Fennel 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.
At https://fennel-lang.org there's a live in-browser repl you can use without installing anything.
- The setup guide is a great place to start
- The tutorial teaches the basics of the language
- The rationale explains the reasoning of why Fennel was created
- The reference describes all Fennel special forms
- The API listing shows how to integrate Fennel into your codebase
- The Lua primer gives a very brief intro to Lua with pointers to further details
The changelog has a list of user-visible changes for each release.
(print "hello, world!")
(fn fib [n] (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2))))) (print (fib 10))
Building Fennel from source
Building Fennel from source allows you to use versions of Fennel that haven't been released, and makes contributing to Fennel easier.
To build Fennel from source
cdto a directory in which you want to download Fennel, such as
git clone https://git.sr.ht/~technomancy/fennel
make fennelto create a standalone script called
- Copy or link the
fennelscript to a directory on your
$PATH, such as
Note: If you copied the
fennel script to one of the
directories on your
$PATH, then you can run
fennel filename.fnl to
run a Fennel file anywhere on your system.
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
- Opt-in mutability for local variables
- Opt-in arity checks for
- Pattern matching
- Ability to extend the syntax with your own macros and special forms
Differences from other lisp languages
- Its 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.)
Why not Fennel?
Fennel inherits the limitations of the Lua runtime, which does not offer pre-emptive multitasking or OS-level threads. Libraries for Lua work great with Fennel, but the selection of libraries is not as extensive as it is with more popular languages. While LuaJIT has excellent overall performance, purely-functional algorithms will not be as efficient as they would be on a VM with generational garbage collection.
Even for cases where the Lua runtime is a good fit, Fennel might not be a good fit when end-users are expected to write their own code to extend the program, because the available documentation for learning Lua is much more readily-available than it is for Fennel.
- Join the
#fennelchat thru IRC on Freenode or on Matrix
- The mailing list has slower-paced discussion and announcements
- You can browse and edit the Wiki
Copyright © 2016-2020 Calvin Rose and contributors
Released under the MIT license.