Getting Started


NOTE: This section is a placeholder for the moment until we get our toolchain packaging implemented

To use Firefly, you’ll need to download our toolchain here, and install it like so:

This will install the firefly executable to /usr/local/bin/firefly and various supporting files to appropriate locations under /usr/local.
If you install to a different target directory, make sure you add the bin folder to your PATH, for example:


NOTE: This section reflects the way Firefly is supposed to work, but the current implementation may
lack functionality described here. However, if you encounter something broken, feel free to open an issue
if there isn’t one already, and we’ll ensure it gets tracked as we continue to improve the project.

You should now be able to run firefly, start by reviewing the output of the help command:

This will print out the various ways you can use the Firefly compiler. Obviously, the most interesting is the ability
to compile some code, so let’s see how that works.

Firefly can compile executables or static/dynamic libraries. By default an executable is produced
which acts very similar to an OTP release, but this behavior can be customized depending on how
you want the executable to be used. For example, when compiling an executable that you wish to use
as a CLI tool, you might want a specific module/function to always be called with the arguments passed
to the program when the system has booted. There are options to the compile command that allow you to
do this, and much more. It is even possible to provide your own init, and handle everything manually,
including management of the core supervision tree.

NOTE: Firefly does not do any dependency management itself, and we have not yet provided Rebar/Mix
shims to integrate Firefly as a compiler with those tools, so compiling an application and all of its
dependencies is still somewhat of a manual process. This will be addressed in the near future.

Firefly allows you to compile Erlang sources a couple of ways, let’s take a look at each:

Compiling Files/Directories

NOTE: Since Firefly is not compiling for a virtual machine, it does not produce BEAM bytecode like erlc
does. Instead, it produces either an executable or a static/dynamic library depending on the type of
application being compiled, unless otherwise overridden by compiler options. By default, for
OTP applications (i.e. apps which define the mod key in their application manifest), Firefly will produce
an executable that runs that application much like an OTP release would. For library applications (i.e. apps
that do not define the mod key in their manifest), Firefly will produce a static library which can be linked
into an executable at another time. You may specify --bin to force production of an executable, or --lib to
force production of a library. If you want to compile a shared library, pass --lib --dynamic.

You can compile one or more Erlang source files by specifying their paths like so:

NOTE: Since there is no application manifest available, these sources will be treated as modules of an anonymous
library application with the same name as the current working directory. As such, the output of the above
command will be a static library.

Alternatively, you can compile all the files found in one or more directories, by specifying their path:

In this case, each directory will be treated as an application; if no .app or .app.src manifest is found in
a directory, then a new anonymous library application with the same name as the directory will be used as the
container for the sources in that directory. Since no root application manifest was provided, an anonymous library
application with the same name as the current working directory will be used as the “root” of the dependency tree.
In other words, if the name of our current directory is myapp, then in the example above, app1 and app2 will
be treated as dependencies of the myapp application, and will result in a static library being produced containing
all three applications.

When specifying files individually on the command line, you can control the default app properties using compiler flags,
and use that to manage what type of output gets produced. For example:

This is equivalent to:

Where src/myapp.app looks like:

{application, myapp, [{mod, {foo, []}}]}.

In both cases, the result is an executable containing the myapp application, which consists of two modules: foo and bar.

Let’s assume that src/foo.erl contains the following:


Then we should see the following when we run our compiled executable:

NOTE: The directory under _build/firefly contains the target triple the executable was compiled for, and
since this example was compiled on an M1, the triple reflects that.

Compiling Projects

Firefly also recognizes the conventional Erlang project structure. For example, let’s say you have an application called hello:


Where hello.app contains:

{application, hello, [{vsn, "1.0"}, {mod, {hello, []}}]}.

and hello.erl contains:



start(_, _) ->
  erlang:display(<<"hello world!">>),

From the root of the hello/ directory, you can compile this to an executable like so:

If we run it, it should print our greeting:


> _build/firefly/arm64-apple-macosx11.0.0/hello
<<"hello world!">>

NOTE: The directory under _build/firefly contains the target triple the executable was compiled for, and
since this example was compiled on an M1, the triple reflects that.

If you instead wish to compile hello as a library, you can compile with:

This will produce the static archive _build/firefly//hello.a.

If you want to compile an application and link in a previously-compiled library, you can do that like so:

This tells the compiler to use _build/firefly// as a search path for the linker, and to link the library named foo.

Replacing Erlc

Now that you’ve learned how to use Firefly to compile Erlang sources, what’s the best approach for compiling a real world Erlang

Let’s assume you are in a directory containing a standard Erlang project called myapp, and all of your dependencies are located
in the _build/default/lib (the default for rebar3), then the following will compile your application and all its declared dependencies
(based on the app manifest) into an executable:

This works because Firefly has an application manifest to work from, and can infer the location of the sources for the dependencies.

However, if your project is less conventional, then you might want to follow a different approach instead, by compiling
each application to a library, and then compiling the root application as an executable while linking in all of the dependencies:

The above assumes that deps/foo and deps/bar are directories containing application manifests, and compiles both to static libraries
in the current working directory. The last line will create an executable containing the foo and bar applications, as well as the application
contained in src.

This method is more manual, but provides a lot of flexibility for those who need it.


NOTE: This is primarily for experimentation and development work, but might be of interest to
those interested in implementing inits, or even alternative Erlang standard libraries.

To compile an executable which simply invokes init:boot/1 and leaves the definition of that function
up to you, you can use the following:

The resulting executable performs none of the default initialization work that the standard runtime normally
does, i.e. there is no init, so no application master/controller, and as a result, none of the normal OTP
startup sequence occurs. This does however provide you an opportunity to handle this yourself, however you like;
albeit with the major caveat that using any standard library modules without doing the proper initialization
or providing the things needed by those modules will almost certainly fail. Erlang without the default
init is a very interesting environment to play in!

As an example, consider if init.erl above is defined as the following:

Running the resulting executable will print the default arguments the runtime provides to the init
and then exit.


In order to build Firefly, or make changes to it, you’ll need the following installed:


First, you will need to install rustup. Follow the instructions at that link.

Once you have installed rustup, you will need to install the nightly version
of Rust (currently our CI builds against the 2022-08-08 nightly, specifically). We require
nightly due to a large number of nightly features we use, as well as some
dependencies for the WebAssembly targets that we make use of.

# to use the latest nightly
rustup default nightly

# or, in case of issues, install the specific nightly to match our CI
rustup default nightly-2022-11-02
export CARGO_MAKE_TOOLCHAIN=nightly-2022-11-02

In order to run various build tasks in the project, you’ll need the cargo-make plugin for Cargo. You can install it with:

You can see what tasks are available with cargo make --print-steps.

You may also want to install the following tools for editor support (rustfmt will be required on
all pull requests!):

rustup component add rustfmt clippy

Next, for wasm32 support, you will need to install the wasm32 targets for the Rust toolchain:


LLVM is used internally for the final code generation stage. In order to build the compiler,
you must have our expected version of LLVM (currently LLVM 15) installed somewhere locally.

LLVM releases are posted here.

Linux (x86_64)

Follow the install instructions here for Debian or Ubuntu.
For other distros, you are going to have to check whether our required LLVM version is
available via your package manager, and either use that, or build LLVM from source. Ideally
you won’t need to do the latter, but you can also try slightly newer LLVM releases as well,
if they are available, which may also work, but is not a supported configuration.

Once installed, make sure you export LLVM_PREFIX in your environment when working in the
Firefly repo, e.g. building the compiler.

macOS (arm64 and x86_64)

Go to the releases page mentioned above, and follow the download link to the GitHub releases page.
From here, you’ll want to select the clang+llvm package which matches your platform, then follow
the instructions below. NOTE: The following instructions use the arm64 release for the example:

mkdir -p $XDG_DATA_HOME/llvm/
cd $XDG_DATA_HOME/llvm/
wget 'https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/clang+llvm-15.0.7-arm64-apple-darwin22.0.tar.xz'
tar -xzf clang+llvm-15.0.7-arm64-apple-darwin22.0.tar.xz
mv clang+llvm-15.0.7-arm64-apple-darwin22.0.tar.xz firefly
rm clang+llvm-15.0.7-arm64-apple-darwin22.0.tar.gz
cd -
export LLVM_PREFIX="${XDG_DATA_HOME}/llvm/firefly"

Building Firefly

Once LLVM is installed, you can build the firefly executable!

NOTE: Firefly has components that need to be compiled with clang; On Linux, the default compiler is generally gcc.
You’ll need to make sure to use clang instead. The LLVM toolchain should include clang, but you may
need to install it separately from your package manager. Then, export the following environment variables
when building Firefly:

export CC=$XDG_DATA_HOME/llvm/firefly/bin/clang
export CXX=$XDG_DATA_HOME/llvm/firefly/bin/clang++

To build Firefly, run the following:

LLVM_PREFIX=$XDG_DATA_HOME/llvm/firefly cargo make firefly

NOTE: If you have .direnv installed, run direnv allow in the project root, and you can omit all
of the above environment variables, and instead modify the .envrc file if needed.

This will create the compiler executable and associated toolchain for the host
machine under bin in the root of the project. You can invoke firefly via the
symlink bin/firefly, e.g.:

You can compile an Erlang file to an executable (currently only on x86_64/AArch64):

This will produce an executable with the same name as the source file in the
current working directory with no extension (except on Windows, where it will
have the .exe extension).

NOTE: Firefly is still in a very experimental stage of development, so stability is not guaranteed.

Project Structure

Firefly is currently divided into three major components:


The Firefly compiler is composed of many small components, but the few most interesting are:

  • firefly is the crate for the firefly executable itself, but it is largely empty,
    most of the meat is in other crates
  • compiler/driver, handles driving the compiler, i.e. parsing arguments, handling commands,
    and orchestrating tasks; it also defines the compiler pipeline. It currently also contains the
    passes which perform code generation.
  • compiler/syntax_base, contains common types and metadata used in many places across the compiler
  • compiler/syntax_*, these crates implement the frontend and middle-tier of the compiler.
    Each one provides an intermediate representation, semantic analysis, transforms to other representations,
    output to a textual format, and in some cases, parsing.

    • syntax_erl is the primary frontend of the compiler, which handles Erlang sources
    • syntax_pp, is a frontend for Abstract Erlang Format, and converts to the AST from syntax_erl
    • syntax_core is a high-level intermediate representation to which the AST is lowered after some initial
      semantic analysis. Core is an extended form of lambda calculus, so is highly normalized, and is where we do
      the remaining semantic analysis tasks. Core is also where comprehension and receive expressions are transformed
      into their lower level representations. Core is logically split into two forms, “internal” and regular. The
      internal form is used during initial lowering, before certain transformations have been applied. The regular form
      of Core has certain properties which must be upheld, and the internal form does not enforce them.
    • syntax_kernel is a specialized intermediate representation to which Core is lowered. It is completely flat,
      i.e. no nested scopes, variables are unique, all function calls are resolved, dynamic apply is converted
      to a call to erlang:apply/2,3, and all closures have been lifted. During the lowering from Core, pattern matching
      compilation is performed, and some liveness analysis is performed.
    • syntax_ssa is a low-level SSA IR to which Kernel is lowered. While Kernel is flat, it is still expression based,
      SSA breaks things down further into blocks, jumps, and instructions for a register-based machine.
      From here, we can perform code generation either to native code, or bytecode.
  • compiler/linker, performs all the work needed to link generated code into objects/libraries/executables.

The remaining crates under compiler/ are support crates of some kind.


There are a number of core libraries that are used by the runtime, but are also in some cases shared
with the compiler. These are designed to either be optional components, or part of a tiered system of
crates that build up functionality for the various runtime crates.

  • library/system, provides abstractions over platform-specific implementation details that most of the runtime
    code doesn’t need to know about. This primarily handles unifying low-level platform APIs.
  • library/alloc, provides abstractions for memory management
  • library/arena, this is a helper crate that provides an implementation of both typed and untyped arenas
  • library/rt, this is the primary core runtime library, hence the name, and provides the implementations of all the
    term types and their native APIs, as well as establishing things like the calling convention for Erlang functions,
    exceptions and backtraces, and other universal runtime concerns that cannot be delegated to a higher-level runtime crate.
  • library/binary, this crate provides all the pieces for implementing binaries/bitstrings, including pattern matching
    primitives and constructors.
  • library/number, this crate provides the internal implementation of numeric types for both the compiler and runtime
  • library/beam, this crate provides native APIs for working with BEAM files
  • library/bytecode, this crate defines the opcodes for the bytecode emulator, as well as support for the binary bytecode format


The runtime is intended to be pluggable, so it consists of a “wrapper” crate that provides the entry point
for executables, and a specific runtime implementation. Currently there is only one of the latter.

  • runtimes/crt, plays the role of crt0 in our system, i.e. it provides the entry point, initializes
    the function and atom tables, and invokes the main function of the linked-in runtime.
  • runtimes/emulator, provides a runtime which operates on the output of the bytecode compiler

Making Changes

At this stage of the project, it is important that any changes you wish to contribute are communicated
with us first, as there is a good chance we are working on those areas of the code, or have plans around
them that will impact your implementation. Please open an issue tagged appropriately based on the part of
the project you are contributing to, with an explanation of the issue/change and how you’d like to approach
implementation. If there are no conflicts/everything looks good, we’ll make sure to avoid stepping on your
toes and provide any help you need.

For smaller changes/bug fixes, feel free to open an issue first if you are new to the project and
want some guidance on working through the fix. Otherwise, it is acceptable to just open a PR
directly with your fix, and let the review happen there.

Always feel free to open issues for bugs, and even perceived issues or questions, as they can be a
useful resource for others; but please do make sure to use the search function to avoid

If you plan to participate in discussions, or contribute to the project, be aware that this project
will not tolerate abuse of any kind against other members of the community; if you feel that someone
is being abusive or inappropriate, please contact one of the core team members directly (or all of
us). We want to foster an environment where people both new and experienced feel welcomed, can have
their questions answered, and hopefully work together to make this project better!

About Firefly

Firefly is not only a compiler, but a runtime as well. It consists of two parts:

  • A compiler for Erlang to native code for a given target (x86, ARM, WebAssembly)
  • An Erlang runtime, implemented in Rust, which provides the core functionality
    needed to implement OTP

The primary motivator for Firefly’s development was the ability to compile Elixir
applications that could target WebAssembly, enabling use of Elixir as a language
for frontend development. It is also possible to use Firefly to target other
platforms as well, by producing self-contained executables on platforms such as x86.

Firefly is different than BEAM in the following ways:

  • It supports compilation to standalone executables
  • It is designed to support targeting WebAssembly, as well as many other types of targets.
  • It is designed to support both ahead-of-time compilation to machine code, and compilation to bytecode
  • It sacrifices some features to enable additional optimizations, in particular we don’t
    have plans currently to support hot code reloading.
  • It is written as a way to better understand the BEAM itself, and one of its goals
    is to provide a more accessible means of learning how the BEAM works internally, to
    the degree that we provide the same functionality as the BEAM. By implementing it in
    Rust, we also hope to learn how implementing something like the BEAM in a much more restrictive,
    but safe language impacts its development.


  • Support WebAssembly/embedded systems as a first-class platforms
  • Produce easy-to-deploy static executables as build artifacts
  • Integrate with tooling provided by BEAM languages
  • Feature parity with mainline OTP (with exception of the non-goals listed below)


  • Support for hot upgrades/downgrades
  • Support for dynamic code loading

Firefly is an alternative implementation of Erlang/OTP, so as a result it is not as battle tested, or necessarily
as performant as the BEAM itself. Until we have a chance to run some benchmarks, it is hard to know
what the difference between the two in terms of performance actually is.

Firefly is not intended to replace BEAM at this point in time. At a minimum, the stated non-goals
of this project mean that for at least some percentage of projects, some required functionality would
be missing. However, it is meant to be a drop-in replacement for applications which are better served
by its feature set.



The compiler frontend accepts Erlang source files. This is parsed into an
abstract syntax tree, then lowered through a set of intermediate representations
where different types of analysis, transformation, or optimization are performed:

  • Core IR (similar to Core Erlang)
  • Kernel IR (similar to Kernel Erlang)
  • SSA IR (a low-level representation used to prepare for code generation)
  • Bytecode/MLIR (where final optimizations and code generation are performed)

The final stage of the compiler depends on whether compiling to bytecode or native
code, but in both cases the output produces LLVM IR that is then compiled to one
or more object files, and linked via our linker into an executable.


The runtime design is mostly the same as OTP, but varies a bit on what type of codegen backend was used.
In general though:

  • The entry point sets up the environment, and starts the scheduler
  • The scheduler is composed of one scheduler per thread
  • Each scheduler can steal work from other schedulers if it is short on work
  • Processes are spawned on the same scheduler as the process they are spawned from,
    but a scheduler is able to steal them away to load balance
  • I/O is asynchronous, and integrates with the signal management performed by the schedulers

The initial version is quite spartan, but should be easy to grow now that we have some of the
fundamentals built out.


Currently it is straightforward to extend the runtime with native functions implemented in Rust,
without all of the extra stuff that goes into port drivers and erl_nif. Currently we have some
of the baseline infrastructure in place to support port drivers, but none of the necessary pieces
to support erl_nif as of yet. We’ll need to start adding some of that in short order however, so
it is on our roadmap to support NIFs to at least a minimum degree needed for things required by
the standard library.


Firefly previously had the name “Lumen”. This was intended to be a temporary name and it was
changed in 2022, partly due to there being numerous other projects also named Lumen.


Apache 2.0

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By |2023-04-23T22:17:16+00:00April 23, 2023|Technology|0 Comments

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