Fennel: Lua-based Lisp Language

Exploring Fennel: A Lua Lisp Language

Fennel is a modern, powerful, and compact Lisp dialect that brings together the syntax and capabilities of the Lisp programming language with the efficiency and flexibility of Lua. This blend allows developers to enjoy the best of both worlds, combining the readability and expressiveness of Lisp with the performance and accessibility of Lua. Created by Calvin Rose in 2016, Fennel offers a unique perspective on the dynamic language ecosystem, providing an alternative for those seeking a more structured yet flexible development environment.

Background of Fennel: A Lua-Based Lisp

Fennel is designed as a Lisp that compiles to Lua code, offering the rich features of a Lisp language such as macros and a powerful functional paradigm, while leveraging the ease of integration with the Lua ecosystem. The development of Fennel began in 2016 with the intention of filling a niche for those who enjoyed the declarative nature of Lisp but needed to work within the Lua runtime environment. Lua, with its lightweight footprint and extensive usage in game development, embedded systems, and scripting, made it an ideal target for this Lisp-inspired language.

One of the key motivations behind Fennel was to make it easier to integrate with existing Lua codebases, which are known for their simplicity and speed. By compiling directly to Lua, Fennel provides a way for developers to use the expressive power of Lisp while still maintaining compatibility with the Lua ecosystem.

Core Features of Fennel

Fennel offers several features that make it an attractive choice for developers looking for a Lisp dialect that is both powerful and efficient:

1. Lisp Syntax with Lua Interoperability

• Fennel adopts the parenthesized, symbolic syntax of Lisp, making it familiar to those already acquainted with Lisp-based languages. The language’s core syntax revolves around expressions, where almost everything is treated as an expression.
• The major strength of Fennel lies in its ability to compile down to Lua. This allows developers to take full advantage of Lua’s existing libraries and tools while writing in a more abstract, high-level style.

2. Comments and Documentation

• Fennel allows for line comments using the semicolon (;) syntax. This is a common feature in many Lisp dialects, which allows developers to annotate their code easily without disrupting the flow of expressions.
• Unlike some other Lisp implementations, Fennel does not feature semantic indentation, meaning that indentation does not influence the interpretation of the code, which keeps the syntax relatively simple and familiar for those coming from other languages.

3. Rich Macro System

• As with most Lisp dialects, Fennel’s macro system is one of its standout features. Macros in Fennel enable developers to define new syntactic constructs, essentially expanding the language itself. This provides a powerful way to create domain-specific languages or to simplify complex patterns in code.

4. Performance and Efficiency

• Fennel compiles directly to Lua code, which then runs on the Lua interpreter or virtual machine. Lua is known for its speed and low overhead, which makes Fennel an efficient choice for scenarios where performance is critical, such as game scripting or embedded systems.
• The language is lightweight, meaning that it can be easily embedded into other applications that already use Lua, which is common in game engines and other performance-sensitive environments.

5. Development Community and Ecosystem

• Fennel is actively developed and maintained, with a growing community of users and contributors. The official website (https://fennel–lang.org/) provides documentation, tutorials, and examples to help new users get started with the language.
• The language’s source code and development process are hosted on GitHub, with the repository description indicating that it is a Lua Lisp language. Although the issues are relatively low, indicating that it’s a mature project, there are always opportunities for collaboration and contributions.
• The community also communicates via the official mailing list, which is hosted at https://lists.sr.ht/~technomancy/fennel, allowing for discussions around the future of the language, troubleshooting, and feature requests.

How Fennel Works: A Closer Look

At the core of Fennel is its ability to compile to Lua code. The Fennel compiler takes in source code written in the Fennel language and transforms it into equivalent Lua code. This process is relatively straightforward, and the generated Lua code is typically easy to understand for those familiar with the Lua language. This feature sets Fennel apart from many other Lisp dialects, as it eliminates the need for a custom runtime and allows Fennel to benefit from the existing performance optimizations present in Lua.

When you write code in Fennel, you’re essentially working within a Lisp environment, where functions are first-class citizens and code is treated as data. However, the difference lies in the fact that Fennel is seamlessly compiled into Lua, making it easy to integrate with Lua-based systems and extend them without needing to learn an entirely new runtime.

Example: A Simple Fennel Program

To better understand how Fennel works, let’s examine a simple program that adds two numbers in Fennel:

fennel
Copy code
(defn add [a b]
  (+ a b))

(print (add 2 3))

In this example, we define a function add using the defn keyword, which takes two arguments, a and b. The body of the function uses the + operator to return the sum of a and b. The print function is used to output the result of calling (add 2 3), which would print 5.

This Fennel code would be compiled into equivalent Lua code that looks like this:

lua
Copy code
local add
add = function (a, b)
  return (a + b)
end

print(add(2, 3))

The generated Lua code is straightforward, and it shows how Fennel closely mirrors Lua’s functionality. The primary difference is the Lisp-style syntax, which offers the benefit of a cleaner, more concise way of writing code, especially for those who prefer the functional programming style.

Integration with Lua

One of the major selling points of Fennel is its ability to work alongside Lua. Lua is widely used in game development (such as in the popular game engine Love2D) and embedded systems, so having a Lisp dialect that can be easily integrated into existing Lua projects is a huge advantage.

Fennel’s integration with Lua also means that developers can use Lua libraries and tools directly from their Fennel code. This provides a seamless experience for anyone working in environments where Lua is already the standard language. Furthermore, since Fennel is transpiled to Lua, it can run in any environment where Lua is supported, which is one of the reasons it has found use in embedded systems and small devices.

Use Cases for Fennel

Fennel’s design makes it an ideal candidate for several key use cases:

1. Game Development

   • Lua’s role in game development, particularly in engines like Love2D, makes Fennel a powerful tool for game developers who want to leverage the expressiveness of Lisp while still working within the Lua ecosystem.

2. Embedded Systems

   • Lua is widely used in embedded systems because of its small footprint and speed. Fennel, as a Lua dialect, inherits these same characteristics, allowing developers to write clean, functional code while maintaining the efficiency of Lua in constrained environments.

3. Scripting and Automation

   • Fennel can be used for scripting tasks in Lua-based applications, offering a more powerful syntax and structure for automation tasks. This could range from server scripting to extending the functionality of Lua-based software applications.

4. Functional Programming Enthusiasts

   • For developers who are familiar with functional programming paradigms and want to work within the Lua ecosystem, Fennel offers a more structured approach compared to traditional Lua. The language’s Lisp roots bring immutability, first-class functions, and a focus on pure functions, which can enhance code quality and reduce side effects.

Conclusion

Fennel is a modern Lua-based Lisp language that brings the power of Lisp syntax and semantics to the Lua runtime. It enables developers to work with a more functional, expressive style of programming while still benefiting from Lua’s performance and compatibility. With its clean syntax, strong macro system, and direct Lua compatibility, Fennel offers a compelling option for anyone working within the Lua ecosystem who seeks the power and elegance of Lisp.

The growing community, active development, and increasing documentation make Fennel an attractive choice for developers looking to explore a new way of programming in the Lua world. Whether you’re building games, embedded systems, or automation scripts, Fennel provides a unique and efficient approach to Lua development.