otter — write fast and efficient Erlang NIFs in Rust

If you're an Erlang developer trying to write NIFs in Rust, you've probably run into a few obstacles. The de facto library for writing Rust NIFs, rustler, has documentation mostly written for Elixir. Its API wraps the native erl_nif API in ways that block significant capabilities, like NIF upgrades, and obscure others, like improper lists. You can get a NIF to build, but you're flying blind, trusting that the abstraction did the right thing underneath without ever being shown what it did.

otter is an Erlang-first NIF library built on the opposite premise: you should be able to see exactly what's happening. Every type and function maps thinly and explicitly onto the erl_nif call beneath it, so you can read otter's docs next to the erl_nif man pages and know precisely what your NIF does — what call fires, when it raises, how a term is really shaped. Improper lists are first-class cons cells; decoder errors name the actual failure; the full message-send matrix is exposed; and every module is hot-code-upgrade-safe.

use otter::types::CallEnv;

#[otter::nif]
fn add(_env: CallEnv<'_>, a: i64, b: i64) -> i64 {
    a + b
}

otter::init!("my_nifs", [add]);

→ Full quick start, types, and feature tour, see otter/README.md

→ For a detailed, point-by-point comparison with rustler, see docs/RUSTLER.md.

→ For a complete working example app, see otter-demo.

This is the monorepo for otter (published as otter-nif on crates.io). For the introduction, quick start, and feature tour:

• README
• crates.io
• docs.rs

Path	Crate / App	Package	Docs	What it is
otter/	otter-nif			Core Rust library — types, codecs, environment, resources (imported as otter)
otter_codegen/	otter-nif-macros		—	Proc macros (#[otter::nif], otter::init!), re-exported through otter-nif
rebar3_otter/	rebar3_otter			rebar3 plugin — drives cargo build/cargo clean and installs the NIF into priv/
otter_test/	otter_test	—	—	In-tree Erlang app — end-to-end test suite exercising the bridge

You only ever depend on otter-nif (imported as otter); the codegen macros are re-exported through it.

otter-nif — the Rust library

• OTP 26 (NIF 2.17) runtime floor; the optional nif_2_18 feature requires OTP 29 (NIF 2.18)
• Rust edition 2021, MSRV 1.82 (otter-nif-macros 1.56)
• cargo on PATH

rebar3_otter — the rebar3 plugin

• OTP 27 (it uses OTP-27 doc syntax)

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.