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Comment by haberman | Hacker News Reader

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.

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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 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

One that comes to mind is Terra: http://terralang.org/

In retrospect Terra seems a clear precursor to Zig (though I don't know if it was a direct influence).

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).

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.

It doesn’t seem as general as MetaOcaml. Just two-stage evaluation, and without quote and splice.

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.

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.

Can you elaborate a bit on the first issue?

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.

    pub fn floor(x: anytype) @TypeOf(x) {
        const T = @TypeOf(x);
        return switch (T) {
            f16 => floor16(x),
            f32 => floor32(x),
            f64 => floor64(x),
            f128 => floor128(x),
            else => @compileError("floor not implemented for " ++ @typeName(T)),
        };
    }

This seems reasonable to me. It seems comparable to the following in C++:

    template <class T> T floor(T x) { /* ... */ }

I think a documentation generator could display your Zig signature verbatim and a user could make sense of that.

Even Java and Rust ended up with Turing-complete generics.

Java turing complete generics are an edge case though, and never hit during normal compilation.

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.

Incorrect Java does not have genetics. That is a C++ concept.

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