How to emulate "downward" closures with Zig

closures.zig

const std = @import("std");

fn printSliceWithFilter(comptime T: type, values: []const T, context: var, filter: fn (T, @typeOf(context)) bool) void {
    for (values) |val, i| {
        if (!filter(val, context))
            continue;
        std.debug.warn("[{}] = {}\n", i, val);
    }
}

pub fn main() void {
    const values = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    printSliceWithFilter(u8, values[0..], {}, struct {
        fn filter(val: u8, _: void) bool {
            return val < 5;
        }
    }.filter);

    const strings = [_][]const u8{
        "Hello, World!",
        "This is stupid.",
        "Hi, Man!",
    };

    const filterStrings = struct {
        fn filter(val: []const u8, prefix: u8) bool {
            return val[0] == prefix;
        }
    }.filter;

    std.debug.warn("strings starting with 'H':\n");
    printSliceWithFilter([]const u8, strings[0..], @as(u8, 'H'), filterStrings);

    std.debug.warn("strings starting with 'T':\n");
    printSliceWithFilter([]const u8, strings[0..], @as(u8, 'T'), filterStrings);
}

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