Comptime Validation and @compileError
Fail the build with clear messages.
Comptime Validation and @compileError is a free Zig Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Zig Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Catch Mistakes Before Run Time
Zig lets you reject bad usage while the code compiles. The built-in @compileError stops the build with a message you write. 🛑
Fail with a Clear Message
@compileError takes a comptime string and aborts compilation, printing exactly that text so the caller knows what went wrong.
@compileError("this type is not supported");Guard a Generic Function
Inside a comptime branch you can reject unwanted types. Here only integers pass; anything else hits @compileError at the call site.
if (@typeInfo(T) != .Int) {
@compileError("T must be an integer");
}Errors Point at the Caller
A comptime check runs when the function is instantiated, so the error appears where the wrong type was actually passed in.
Assert Facts with comptime
For simple invariants, comptime assert stops the build when a condition is false, similar to a runtime assert but earlier.
comptime std.debug.assert(@sizeOf(T) <= 8);Check for a Declaration
Use @hasDecl to confirm a type provides a method or constant before you call it, then fail clearly if it does not.
if (!@hasDecl(T, "init")) {
@compileError("T needs an init function");
}Check for a Field
Likewise @hasField tells you whether a struct has a named field, which is handy when validating shapes at compile time.
const ok = @hasField(Config, "port");Build Messages from Type Names
Combine @typeName with std.fmt to embed the offending type in your message, giving callers a precise, readable error.
@compileError("unsupported type: " ++ @typeName(T));Strings Join at Compile Time
The ++ operator concatenates comptime strings, so you can compose detailed error text without any heap allocation.
Zero Runtime Cost
All of this validation happens during compilation. A program that builds successfully carries no checking overhead at run time. ⚡
Better Than a Runtime Crash
Comptime validation turns a possible crash into a friendly build-time message, so misuse is caught early and explained well.
Quick Check
You want to abort the build with a custom message when a type is unsupported. What do you call?
Recap
@compileError plus checks like @hasDecl and comptime assert reject bad usage during the build, with zero runtime cost. 🎯
Frequently asked questions
Is the “Comptime Validation and @compileError” lesson free?
Yes — the full text of “Comptime Validation and @compileError” is free to read here on the web, and the Zig Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Zig Academy course, upgrade to CoddyKit PRO.
What will I learn in “Comptime Validation and @compileError”?
Fail the build with clear messages. You practise Zig Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Zig Academy?
No prior experience is required. Zig Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Comptime Validation and @compileError” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Zig Academy lesson?
Yes. Every Zig Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Reflecting Fields with @typeInfo
- Building Types with @Type
- Comptime Validation and @compileError
- Generating Code with comptime