Setting up Fennel

This document will guide you through setting up Fennel on your computer. This document assumes you know how to run shell commands and edit configuration files in a UNIX-like environment.

Note: Fennel can be used in non-UNIX environments, but those environments will not be covered in this document.

Downloading Fennel

Downloading Fennel on your computer allows you to run Fennel code and compile to Lua. You have a few options for how to install Fennel.

Downloading the fennel script

Downloading the fennel script allows you to place the script in convenient locations for running Fennel code.

This method assumes you have Lua 5.1, 5.2, 5.3, 5.4, or LuaJIT installed on your system.

This method requires you to manually update the fennel script when you want to use a newer version that has come out.

1. Download the fennel script
2. Run chmod +x fennel-0.6.0 to make it executable
3. Download the signature
4. Run gpg --verify fennel-0.6.0.asc to verify that the fennel script is from the Fennel creators (optional but recommended)
5. Move fennel-0.6.0 to a directory on your $PATH, such as /usr/local/bin

Note: You can rename the script to fennel for convenience. Or you can leave the version in the name, which makes it easy to keep many versions of Fennel installed at once.

Downloading a Fennel binary

Downloading a Fennel binary allows you to run Fennel on your computer without having to download Lua, if you are on a supported platform.

1. Choose one the options below, depending on your system:
   • GNU/Linux x86_64 (signature)
   • GNU/Linux arm32 (signature)
   • Windows x86 32-bit (signature)
2. Run chmod +x fennel-0.6.0* to make it executable (not needed on Windows).
3. Download the signature and confirm it matches using gpg --verify fennel-0.6.0*.asc (optional but recommended).
4. Move the downloaded binary to a directory on your $PATH, such as /usr/local/bin

Downloading Fennel using LuaRocks

LuaRocks contains a repository of Lua software packages. LuaRocks is convenient because it automates the downloading, installation, updating, and uninstallation of Lua software packages.

1. Ensure the luarocks package is installed on your system.
2. Ensure the ~/.luarocks/bin directory is added to your shell’s $PATH.
3. Run one of the following commands:
   • luarocks --local install fennel to install locally (recommended)
   • luarocks install fennel to install globally (requires root or admin)
4. Run fennel --help to confirm the installation succeeded.

Note: When using luarocks with --local, you’ll need to ensure you have a $PATH which contains the LuaRocks bin/ directory. You can add eval $(luarocks path --bin) to your shell’s init file to do this. See the LuaRocks path docs for more information.

Downloading Fennel using Homebrew

Homebrew aims to be the missing package manager for macOS (and Linux). Its primary goal is to be useful to as many people as possible, while remaining maintainable to a professional, high standard by a small group of volunteers.

To download Fennel using Homebrew:

1. Ensure the brew package is installed on your system
2. Run brew install fennel to install fennel
3. Run fennel --help to confirm the installation succeeded

Embedding Fennel

Fennel code can be embedded inside of applications that support Lua either by including the Fennel compiler inside of the application, or by performing ahead-of-time compilation. Embedding Fennel in a program that doesn’t already support Lua is possible but outside the scope of this document.

Note: Embedding the Fennel compiler in an application is the more flexible option, and is recommended. By embedding the Fennel compiler in an application, users can write their own extension scripts in Fennel to interact with the application, and you can reload during development. If the application is more restricted, (for instance, if you can only embed one Lua file into the application and it cannot load further files) then compiling Fennel code to Lua during the build process and including the Lua output in the application may be easier.

Embedding the Fennel compiler in a Lua application

The Fennel compiler can be added to your code repository, and then loaded from Lua.

1. Add fennel.lua to your code repository
2. Add the following lines to your Lua code:

local fennel = require("fennel")
table.insert(package.loaders or package.searchers, fennel.searcher)
local mylib = require("mylib") -- will compile and load code in mylib.fnl

Note: Optionally, if you want the Fennel REPL to print tables in a more readable format, you can add fennelview.fnl to your code repository.

Performing ahead-of-time compilation

If the target system of your application does not make it easy to add the Fennel compiler but has Lua installed, Fennel offers ahead-of-time compilation. This allows you to compile .fnl files to .lua files before shipping an application.

This section will guide you through updating a Makefile to perform this compilation for you.

1. Add the following lines to your Makefile:

   %.lua: %.fnl fennel
       ./fennel --compile $< > $@

2. Ensure your build target depends on the .lua files you need.

Note 1: Ahead-of-time compilation is also useful if what you are working with requires optimal startup time. “Fennel compiles fast, but not as fast as not having to compile.” – jaawerth

Note 2: It’s recommended you include the fennel script in your repository to get consistent results rather than relying on an arbitrary version of Fennel that is installed on your machine at the time of building.

Adding Fennel support to your text editor

You can write Fennel code in any editor, but some editors make it more comfortable than others. Most people find support for syntax highlighting, automatic indentation, and delimiter matching convenient, as working without these features can feel tedious.

Other editors support advanced features like an integrated REPL, live reloading while you edit the program, documentation lookups, and jumping to source definitions.

If your favorite editor isn’t listed here, that’s OK; stick with what you’re most comfortable. You can usually get decent results by telling your editor to treat Fennel files as if they were Clojure or Scheme files.

Adding Fennel support to Emacs

Installing fennel-mode gives you syntax highlighting, indentation, paren-matching, a repl, reloading, documentation lookup, and jumping to source definitions.

For more information, see the fennel-mode readme.

Adding Fennel support to Vim

The fennel.vim plugin offers syntax highlighting and indentation support.

Adding Fennel support to Neovim

• For syntax highlighting and indentation, install fennel.vim
• To spin up a REPL in the terminal buffer, you can install a REPL plugin like iron.nvim and follow the documentation to add fennel as an executable REPL command.

Adding Fennel support to Visual Studio Code

Search in the built-in extension manager for “Fennel” to install the vsc-fennel extension. At the time of this writing it only provides syntax highlighting.

Adding readline support to Fennel

The command-line REPL that comes with the fennel script works out of the box, but the built-in line-reader is very limited in user experience. Adding GNU Readline support enables user-friendly features, such as:

• tab-completion on the REPL that can complete on all locals, macros, and special forms
• a rolling history buffer, which can be navigated, searched (ctrl+r), and optionally persisted to disk so you can search input from previous REPL sessions
• Emacs (default) or vi key binding emulation via readline’s custom support for better line navigation
• optional use of additional readline features in ~/.inputrc, such as blinking on matched parentheses or color output (described below)

Requirements for readline support

• GNU Readline (installation steps vary for different operating systems, but you may already have it!)
• readline.lua Lua bindings to libreadline

Note: The stock Fennel REPL will automatically load and use the readline bindings when it can resolve the readline module, so that’s all you need to get started.

Installing readline.lua with LuaRocks

The easiest way to get readline.lua is to install it with LuaRocks, which will fetch the package and automatically compile the native bindings for you.

Tip: If you don’t want to use LuaRocks, you can manually install readline.lua by following the guide here.

To install readline.lua with LuaRocks:

1. Ensure libreadline is installed for the Lua version you intend to use.
2. Run one of the following commands:
   • luarocks install --local readline (recommended)
   • luarocks install --lua-version=5.1 readline (for a non-default Lua version)
   • luarocks install readline (requires root or admin)

Note: If you’ve installed with the --local flag, you may need to ensure your package.path and package.cpath contain its location. See the note on LuaRocks installation for details.

Configuring readline.lua

You can configure readline.lua using one of the following options:

• the readline.lua API in fennelrc
• the readline.lua ~/.inputrc file

Enabling persistent history using fennelrc

To configure the REPL to save the rolling history to file at the end of every session, add the following to your fennelrc with your desired filename:

See the readline.lua documentation for information on its API, most notably other parameters that can be set via rl.set_options.

; persist repl history
(match package.loaded.readline
  rl   (rl.set_options {:histfile  "~/.fennel_history" ; default:"" (don't save)
                        :keeplines 1000}))             ; default:1000

Configuring readline in ~/.inputrc

See the documentation on the readline init file for the full set of options and a sample inputrc.

The following example adds these behaviors:

• Blink on a matching parenthesis when entering ). Useful in a Lisp REPL, where the parens are plentiful!
• Enable bracketed paste mode for more reliable pasting from clipboard
• When tab-completing on a term with more than one possible match, display all candidates immediately instead of ringing the bell + requiring a second <tab>

Create a ~/.inputrc file with the following contents:

set enable-bracketed-paste on
set blink-matching-paren on
set show-all-if-ambiguous on

As of Fennel 0.4.0 and readline.lua 2.6, you can make use of a conditional directive your inputrc if you would like certain settings to only apply to Fennel.

Making games with Fennel

The two main platforms for making games with Fennel are TIC-80 and LÖVE.

TIC-80 is software that acts as a simulated computer in which you can write code, design art, compose music, and lay out maps for games. TIC-80 also makes it easy for you to publish and share the games you make with others. TIC-80 introduces restrictions such as low resolution and limited memory to emulate retro game styles.

LÖVE is a game-making framework for the Lua programming language. Because Fennel compiles to Lua, you can reference the LÖVE wiki when making games with Fennel. LÖVE is more flexible than TIC-80 in that it allows you to import from external resources and use any resolution or memory you like, but at a cost in that it is more complicated to make games in.

Both TIC-80 and LÖVE offer cross-platform support across Windows, Mac, and Linux systems, but TIC-80 games can be played in the browser and LÖVE games cannot.

The Fennel wiki links to many games made in both systems you can study.

Using Fennel in TIC-80

Support for Fennel is built into TIC-80. If you want to use the built-in text editor, you don’t need any other tools, just launch TIC-80 and run new fennel to get started.

• The TIC-80 wiki documents the functions to use and important concepts.
• See this project skeleton repo for information on using external editors, instead of the built-in TIC-80 editor.

All TIC-80 games allow you to view and edit the source and assets. Try loading this Conway’s Life game to see how it’s made:

• Click “start” to begin
• Press the Esc key to open a menu
• Use the arrow keys to navigate the menu
• Press the Z key to open the console, followed by Esc to see the source code.

Using Fennel with LÖVE

LÖVE has no built-in support for Fennel, so you will need to setup support yourself, similar to Embedding Fennel above.

This project skeleton for LÖVE shows you how to setup support for Fennel and how to setup a console-based REPL for debugging your game while it runs.

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