Zig: When to Use comptime Metaprogramming vs. Runtime Error Unions

Zig gives you two very different tools for making code correct and fast: comptime, which runs code during compilation, and error unions (!T), which force callers to deal with failures at runtime. They solve different problems, but new Zig code often overuses one and underuses the other. This guide helps you pick, then shows a minimal example combining both.

The decision and its constraints

The core question: can the work be done before the program runs, or does it depend on data only known at runtime?

comptime is for anything fully determined by types, constants, or build-time configuration: generating lookup tables, specializing generic code, validating invariants, computing sizes. It costs compile time and can grow the binary, but produces zero runtime overhead.

Error unions are for failures that only exist at runtime: I/O, allocation, parsing user input, network calls. Zig has no exceptions and no hidden control flow — a function returning !T must have its error handled or explicitly propagated with try.

Comparing the options

Rules of thumb

• If the input is a type or a literal known at build time, prefer comptime. Example: precomputing a CRC table or a perfect-hash map of keywords.
• If the input comes from a file, socket, user, or allocator, use an error union. Do not try to comptime your way around runtime failure.
• If a value could be comptime-known but usually isn't, write a normal runtime function — Zig will evaluate it at comptime automatically when called in a comptime context. This is a key Zig strength: the same function serves both worlds.
• If error propagation is making call sites noisy, introduce a narrower error set or handle-and-log at one layer rather than threading try through five levels.

A concrete example: comptime table, runtime parsing

This example (Zig 0.11/0.12 era syntax) builds a command lookup table at compile time, then uses an error union for runtime parsing. Save as main.zig:

const std = @import(\"std\");\n\nconst Command = struct {\n    name: []const u8,\n    id: u32,\n};\n\n// Built entirely at compile time: zero runtime cost.\nconst commands = comptime blk: {\n    var list: []const Command = &.{};\n    for ([_][]const u8{ \"start\", \"stop\", \"status\" }, 0..) |name, i| {\n        list = list ++ [_]Command{.{ .name = name, .id = @intCast(i) }};\n    }\n    break :blk list;\n};\n\nconst ParseError = error{ UnknownCommand, EmptyInput };\n\nfn parse(input: []const u8) ParseError!u32 {\n    if (input.len == 0) return error.EmptyInput;\n    inline for (commands) |cmd| {\n        if (std.mem.eql(u8, input, cmd.name)) return cmd.id;\n    }\n    return error.UnknownCommand;\n}\n\npub fn main() !void {\n    const id = parse(\"stop\") catch |err| {\n        std.debug.print(\"parse failed: {s}\\n\", .{@errorName(err)});\n        return err;\n    };\n    std.debug.print(\"id = {}\\n\", .{id});\n}

inline for unrolls the loop over the comptime-known table, so the runtime code is a short sequence of string comparisons with no allocation and no table construction.

Building and validating

Run these in the directory containing main.zig, with the Zig toolchain on your PATH (no special permissions needed):

zig build-exe main.zig\n./main          # expected: id = 1

To confirm the error path is real and checked, change the call to parse(\"\") and rebuild — you should see parse failed: EmptyInput. To see the compiler enforce error handling, remove the catch block and call parse(\"stop\") bare: compilation should fail with an error about an unhandled error union. That rejection is the feature working as designed.

To confirm the table really is comptime-generated, check that the program compiles and runs identically with -O ReleaseSmall; there is no initialization code to measure. If you want harder evidence, inspect the disassembly (objdump -d main) and look for the inlined comparisons rather than a loop over data.

Limitations and caveats

• Comptime blocks that build large data structures can noticeably slow compilation and inflate the binary; measure with zig build-exe --verbose timing if a build feels slow.
• Comptime code that depends on unresolved or circular declarations fails with errors that can be hard to read. Keep comptime logic small and self-contained.
• Error unions add a branch at every call site. In hot loops where failure is impossible (e.g., writing to a fixed buffer), prefer APIs that cannot fail over wrapping everything in try.
• Syntax details (e.g., @intCast form) changed between Zig 0.10 and 0.11; check against your installed version with zig version if an example fails to compile.

The short version: push everything knowable at build time into comptime, and let error unions make every runtime failure explicit. Zig rewards using both — the mistake is forcing one tool to do the other's job.