Build and Run

Added in v5.0.0-rc.1

Erlang/OTP Version#

Fable targets Erlang/OTP 25 or higher.

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 erlc 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#

The output directory will contain:

output/
  program.erl                              # Your compiled F# modules
  fable_modules/
    fable-library-beam/
      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

Compiling Erlang#

After Fable generates .erl files, compile them with erlc:

# Compile the runtime library
erlc -o output/fable_modules/fable-library-beam output/fable_modules/fable-library-beam/*.erl

# Compile your project files
erlc -pa output/fable_modules/fable-library-beam -o output output/*.erl

Running Erlang Code#

Run your compiled module using the Erlang shell:

erl -pa output -pa output/fable_modules/fable-library-beam \
    -noshell -eval 'program:main(), halt().'

The -pa flag adds 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