Generating types at runtime

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1

I have been trying to create a simple function to generate type at runtime, here’s the following code:

const std = @import("std");

const Type = std.builtin.Type;

const print = std.debug.print;

const anonymous_struct = struct { []const u8, u8, u8 };

fn create_tuple(allocator: std.mem.Allocator, input: []u8) !type {
    var fields = std.ArrayList(Type.StructField).init(allocator);
    defer fields.deinit();

    var index: u8 = '0';

    var i: usize = 0;

    for (input) |field| {
        try fields.append(.{
            .name = &[_:0]u8{index},
            .type = @TypeOf(field),
            .default_value = @as(*const anyopaque, @ptrCast(&(field + 1))),
            .is_comptime = false,
            .alignment = @alignOf(@TypeOf(field)),
        });

        i += 1;
        index += 1;
    }

    return @Type(.{
        .Struct = Type.Struct{
            .is_tuple = true,
            .decls = &[_]Type.Declaration{},
            .backing_integer = null,
            .fields = try fields.toOwnedSlice(),
            .layout = .auto,
        },
    });
}

pub fn main() !void {
    const input = [_]u8{ 'a', 'b', 'c' };

    const slice = input[0..];

    const allocator = std.heap.page_allocator;

    const tuple_type = try create_tuple(allocator, @constCast(slice));

    const tuple = tuple_type{};

    print("{any}: {any}\n", .{ slice, tuple });
}

I am getting the following error:

run
└─ run zig_project_02
   └─ zig build-exe zig_project_02 Debug native 1 errors
C:\zig\lib\std\os\windows.zig:1745:33: error: comptime call of extern function
    return kernel32.VirtualAlloc(addr, size, alloc_type, flProtect) orelse {
           ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\heap\PageAllocator.zig:24:42: note: called from here
        const addr = windows.VirtualAlloc(       
                     ~~~~~~~~~~~~~~~~~~~~^       
C:\zig\lib\std\mem\Allocator.zig:86:29: note: called from here
    return self.vtable.alloc(self.ptr, len, ptr_align, ret_addr);
           ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\mem\Allocator.zig:225:35: note: called from here
    const byte_ptr = self.rawAlloc(byte_count, log2a(alignment), return_address) orelse return Error.OutOfMemory;
                     ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\mem\Allocator.zig:211:40: note: called from here
    return self.allocBytesWithAlignment(alignment, byte_count, return_address);
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\mem\Allocator.zig:205:75: note: called from here
    const ptr: [*]align(a) T = @ptrCast(try self.allocWithSizeAndAlignment(@sizeOf(T), a, n, return_address));
                                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\mem\Allocator.zig:193:41: note: called from here
    return self.allocAdvancedWithRetAddr(T, alignment, n, @returnAddress());
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\zig\lib\std\array_list.zig:457:67: note: called from here
                const new_memory = try self.allocator.alignedAlloc(T, alignment, new_capacity);   
                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~    
C:\zig\lib\std\array_list.zig:434:51: note: called from here
            return self.ensureTotalCapacityPrecise(better_capacity~
src\main.zig:18:26: note: called from here
        try fields.append(.{
            ~~~~~~~~~~~~~^
src\main.zig:48:40: note: called from here
    const tuple_type = try create_tuple(allocator, @constCast(slice));
                           ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    callMain: C:\zig\lib\std\start.zig:524:32
    WinStartup: C:\zig\lib\std\start.zig:363:45
    remaining reference traces hidden; use '-freference-trace' to see all reference traces
error: the following command failed with 1 compilation errors:    
C:\zig\zig.exe build-exe -ODebug -Mroot=C:\Learning\Zig\reflections\src\main.zig --cache-dir C:\Learning\Zig\reflections\.zig-cache --global-cache-dir C:\Users\legend\AppData\Local\zig --name zig_project_02 -flto --listen=-
Build Summary: 0/5 steps succeeded; 1 failed (disable with --summary none)
run transitive failure
└─ run reflections transitive failure
   ├─ zig build-exe reflections Debug native 1 errors
   └─ install transitive failure
      └─ install reflections transitive failure
         └─ zig build-exe reflections Debug native (reused)    
error: the following build command failed with exit code 1:       
C:\Learning\Zig\reflections\.zig-cache\o\97c05016496160a42f4b765de720eb37\build.exe C:\zig\zig.exe C:\Learning\Zig\reflections C:\Learning\Zig\revlections\.zig-cache C:\Users\legend\AppData\Local\zig --seed 0xf7c7b2b7 -Z7276550ef218267c run

And how do I cleanup the arraylist memory?

2

You can’t generate types at runtime in Zig.

The fact that your function returns type makes it a comptime-only function, which is why you’re getting an error when accessing a heap allocator at comptime.

If you want to do something of this sort, you will have to generate your own runtime abstractions where your “types” are represented as data.

1 Like
3

Is there any chance of zig getting runtime reflections in the future?

4

Most probably not.

5

What is your reason for wanting runtime?
Comptime has way more support.

If you describe what you actually want to do, maybe it could be achieved in a different way.

6

The function create_tuple for example could be made runtime by not doing @Type and just returning the Type struct, which can totally be used at runtime.

7

So, I was working on creating a serializer for json string. Since there is no strict typing, arrays/lists in json have dynamic type and length, so was trying to generate a type based on the parsed array/list in json string into a tuple type so that I can access it like:

_ = field[0];
_ = field[1];

Note, my intention was only for indexing the array/list in json.

9

use a tagged union, this is essential what runtime types are anyway, its also how std.json handles that problem

4 Likes