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Bytecode-compiling Unlambda 2.0 interpreter
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An Unlambda interpreter in Rust

Relambda is a bytecode-compiling Unlambda interpreter in Rust.

$ relambda
>> `r`.!`.d`.l`.r`.o`.w`. `.,`.o`.l`.l`.e`.Hi
Hello, world!

You can find more sample programs at the comprehensive Unlambda archive network, and a tutorial and language spec on the Unlambda homepage.

Try it out!

Make sure you have cargo installed.

git clone
cd relambda
cargo run

Language support

Relambda supports Unlambda 2.0. It supports arbitrary Unicode characters after ., where the standard supports ASCII. This means that you cannot print out raw bytes in the 127-255 range. Code is case insensitive, except for . characters. Comments are supported.

Design notes

Unlambda is compiled to a 6-instruction bytecode. Due to its very dynamic nature, most of the work is dynamically dispatched by the Invoke opcode.


Some integ tests are included. I've tested most of the programs in the CUAN ( The test-runner script will run the suite of tests at (which you need to download if you want to run them—I am not including them here as this file has no copyright information.)


Very informal testing suggests that this interpreter is quite a bit faster than the C-refcount interpreter included in the official CUAN distribution. It's 2/3 as much code, but Rust is a higher-level language than C, and uses dependencies to manage argument parsing and output level control.

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