This comparative study of macros in Lisp and Rust highlights not only their technical differences but also the philosophies that guide their design.
Metaprogramming is the technique of writing programs that can generate, transform, or analyse other programs—or even themselves. It allows developers to operate at a higher level of abstraction, automate boilerplate code, and implement custom behaviours that the base language may not natively support. By shifting some work from runtime to compile time (or from manual labour to automation), metaprogramming plays a critical role in enhancing code efficiency, reusability, and expressiveness.
One of the most powerful tools for metaprogramming is the macro. Macros allow developers to manipulate code as data—intercepting and transforming code before it is executed. Unlike functions, which operate on values, macros operate on the structure of code itself. This capability makes them ideal for defining domain-specific languages (DSLs), implementing compile-time checks, or eliminating repetitive code patterns. Let’s compare Rust and Lisp, two languages known for their powerful macro systems but with drastically different approaches.
Why compare Rust and Lisp specifically
Rust and Lisp offer two of the most distinctive and influential macro systems in modern programming, making them ideal for a comparative study of metaprogramming. In contrast to Lisp, Rust is a modern systems programming language designed with an emphasis on safety, concurrency, and performance. Despite its strict and static type system, Rust embraces metaprogramming through two macro systems: declarative macros (macro_rules!) and procedural macros, which operate at the abstract syntax tree (AST) level. Rust’s macros are carefully engineered to integrate tightly with the compiler’s hygiene, type-checking, and error reporting systems—offering powerful compile-time capabilities while minimising unintended side effects.
Understanding macros
In programming, a macro is a construct that allows code to be generated, transformed, or manipulated before it is executed or compiled. In essence, macros are code that writes code. Unlike functions, which operate at runtime on values, macros operate at compile time (or macro-expansion time) on the structure of the code itself, often before any type checking or execution occurs.
Macros enable developers to work at a higher level of abstraction, eliminate repetitive patterns, and implement custom language features. Their power lies in their ability to modify how code is interpreted or generated, making them indispensable tools for advanced metaprogramming.
Types of macros
Macros can generally be classified into two main categories:
Textual macros (simple substitution)
Textual macros are the simplest form of macros, often found in languages like C and C++ through the #define directive. These macros perform direct text substitution, replacing identifiers or expressions with predefined code fragments.
Structural macros (AST-based macros)
Structural macros, in contrast, operate on the abstract syntax tree (AST) of the code. Instead of performing simple text substitution, these macros manipulate the parsed representation of code. This is the approach taken by modern macro systems in languages like Lisp and Rust.
Structural macros are significantly more powerful and safer than textual macros. They support syntax-aware transformations, enforce scoping rules, and allow compile-time validation of the generated code.
Macros in Lisp
Lisp, one of the oldest high-level programming languages (dating back to the late 1950s), introduced a revolutionary idea that remains influential to this day: code as data. From its inception, Lisp embraced a minimal, consistent syntax built around S-expressions (symbolic expressions), allowing programs to be represented as nested lists. This design decision enabled one of Lisp’s most powerful and enduring features: macros.
The different types of Lisp macros are:
- Standard macros (defmacro)
- Backquote/Quasiquotation macros
- Reader macros
- Hygienic macros (Scheme: syntax-rules, syntax-case)
- Macrolet (Local macros)
- Symbol macros (symbol-macrolet)
- Compiler macros (define-compiler-macro)
Strengths of Lisp macros
Extreme flexibility: Thanks to homoiconicity, macros can manipulate code with the same ease as any data structure. This allows developers to reshape the language to suit their needs.
Ease of writing DSLs: Lisp’s uniform syntax and powerful macro system make it exceptionally well-suited for developing domain-specific languages. Developers can define constructs that closely mirror the problem domain, resulting in more readable and maintainable code.
Weaknesses of Lisp macros
Despite their power and flexibility, Lisp macros come with several notable drawbacks.
Harder to debug: Macro expansion occurs before runtime, and the transformed code can be difficult to trace back to the original source. Errors often appear in the expanded code, not in the macro definition or usage, making debugging more challenging.
Can make code less readable for non-experts: Macros can obscure program logic, especially when overused or poorly documented. Complex macros, especially those that redefine control flow or syntax, may confuse readers unfamiliar with the transformations being performed.
Key comparative dimensions: Rust vs Lisp macros
Ease of writing
Lisp: Writing macros in Lisp is relatively natural due to its code-as-data philosophy. Because Lisp code is represented as S-expressions (lists), creating macros feels almost like manipulating regular data structures. The syntax is minimalistic, and the process is straightforward for those familiar with Lisp’s paradigms.
Rust: Writing macros in Rust is more verbose and structured, reflecting Rust’s strict syntax and static typing. While macro_rules! allows for declarative macros that match patterns, the syntax can quickly become complex for advanced use cases. Procedural macros, which involve interacting with the abstract syntax tree (AST), require deeper understanding and tooling.
Flexibility
Lisp: Lisp has near-unlimited flexibility. Its homoiconicity (code-as-data) allows macros to manipulate code in any way the programmer desires. Because Lisp code is just data, macros can do anything from simple substitutions to complex transformations, including generating entire control structures or DSLs.
Rust: Rust macros are more controlled. While Rust’s macro system is powerful, it is still bounded by compiler rules and its type system. Macros in Rust cannot bypass the type checker, making them safer but limiting their ability to manipulate code freely compared to Lisp.
Safety and hygiene
Rust: Rust macros emphasise hygiene, meaning they avoid name conflicts and ensure that variable scopes are preserved. Rust’s macro system is designed to prevent unintended clashes and maintains strict scope rules, making them safer and more predictable. Procedural macros are also hygienic by design, ensuring that the generated code does not accidentally overwrite or conflict with existing variables.
Lisp: In traditional Lisp, macros are not hygienic by default. Programmers must manage hygiene manually, meaning they need to be cautious about variable name collisions or scope issues when writing macros. However, some modern Lisp implementations (like Scheme) provide hygiene features, though they are not always present in older dialects.
Debuggability
Rust: Rust offers strong compiler tooling, like cargo expand, which helps visualise and inspect macro expansions. This makes it much easier to debug macros, as developers can see exactly how their macros expand at compile-time, helping them diagnose issues early in the development process.
Lisp: Debugging macros in Lisp can be harder without good REPL tools. While Lisp’s REPL (Read-Eval-Print Loop) can facilitate quick testing of macros, it doesn’t offer as seamless a debugging experience as Rust’s tooling. Moreover, the lack of a standardised macro inspection tool means that developers often have to rely on custom approaches or manual inspection of expanded code.
Performance impact
Rust: Macros in Rust are expanded at compile time, meaning they incur no runtime cost. This makes Rust macros highly performant, as all the code generation and transformations happen during compilation. The result is efficient, optimised code at runtime.
Lisp: The performance impact of macros in Lisp is generally minimal, as the macro expansion is done at compile time (in interpreted environments) or when the code is loaded. However, in certain cases where macros generate complex or inefficient code, there could be some performance overhead, but this is generally not a concern for most use cases.
Here are a few case studies of real-world use of macros in Lisp and Rust.
DSL creation
Lisp: Seamless embedded DSLs: Lisp’s homoiconic syntax and macro flexibility make it exceptionally well-suited for building embedded domain-specific languages (DSLs). Because code and data share the same structure (S-expressions), new syntactic constructs can be defined with minimal friction.
Here’s an example of HTML DSL using common Lisp:
(defmacro html (&body body)
`(format t “<html>~{~A~}</html>” (list ,@body)))
(html “<head><title>My Page</title></head>”
“<body><p>Hello, Lisp!</p></body>”)
This macro creates a simple HTML DSL by generating and formatting HTML code. Developers can easily extend it with more tags or nesting structures, mimicking native language features.
Metaprogramming in libraries
Lisp: Macros in core libraries (e.g., CLOS): The Common Lisp Object System (CLOS) relies heavily on macros for defining classes, methods, and multiple dispatch. Macros enable CLOS to extend the language with object-oriented features that behave like native constructs.
Here’s an example of defining a class in CLOS:
(defclass person () ((name :initarg :name :accessor person-name) (age :initarg :age :accessor person-age)))
This macro expands into code that sets up a full-featured object system, allowing inheritance, dynamic dispatch, and method combination. This would be extremely verbose to implement without macros.
At the heart of the macro systems in Lisp and Rust lies a deeper philosophical divergence—how much power to give the programmer, and under what constraints.
Lisp: Freedom with responsibility
Lisp embodies the ethos of trusting the programmer. Its macro system offers near-total freedom to manipulate and reshape the language. Thanks to homoiconicity, macros in Lisp can seamlessly generate code that feels indistinguishable from hand-written constructs. This freedom enables expressive DSLs, new control structures, and domain-specific syntactic layers.
But with that power comes responsibility:
- Macros can introduce subtle bugs (e.g., variable capture) if not carefully written.
- There are few built-in constraints to prevent misuse or misdesign.
- Debugging and readability rely heavily on the discipline and clarity of the programmer.
Rust: Power with guardrails
Rust approaches metaprogramming through the lens of structured safety. Its macro system—especially procedural macros—grants substantial power, but within the boundaries of the compiler’s guarantees:
- Hygiene is enforced automatically.
- Types must be respected, even in generated code.
- Syntax correctness is validated early.
This philosophy reflects Rust’s overall design: give the programmer tools to write efficient, expressive code, but always under a strong safety net. Metaprogramming in Rust is powerful, but it’s curated and constrained, aiming to eliminate entire classes of bugs before runtime.
Lisp macros: Further readingLisp macros are powerful tools that operate on the code-as-data principle, enabling metaprogramming and domain-specific language design. Here’s where to go deeper:
Rust macros: ResourcesRust supports two macro systems — macro_rules! for pattern matching and procedural macros for powerful compile-time code generation.
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Freedom vs safety
This contrast—Lisp’s freedom vs Rust’s safety—represents two visions of metaprogramming:
- Lisp invites programmers to shape the language as they see fit, trusting their understanding and discipline.
- Rust provides structured metaprogramming that balances power with predictability, aligning with its goals of memory safety, concurrency, and compile-time guarantees.
Neither philosophy is superior in isolation. Instead, they serve different kinds of projects and mindsets: Lisp excels in exploratory programming, rapid DSL prototyping, and extensibility, while Rust shines in systems programming, correctness, and maintainability at scale.
Both languages use macros as powerful tools for metaprogramming—enabling abstraction, code reuse, and language extension—but they do so in markedly different ways.
We’ve seen that Lisp macros offer near-total flexibility, driven by the language’s homoiconicity and minimalist syntax. They empower developers to mould the language itself, creating DSLs and custom constructs with ease. In contrast, Rust macros, while more verbose and structured, integrate deeply with the compiler and enforce strong guarantees of hygiene, type safety, and tooling support.
There is no definitive ‘winner’ in this comparison. Each language reflects the priorities of its broader ecosystem:
- Lisp favours freedom and expressiveness, trusting developers to wield that power responsibly.
- Rust prioritises safety and maintainability, ensuring macro-generated code adheres to the same rigour as handwritten code.
Finally, here’s some advice for developers.
Choose your metaprogramming approach based on the needs of your project and the experience of your team. For rapid prototyping, DSL development, or language experimentation, Lisp may be ideal. For performance-critical, large-scale, or safety-sensitive systems, Rust’s macro system offers the reliability and structure needed to scale confidently.
In the end, both macro systems showcase the strength of metaprogramming—not just as a technical feature but as a design philosophy tailored to the goals of each language.
