Constraints and Static Checks
Validate parameters before runtime.
Constraints and Static Checks is a free Mojo Academy lesson on CoddyKit — lesson 4 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 Mojo Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Catching Mistakes Before Runtime
A constraint is a rule the compiler verifies about your parameters, stopping a bad build before the program ever runs. ⚙️
The constrained Function
Call constrained with a compile-time condition; if it is false, compilation fails with the message you supply.
fn half[n: Int]():
constrained[n % 2 == 0, "n must be even"]()Conditions Are Compile-Time
The condition must use parameters or aliases, so the check is settled while building, not while the program executes.
fn lanes[w: Int]():
constrained[w > 0, "width must be positive"]()Clear Error Messages
The string you pass becomes the error shown at the call site, so misuse is explained instead of failing in some confusing later way.
Guarding Valid Widths
Constraints shine for SIMD code: require a power-of-two width so the kernel never gets an impossible vector size.
constrained[w & (w - 1) == 0, "width must be a power of two"]()Constraints Document Intent
A clear constraint tells future readers exactly what a parameter expects, acting as machine-checked documentation that can never drift.
Static Asserts in Algorithms
Sprinkle checks through a parametric algorithm so every specialization is validated, catching one bad combination without testing every input.
Cheaper Than Runtime Checks
Because the test runs at compile time, the shipped code carries no if-statement to verify the rule, so there is zero runtime cost.
Pair With Conditional Compilation
Combine constraints with @parameter if: branch on what is supported, and reject the unsupported cases outright with a clear error.
Safer Metaprogramming
Constraints turn flexible parameters into a contract: powerful specialization stays safe because the compiler enforces every rule for you. 🚀
Fail Early, Fail Clearly
Place a constrained check near the top of a function so an invalid parameter is rejected immediately, with the reason stated up front.
Quick Check
Recall what constrained does in Mojo.
Recap
You used constraints: constrained validates parameters at compile time with clear errors, making powerful metaprogramming safe at zero runtime cost. 🎯
Frequently asked questions
Is the “Constraints and Static Checks” lesson free?
Yes — the full text of “Constraints and Static Checks” is free to read here on the web, and the Mojo 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 Mojo Academy course, upgrade to CoddyKit PRO.
What will I learn in “Constraints and Static Checks”?
Validate parameters before runtime. You practise Mojo 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 Mojo Academy?
No prior experience is required. Mojo Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Constraints and Static Checks” 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 Mojo Academy lesson?
Yes. Every Mojo 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
- Parametric Algorithms
- Compile-Time Loop Unrolling
- Conditional Compilation
- Constraints and Static Checks