You don't need interfaces in Zig

main.zig

const std = @import("std");

fn Writer(comptime T: anytype) type {
    return struct { Write: fn (t: *T, b: []const u8) anyerror!usize };
}

fn writeHello(comptime T: anytype, comptime writer: Writer(T), t: *T) anyerror!usize {
    return writer.Write(t, "Hello there");
}

fn writeArrayList(t: *std.ArrayList(u8), b: []const u8) anyerror!usize {
    try t.appendSlice(b);
    return b.len;
}

fn writeByteSlice(t: *[]u8, b: []const u8) anyerror!usize {
    @memcpy(t.*[0..b.len], b);
    return b.len;
}

pub fn main() anyerror!void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();

    var al = std.ArrayList(u8).init(arena.allocator());
    _ = try writeHello(std.ArrayList(u8), Writer(std.ArrayList(u8)){ .Write = writeArrayList }, &al);
    std.debug.print("Result: {s}\n", .{al.items});

    var bs: []u8 = try arena.allocator().alloc(u8, 64);
    const len = try writeHello([]u8, Writer([]u8){ .Write = writeByteSlice }, &bs);
    std.debug.print("Result: {s}\n", .{bs[0..len]});
}