Zig's comptime is powerful but has limitations. It's not a general-purpose Turing-complete language. It cannot perform arbitrary I/O operations like reading files or making network requests. Loop bounds and recursion depth must be known at compile time, preventing dynamic computations based on runtime data. While it can generate code, it can't introspect or modify existing code, meaning no macros in the traditional C/C++ sense. Finally, comptime doesn't fully eliminate runtime overhead; some checks and operations might still occur at runtime, especially when interacting with non-comptime code. Essentially, comptime excels at manipulating data and generating code based on compile-time constants, but it's not a substitute for a fully-fledged scripting language embedded within the compiler.
This blog post by Aleksey Kladov explores the limitations of comptime in the Zig programming language, specifically focusing on what cannot be done during compile time despite the powerful metaprogramming capabilities Zig offers. The author emphasizes that while comptime allows for impressive compile-time execution of arbitrary Zig code, it doesn't transform Zig into a fully dependent type system or a general-purpose compile-time computation engine akin to something like Lisp macros.
The core argument revolves around the distinction between values known at compile time and types. While comptime lets you compute values at compile time, these values do not automatically become part of the type system. This means you cannot, for instance, create a type based on the result of a comptime calculation. A concrete example provided is the inability to define an array type where the size is determined by a comptime function. Even if the function's result is known at compile time, the compiler treats it as a value, not a type parameter, and thus cannot be used to specify the array's size.
The post further elaborates on this limitation by illustrating scenarios involving generics. Even when a function's type parameters are known at compile time, comptime doesn't provide a mechanism to intrinsically reflect this knowledge within the type system. This means that operations within a comptime function that depend on these type parameters are still constrained by the general rules of compile-time evaluation, and the resulting values, though computed at compile time, do not influence the type itself.
The author highlights that this behavior is a deliberate design choice in Zig. The intention is to maintain a clear separation between values and types, preventing the type system from becoming overly complex and difficult to reason about. While this restricts certain metaprogramming possibilities, it contributes to Zig's overall goal of simplicity and predictability. The post concludes by acknowledging that future iterations of Zig might explore more advanced compile-time features, but the current philosophy prioritizes the clarity and maintainability of the language. The underlying message is that while comptime is undoubtedly powerful, it's essential to understand its boundaries and not expect it to behave like a fully-fledged dependently typed system.
Summary of Comments ( 160 )
https://news.ycombinator.com/item?id=43744591
HN commenters largely agree with the author's points about the limitations of Zig's
comptime, acknowledging that it's not a general-purpose Turing-complete language. Several discuss the tradeoffs involved in compile-time execution, citing debugging difficulty and compile times as potential downsides. Some suggest that aiming for Turing completeness at compile time is not necessarily desirable and praise Zig's pragmatic approach. One commenter points out thatcomptimeis still very powerful, highlighting its ability to generate optimized code based on input parameters, which allows for things like custom allocators and specialized data structures. Others discuss alternative approaches, such as using build scripts, and how Zig's features complement those methods. A few commenters express interest in seeing how Zig evolves and whether future versions might address some of the current limitations.The Hacker News post "Things Zig comptime won't do" has generated a moderate amount of discussion, with several commenters offering their perspectives on the article's points and the nature of Zig's
comptimefeature.Several commenters discuss the trade-offs inherent in Zig's design. One points out that the limitations of
comptimeare a deliberate choice, contrasting it with C++'s templates, which are Turing-complete and thus capable of much more, but at the cost of greater complexity and often inscrutable error messages. They argue that Zig's approach prioritizes simpler mental model and easier debugging. Another commenter elaborates on this by highlighting the difference between compile-time execution (which Zig does) and compile-time code generation (which C++ templates enable).Another thread of discussion centers around reflection and code generation. The author of the original article states that
comptimedoes not currently offer robust reflection capabilities, limiting its potential for generating code dynamically. A commenter suggests that the "comptime is not a code generator" position in the article feels like a moving target, citing recent additions to Zig'scomptimecapabilities. Another commenter agrees, pointing out that macros and procedural macros are planned additions that could expandcomptime's power.One commenter suggests that the article's complaints might not be as significant as they appear. They argue that many of the use cases mentioned, such as static polymorphism through traits, can be achieved through other means within Zig, even if not directly through
comptime.The discussion also touches upon the specific examples presented in the article. One commenter questions the practicality of the "generic map" example, suggesting it's a contrived scenario. Another commenter delves into the example concerning statically sized arrays, offering an alternative solution using
std.ArrayListthat avoids the need for complexcomptimemanipulation.Finally, a few comments compare Zig to other languages, particularly C++ and Rust. One commenter notes the differences in compile-time metaprogramming capabilities between Zig and C++, reinforcing the point about Zig's focus on simplicity. Another commenter discusses how Rust's trait system, while powerful, can lead to complex error messages and compilation times, similar to C++ templates. They suggest that Zig's more constrained approach might be preferable in some cases.
In summary, the comments on Hacker News reflect a nuanced understanding of Zig's
comptimeand its limitations. The discussion is generally supportive of Zig's design choices, recognizing the trade-offs between power and complexity. While some commenters point out potential areas for improvement or alternative solutions, the overall sentiment seems to be that Zig's approach is valuable for its simplicity and predictability.