Minimal Zig of a custom formatter

optional_u64_formatter.zig

const std = @import("std");

pub fn main() void {
    var foo: ?u64 = 101;
    std.debug.print("optional u64 foo = '{}'\n", .{optionalU64(foo)});
    foo = null;
    std.debug.print("optional u64 foo = '{}'\n", .{optionalU64(foo)});
    foo = 1000000;
    std.debug.print("optional u64 foo = '{}'\n", .{optionalU64(foo)});
}

fn formatOptionalU64(val: ?u64, comptime _: []const u8, _: std.fmt.FormatOptions, writer: anytype) !void {
    if (val) |v| try writer.print("{d}", .{v});
}

fn optionalU64(val: ?u64) std.fmt.Formatter(formatOptionalU64) {
    return .{ .data = val };
}