Conditional Compilation
Branch code paths at compile time.
Conditional Compilation is a free Mojo 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 Mojo Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Choosing Code at Compile Time
Conditional compilation picks which code path to build based on compile-time values, so the unused branch never reaches the final program. ⚙️
The @parameter if Statement
Add @parameter to an if whose condition is known at compile time, and Mojo keeps only the branch that matches.
@parameter
if width == 1:
scalar_path()
else:
vector_path()The Condition Must Be Static
The test has to use parameters or aliases, values the compiler already knows, not data that only appears while the program runs.
alias DEBUG = False
@parameter
if DEBUG:
print("tracing")Dead Branches Disappear
The branch that fails the condition is simply not compiled, so it adds no size and costs nothing at runtime.
Specialize by Type
Use it to switch logic based on a type parameter, choosing the best implementation for floats versus integers, for example.
@parameter
if T.is_floating_point():
use_fma()
else:
use_int_mul()Branch on Hardware Facts
Pair conditional compilation with platform checks to emit the right kernel for the CPU or accelerator you are targeting.
Feature Flags Without Cost
Gate optional features behind an alias flag; when it is False the whole feature compiles away, leaving a lean binary.
Different From a Runtime if
A normal if always keeps both branches in the binary and tests at runtime; @parameter if decides once, at build time, and drops the rest.
Combine With Other Metaprogramming
Conditional compilation mixes well with parameterized functions and unrolled loops, letting each specialization take its own optimal path.
One Source, Many Targets
From a single definition you generate tailored builds for each platform or type, with no runtime branching tax to pay. 🚀
Keep Both Paths Valid
Each branch must still compile on its own when chosen, so write every path to be correct for the parameter values that select it.
Quick Check
Compare @parameter if with a normal if.
Recap
@parameter if chooses code paths at compile time: dead branches vanish, you specialize by type or platform, and the binary stays lean. 🎯
Frequently asked questions
Is the “Conditional Compilation” lesson free?
Yes — the full text of “Conditional Compilation” 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 “Conditional Compilation”?
Branch code paths at compile time. 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 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Conditional Compilation” 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