eetf

Imported from sile/eetf (skills/eetf/SKILL.md). Install upstream with npx skills add sile/eetf --skill eetf. Copyright stays with the author (MIT).

eetf — Erlang External Term Format for Rust

eetf is a pure-Rust implementation of Erlang's External Term Format. It encodes and decodes the same binary payloads that erlang:term_to_binary/1 and :erlang.term_to_binary/1 (Elixir) produce.

Use this skill when the user is:

• Decoding bytes that start with 131 (0x83, the ETF version tag) into a Rust value.
• Building a payload to hand back to Erlang/Elixir (Erlang distribution, Erlang ports, ErlPort, Mnesia, :erlang.binary_to_term/1).
• Pattern-matching against decoded terms (e.g. {ok, Value}-shaped tuples).
• Converting between BEAM types (atoms, binaries, pids, refs, big integers, maps, improper lists) and Rust types.

Setup

Add to Cargo.toml:

[dependencies]
eetf = "0.11"

The crate uses Rust edition 2024. It pulls in num-bigint, num-traits, and noflate (zlib decompression for the COMPRESSED ETF tag). No unsafe, no async, no feature flags.

Core API at a glance

Everything lives on Term. Decoding takes any io::Read; encoding takes any io::Write.

use eetf::Term;
use std::io::Cursor;

// Decode
let term = Term::decode(Cursor::new(&bytes))?;

// Encode
let mut buf = Vec::new();
term.encode(&mut buf)?;

Term is an enum over every ETF type:

Every concrete type implements Display, Debug, PartialEq, Hash (except Float needs care), and Clone. Most also impl From<...> from common Rust types — Atom::from("foo"), FixInteger::from(42i32), Binary::from(vec![1,2,3]), Tuple::from(vec![Term::from(...)]), etc.

Errors are DecodeError and EncodeError (both std::error::Error). Their type aliases are DecodeResult = Result<Term, DecodeError> and EncodeResult = Result<(), EncodeError>.

Decode example

use eetf::{Term, Atom};
use std::io::Cursor;

let bytes = vec![131, 119, 3, 102, 111, 111]; // term_to_binary(foo)
let term = Term::decode(Cursor::new(&bytes)).unwrap();
assert_eq!(term, Term::from(Atom::from("foo")));

Reading from a file:

use std::fs::File;
let term = Term::decode(File::open("snapshot.etf")?)?;

The decoder requires the leading version byte 131. Strip it yourself if you receive the content without the version prefix (rare).

Encode example

use eetf::{Term, Tuple, Atom, FixInteger};

let term = Term::from(Tuple::from(vec![
    Term::from(Atom::from("ok")),
    Term::from(FixInteger::from(42)),
]));

let mut buf = Vec::new();
term.encode(&mut buf).unwrap();
// buf[0] == 131

Writing to a TcpStream, BufWriter, etc. is identical — anything implementing io::Write.

Pattern matching

The pattern module and Term::as_match give a typed shape-matching API. Useful for picking apart {ok, Value} / {error, Reason} style replies without writing nested if let chains.

use eetf::{Term, Tuple, Atom, FixInteger};
use eetf::pattern::any;

let t = Term::from(Tuple::from(vec![
    Term::from(Atom::from("ok")),
    Term::from(FixInteger::from(7)),
]));

// Match against (literal-atom, any-FixInteger)
let (_ok, n) = t.as_match(("ok", any::<FixInteger>())).unwrap();
assert_eq!(n.value, 7);

Useful pattern building blocks (in eetf::pattern):

• Tuple-shaped patterns via Rust tuples — ("foo", any::<Atom>()) matches a 2-tuple of literal atom foo and any atom.
• any::<T>() — accept any value of type T (Atom, FixInteger, BigInteger, Float, Binary, Tuple, List, Map, etc.).
• Union2 … Union6 — alternation between several pattern shapes; returns a tagged enum.
• U8, I32, etc. — integer-range patterns.
• Unmatch carries input, pattern, and an optional cause chain so failures are debuggable.

For ad-hoc extraction, convert::TryAsRef is the lower-level escape hatch:

use eetf::convert::TryAsRef;
use eetf::{Term, Atom};

let t: Term = /* ... */;
if let Some(atom) = <Term as TryAsRef<Atom>>::try_as_ref(&t) {
    println!("{}", atom.name);
}

Term also supports TryInto<T> for each variant — let atom: Atom = term.try_into()?; (errors return the original term back, not a typed error).

Common gotchas

1. ByteList vs List<FixInteger>

Erlang represents short lists of bytes ([1,2,3] where every element fits in u8) using STRING_EXT. eetf decodes those into Term::ByteList(ByteList { bytes: Vec<u8> }), not Term::List. If you blindly match on Term::List, you'll miss byte-list payloads.

To normalize, convert: List::from(byte_list) produces a List of FixIntegers. To accept either, match both variants or pattern with a union.

String::from(...) -> Term::ByteList(...) (i.e. Term::from("hello") becomes ByteList, not a binary). If you want a <<"hello">> Erlang binary, use Term::from(Binary::from(b"hello".as_slice())).

2. Atoms are NOT strings

Erlang atoms (foo) and binaries (<<"foo">>) are distinct types. Sending Term::from(Atom::from("foo")) is not the same as Term::from(Binary::from(b"foo".as_slice())) on the receiving Erlang side. When in doubt, mirror the producer side.

3. Float rejects non-finite values

Float only stores finite numbers. Both TryFrom<f32> and TryFrom<f64> return Err(DecodeError::NonFiniteFloat) for NaN/±∞ — there is no From<f64> for Float. ETF itself has no representation for non-finite floats, so this matches the wire format.

use eetf::Float;
let f = Float::try_from(1.5_f64)?; // OK
assert!(Float::try_from(f64::NAN).is_err());

4. Atom name length cap on encode

EncodeError::TooLongAtomName fires if an atom exceeds 255 UTF-8 bytes. Erlang atoms have the same limit, so this is a real wire-format constraint, not a library quirk.

5. Map does not preserve insertion order

Internally Map wraps HashMap<Term, Term>. Round-tripping a map will re-encode in arbitrary key order. Tests that assert on raw byte equality of encoded maps are flaky; assert on the decoded Term instead.

6. Compressed payloads are handled transparently

ETF can wrap a payload with the COMPRESSED (zlib) tag. Term::decode decompresses automatically via noflate; you don't need to detect it yourself. Term::encode does not compress — if you need a compressed wire format, you must wrap externally.

7. PID/Port/Reference equality includes creation

Two PIDs from different node restarts have different creation values and won't compare equal. If you want to ignore creation (e.g. to dedupe across restarts), strip it before comparing.

8. The version byte is required

Decoding bytes that don't start with 131 yields DecodeError::UnsupportedVersion { version }. If you receive an Erlang distribution payload that has been pre-stripped, prepend 131 before calling decode.

Constructing common shapes (cheat sheet)

use eetf::{Term, Atom, FixInteger, Tuple, List, Binary, Map};

// {ok, 42}
Term::from(Tuple::from(vec![
    Term::from(Atom::from("ok")),
    Term::from(FixInteger::from(42)),
]));

// [1, 2, 3] as a proper list (NOT ByteList)
Term::from(List::from(vec![
    Term::from(FixInteger::from(1)),
    Term::from(FixInteger::from(2)),
    Term::from(FixInteger::from(3)),
]));

// <<"hello">>
Term::from(Binary::from(b"hello".as_slice()));

// #{name => "alice", age => 30}
Term::from(Map::from([
    (Term::from(Atom::from("name")), Term::from(Binary::from(b"alice".as_slice()))),
    (Term::from(Atom::from("age")),  Term::from(FixInteger::from(30))),
]));

Error handling

use eetf::{Term, DecodeError};
use std::io::Cursor;

match Term::decode(Cursor::new(&bytes)) {
    Ok(t) => { /* ... */ }
    Err(DecodeError::UnsupportedVersion { version }) => { /* not 131 */ }
    Err(DecodeError::UnknownTag { tag }) => { /* corrupt or future tag */ }
    Err(DecodeError::UnexpectedType { value, expected }) => { /* shape mismatch */ }
    Err(DecodeError::OutOfRange { value, range }) => { /* bigint exceeded i32 etc. */ }
    Err(DecodeError::NonFiniteFloat) => { /* unreachable for valid ETF, but possible from TryFrom */ }
    Err(DecodeError::Io(e)) => { /* underlying reader error */ }
}

EncodeError is much smaller: Io, TooLongAtomName, TooLargeInteger, TooLargeReferenceId. The last three correspond to wire-format limits — there is no recovery besides changing the input.

Interop notes

• Producing input for :erlang.binary_to_term/1 / binary_to_term/1: Encode with eetf, send the raw bytes (including the leading 131). For :erlang.binary_to_term(bin, [:safe]), ensure all atoms in the payload already exist in the receiving VM, or the call fails.
• Distribution protocol payloads: ETF is the carrier, but distribution adds its own framing on top. eetf only handles the term encoding, not the framing.
• .beam files: BEAM chunks are not raw ETF; they have their own container format. Use a dedicated BEAM parser for those (eetf only decodes the embedded ETF chunks if you locate them yourself).

Verifying your work

The repo's own test suite is a good reference for byte-level expectations: see tests/lib.rs for round-trip assertions per term type. When debugging an unexpected encoding, compare bytes against :erlang.term_to_binary(value) from an iex/erl shell.

# Quick sanity check from an Erlang shell:
# erl -noshell -eval 'io:format("~w~n", [binary_to_list(term_to_binary({ok,42}))]), halt().'