Zig compile-time evaluation even allows to construct an arbitrary type like a struct with a platform-specific members all using the same core language. This beats even Rust macros and in retrospect I wonder why this not used in other languages? Even Lisp ended up with macros with own sub language instead of making the core language more useful for code generation. Or consider Go attempts at generics. Something like Zig approach will fit it more I think than various generics proposals.
"Even Lisp ended up with macros with own sub language..." I assume you're talking about Scheme approaches like `syntax-case` and `syntax-rules`? In Common Lisp, macros are written in Lisp, the same way you would write runtime code. Unquoting and splicing are part of the core language (e.g. they can be applied to data as well as to code).
I was not aware that unquoting in Common Lisp can be applied to data. So it is not a specialized sub language just for macros, but a general purpose template system that could also generate code.
It's really a shorthand for writing lists - sexp's are lists, so can use the same syntax.
Macro's in Lisp are essentially "just" functions where the arguments are un-evaluated - you can use all the same functions etc...
Writing a simple Lisp interpreter is really quite educational - when you add macros and quoting into the language, you suddenly have a realisation at how simple it is.
I agree it's an idea that seems so natural in retrospect that I'm also curious why it hasn't traditionally been more popular. One possible reason that comes to mind is that an imperatively-constructed type is harder to statically analyze.
But on the other hand, even C++ templates are Turing complete and even C++ parse trees can depend on the output of a Turing-complete evaluation. So it is hard to see what benefit a C++-like template feature is buying in comparison.
Not only was it not "more popular," I am not aware of even research languages taking the idea of general-purpose partial evaluation not through syntax macros to the same lengths as Zig (although that doesn't mean there weren't any). It's possible that no one had thought of that, perhaps because type systems, while known to be possibly considered as a form of partial evaluation, are generally treated and studied as a completely separate concept: partial evaluation is a compilation technology while types are part of the theory.
I’m not clear on what you define syntax macros, but the compile time evaluation that is present in Zig (based on the example and documentation I’ve seen) are an almost direct implementation of the multistage computation research done late 90s-mid 00s. The papers and such are typically ML based (with a caveat for the early Template Haskell paper in 02) and the underlying constructs of MetaML are very close to what Zig implements. All this work is retroactively semantically underpinned by Rowan Davies and Frank Pfenning work on Modal S4 logic. I don’t know if Andy based Zig on this research, but if he didn’t, the convergence toward the same theory is a great thing to see.
My current work on a visual programming compatible language also uses this strategy for meta programming, so I’m very familiar with the academic literature. It certainly seems that Zig got this right and is doing well implementing it in a way that is usable and understandable.
By syntax macros I mean expressions that can be referentially opaque, i.e. m(a) and m(b) might have different semantics even though a and b have the same reference. Lisp (and C, and Haskell, and Rust) is referentially opaque while Zig is transparent. Opacity is strictly more expressive, but is harder to understand; Zig strives to be the "weakest" language possible to get the job done.
MetaML certainly does seem to be similar to Zig, although the one paper I've now read thanks to your suggestion (https://www.sciencedirect.com/science/article/pii/S030439750...) does not mention introspection and type functions, and another commenter here suggests that it is, actually, referentially opaque.
Suppose a and b have the same reference. Zig allows something like { const a = 42; const b = 72; ... } which gives these names new references which they no longer share, in a scope. So the opacity is there in some built-in operators; the only question is whether macros can imitate that.
I think Zig's comptime has three interesting features:
1. It is referentially transparent, i.e., unlike Lisp, i.e. nothing in the language can distinguish between `1 + 2` and `3`. This means that the semantics of the language is the same as if everything was evaluated at runtime, and the comptime annotation has no impact on semantics (assuming syntax terms aren't objects and there's no `eval`).
2. It supports type introspection and construction.
3. It doesn't have generics and typeclasses as separate constructs but as applications of comptime.
I think 3 is unique to Zig, but I wonder about 1: is it possible in MetaOCaml, as it is in Lisp, C, C++, Rust and Haskell -- but not in Zig! (or, say, Java) -- to write a unit `m`, such that m(1 + 2) is different from m(3)?
I don’t know. I think MetaOcaml is strict about inspecting Code terms like Code (1 + 2) and Code 3 (as far as I remember it is not allowed). So no, I don’t think you can distinguish between them.
I think (again based on examples and docs) that it is actually multistaged. However, this is not apparent because the quote and splice are semi-implicit. The ‘comptime’ keyword is essentially a the quote operator, ie. it produces what is essentially a compile time thunk, lazily capturing the ‘code’ value of the expression defined under the comptime keyword. This thunk is directly analogous to a value of type ‘code a’ in MetaML. Then there is an implicit splice operator in place any time a value defined under comptime is used subsequently. I say it is multistaged because it certainly appears that one can define a comptime variable for some computation and then use that variable in a later comptime definition or expression. So, it looks like a basic data flow analysis to determine dependency is done on the comptime evaluations and those are ordered implicitly. This order would correspond to the more explicit staging syntax of MetaML and the operation semantics that underpin it.
I've heard of Terra, but I didn't know it had predated Zig by a couple of years; thought they were introduced at about the same time. And yes, there are certainly similarities, although Terra thinks of itself as two interoperating languages, while in Zig there is just one (although there are things you can only do in comptime so perhaps it's more of a difference in perspective).
The term partial evaluation tends to be used for research that focuses on automatic evaluation of the static part of the program. When considered as metaprogramming the research would probably discuss type-checking + evaluation as two-stage evaluation, falling under the general umbrella of multi-stage programming.
Tow issues I have noticed with Zig: not as good type signatures for functions and not as good error messages. I am not sure if those are fundamental issues with Zig's apporach or if they can be fixed with more work.
Take for example the definition of floor(), how would an IDE or documentation generator see which types are supported? This is just imperative code which is executed at compile time and at least to me it is not obvious how to interpret it when generating the documentation. On the other hand in Rust it is almost always easy to see which types can be used where.
Still the compiler has to deal with that. So one ends up with an interpreter in the compiler for very esoteric and hard to write/read language instead of just embedding an interpreter of a subset of Java.
One alleged benefit of Java constrained genetics is that they allow in theory better error messages. Still in complex cases even after over 15 years with genetics in Java the error messages are not that great. Zig to some extend delegates to the compile-time library authors responsibility of producing good error messages. But then the error messages can be improved without waiting for the compiler to improve its reporting heuristics to cover all edge cases.
We have a slightly vague proposal in D to basically construct programs as a library at compile time (i.e. we give you an AST, you play with it and give it back, if it's valid it.gets turned into a type).
D has been making structs at compile time for years now, however.
I would guess one reason it wasn't done in other languages was because people simply didn't have a good cross-architecture JIT compilation resource, and didn't want to write it themselves. LLVM makes this _really_ easy. I realize that Zig is transitioning to an LLVM-optional model now. But, for instance, I've been working on an s-expr language with f-expr application, and this combined with LLVM c bindings allows you to generate arbitrary compile time or runtime code. The JIT portion for compile time code was a single function call in LLVM! I started this a while before Zig came out, but alas I haven't devoted enough time to it over the years...
This is very common in D and D doesn't not have any macro system. It uses regular syntax (more or less). D was doing this way before Zig existed and before C++ had constexpr. Simple example of platform specific members:
struct Socket
{
version (Posix)
int handle;
else version (Windows)
SOCKET handle;
else
static assert(false, "Unsupported platform");
}
Another example is the checked numeric type [1] in the D standard library. It takes two types as parameters. The first being the underlying type and the second being a "hook" type. The hook type allows the user to decide what should happen in various error conditions, like overflow, divide by zero and so on. The implementation is written in a Design By Introspection style and inspects the hook type and adopts its implementation depending on what hooks it provides. If the hook type implements the hook for overflow, that hook will be executed, otherwise it falls back to some default behavior.
