestConfig) : TestConfig =
         { config with
             TimeoutMs = Some ms

src/Scriptorium.Quill/Prelude.fs

@@ -18,7 +18,12 @@ module Prelude =
         Fable.Core.PyInterop.emitPyExpr () "__import__('os').getcwd()"
     #endif
 
-    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON)
+    #if FABLE_COMPILER_BEAM
+        // file:get_cwd() returns {ok, Dir} where Dir is a charlist; F# strings are Erlang binaries.
+        Fable.Core.BeamInterop.emitErlExpr () "list_to_binary(element(2, file:get_cwd()))"
+    #endif
+
+    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
         System.Environment.CurrentDirectory
     #endif
 
@@ -43,7 +48,16 @@ module Prelude =
         member _.ElapsedMs() : int = int ((now () - startTime) * 1000.0)
     #endif
 
-    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON)
+    #if FABLE_COMPILER_BEAM
+    type UniversalStopwatch() =
+        let now () : int =
+            Fable.Core.BeamInterop.emitErlExpr () "erlang:monotonic_time(millisecond)"
+
+        let startTime = now ()
+        member _.ElapsedMs() : int = now () - startTime
+    #endif
+
+    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
     type UniversalStopwatch() =
         let sw = System.Diagnostics.Stopwatch.StartNew()
         member _.ElapsedMs() : int = int sw.ElapsedMilliseconds
@@ -55,6 +69,8 @@ module Prelude =
             DotNet
         elif Compiler.isPython then
             Python
+        elif Compiler.isBeam then
+            Beam
         else
             JavaScript
 
@@ -67,6 +83,11 @@ module Prelude =
         Fable.Core.PyInterop.emitPyExpr () "__import__('os').environ.get('CI') is not None"
     #endif
 
-    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON)
+    #if FABLE_COMPILER_BEAM
+        // os:getenv/1 returns the value (charlist) or the atom false when unset.
+        Fable.Core.BeamInterop.emitErlExpr () "os:getenv(\"CI\") =/= false"
+    #endif
+
+    #if !(FABLE_COMPILER_JAVASCRIPT || FABLE_COMPILER_TYPESCRIPT || FABLE_COMPILER_PYTHON || FABLE_COMPILER_BEAM)
         System.Environment.GetEnvironmentVariable("CI") |> isNull |> not
     #endif