[msteffen]

Here is, I think, an interesting example of the kind of thing TFA is talking about. In case you’re not already familiar, there’s an issue that game devs sometimes struggle with, where, in C/C++, an array of structs (AoS) has a nice syntactic representation in the language and is easy to work with/avoid leaks, but a struct of arrays (SoA) has a more compact layout in memory and better performance.

Zig has a library to that allows you to have an AoS that is laid out in memory like a SoA: https://zig.news/kristoff/struct-of-arrays-soa-in-zig-easy-i... . If you read the implementation (https://github.com/ziglang/zig/blob/master/lib/std/multi_arr...) the SoA is an elaborately specialized type, parameterized on a struct type that it introspects at compile time.

It’s neat because one might reach for macros for this sort of the thing (and I’d expect the implementation to be quite complex, if it’s even possible) but the details of Zig’s comptime—you can inspect the fields of the type parameter struct, and the SoA can be highly flexible about its own fields—mean that you don’t need a macro system, and the Zig implementation is actually simpler than a macro approach probably would be.

[WalterBright]

D doesn't have a macro system, either, so I don't understand what you mean.

[msteffen]

IIUC, it does have code generation—the ability to generate strings at compile-time and feed them back into the compiler.

The argument that the author of TFA is making is that Zig’s comptime is a very limited feature (which, they argue, is good. It restricts users from introducing architecture dependencies/cross-compilation bugs, is more amenable to optimization, etc), and yet it allows users to do most of the things that more general alternatives (such as code generation or a macro system) are often used for.

In other words, while Zig of course didn’t invent compile-time functions (see lisp macros), it is notable and useful from a PL perspective if Zig users are doing things that seem to require macros or code generation without actually having those features. D users, in contrast, do have code generation.

Or, alternatively, while many languages support metaprogramming of some kind, Zig’s metaprogramming language is at a unique maxima of safety (which macros and code generation lack) and utility (which e.g. Java/Go runtime reflection, which couldn’t do the AoS/SoA thing, lack)

Edit Ok, I think Zig comptime expressions are just like D templates, like you said. The syntax is nicer than C++ templates. Zig’s “No host leakage” (to guarantee cross-compile-ability) looks like the one possibly substantively different thing.

[WalterBright]

> Zig’s “No host leakage” (to guarantee cross-compile-ability) looks like the one possibly substantively different thing.

That is a good idea, but could be problematic if one relies on size_t, which changes in size from 32 to 64 bit. D's CTFE adds checks for undefined behavior, such as shifting by more bits than are in the type being shifted. These checks are not done at runtime for performance reasons.

D's CTFE also does not allow calling the operating system, and only works on functions that are "pure".

[pron]

Because Zig supports cross-compilation, what you care about isn't the host -- the machine that runs the compiler -- but the target, which is not (necessarily) the same as the host. While information about the host isn't made available, information about the compilation target is: https://ziglang.org/documentation/master/#Compile-Variables