Build and Run

Added in v5.0.0-rc.1

Prerequisites#

• Erlang/OTP 25 or higher
• rebar3 (the standard Erlang build tool)

Architecture#

Fable compiles F# to Erlang source files (.erl), following the same pipeline as all Fable targets:

F# Source
    |  FSharp2Fable
Fable AST
    |  FableTransforms
Fable AST (optimized)
    |  Fable2Beam
Erlang AST
    |  ErlangPrinter
.erl source files

The generated .erl files are standard Erlang that can be compiled with rebar3 and run on the BEAM VM.

Compiling to Erlang#

dotnet fable --lang beam

By default, output goes to the project directory (where the .fsproj is). You can change this with --outDir:

dotnet fable --lang beam --outDir /path/to/output

Output Structure#

Fable automatically generates a complete rebar3 project scaffold alongside the compiled .erl files. The output directory will contain:

output/
  rebar.config                             # rebar3 project config (auto-generated)
  src/
    program.erl                            # Your compiled F# modules
    my_project.app.src                     # OTP application resource file
  fable_modules/
    fable-library-beam/
      rebar.config                         # Per-dependency rebar3 config
      src/
        fable_library_beam.app.src         # OTP app resource for the library
        fable_list.erl                     # F# List runtime
        fable_map.erl                      # F# Map runtime
        fable_string.erl                   # String utilities
        fable_seq.erl                      # Seq/IEnumerable support
        ...                                # Other runtime modules

Rebar3 Scaffold Details#

— if you create your own rebar.config (without the Fable marker), it will be left untouched (a warning is emitted) — project and dependency names are normalized to valid OTP application names (e.g., Fable.Logging.0.10.0 → fable_logging, fable-library-beam → fable_library_beam) — the generated rebar.config uses {project_app_dirs, [".", "fable_modules/*"]} so rebar3 treats each NuGet dependency as a sub-application

Compiling with rebar3#

After Fable generates the scaffold, compile everything with rebar3:

rebar3 compile

This compiles all .erl files (your project and all dependencies) and places .beam files in _build/default/lib/*/ebin/.

Running Erlang Code#

Run your compiled module using the Erlang shell:

erl -noshell -pa _build/default/lib/*/ebin \
    -eval 'program:main(), halt().'

The -pa flag adds the rebar3 output directories to the code path so Erlang can find both your modules and the Fable runtime library.

Module Naming#

Fable converts F# filenames to snake_case to match Erlang conventions:

• Program.fs becomes program.erl with -module(program).
• MyModule.fs becomes my_module.erl with -module(my_module).

Erlang requires the module name to match the filename, so this conversion is automatic.

Program vs Library#

The Beam backend currently does not differentiate between Exe and Library output types. All top-level code is compiled into a main/0 Erlang function regardless of the OutputType setting. The [<EntryPoint>] attribute is not used.

Watch Mode#

For development, use watch mode to recompile on changes:

dotnet fable watch --lang beam

Custom fable-library Path#

If you need a custom version of the runtime library (e.g., for development), use the --fableLib option:

dotnet fable --lang beam --fableLib /path/to/custom/fable-library-beam