Fennel

Fennel (formerly fnl) is a lisp that compiles to Lua. It aims to be easy to use, expressive, and has almost zero overhead compared to handwritten Lua.

Full Lua compatibility - You can use any function or library from Lua.
Zero overhead - Compiled code should be just as or more efficient than hand-written Lua.
Compile-time macros - Ship compiled code with no runtime dependency on Fennel.
Embeddable - Fennel is a one-file library as well as an executable. Embed it in other programs to support runtime extensibility and interactive development.

Documentation

The tutorial is a great place to start
The reference describes all Fennel special forms
The API listing shows how to integrate Fennel into your codebaes
The Lua primer gives a very brief intro to Lua with pointers to further details
The test suite has basic usage examples for most features.

For a small complete example that uses the LÖVE game engine, see pong.fnl.

Example

Hello World

(print "hello, world!")

Fibonacci sequence

(fn fib [n]
 (if (< n 2)
  n
  (+ (fib (- n 1)) (fib (- n 2)))))

(print (fib 10))

Try it

At https://fennel-lang.org there's a live in-browser repl you can use without installing anything.

Otherwise clone this repository, and run ./fennel --repl to quickly start a repl.

The repl will load the file ~/.fennelrc on startup if it exists.

Differences from Lua

Syntax is much more regular and predictable (no statements; no operator precedence)
It's impossible to set or read a global by accident
Pervasive destructuring anywhere locals are introduced
Clearer syntactic distinction between sequential tables and key/value tables
Separate looping constructs for numeric loops vs iterators instead of overloading for
Opt-in mutability for local variables
Opt-in arity checks for lambda functions
Ability to extend the syntax with your own macros and special forms

Differences from other lisp languages

Lua VM can be embedded in other programs with only 180kb
Access to excellent FFI
LuaJIT consistently ranks at the top of performance shootouts
Inherits aggressively simple semantics from Lua; easy to learn
Lua VM is already embedded in databases, window managers, games, etc
Low memory usage
Readable compiler output resembles input

(Obviously not all these apply to every lisp you could compare Fennel to.)

Install with Luarocks

You can install the dev package from luarocks via

luarocks install --server=http://luarocks.org/dev fennel

This will install both the fennel module, which can be required into via local fennel = require 'fennel', as well as the fennel executable which can be used to run a repl or compile Fennel to Lua.

To start a repl:

fennel --repl

To compile a file:

fennel --compile myscript.fnl > myscript.lua

When given a file without a flag, it will simply load and run the file.

Resources

Emacs support
Wiki
Build:
The #fennel IRC channel is on Freenode

License

Released under the MIT license

ntdef/Fennel