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Conditional compilation picks which code path to build based on compile-time values, so the unused branch never reaches the final program. ⚙️

Add @parameter to an if whose condition is known at compile time, and Mojo keeps only the branch that matches.

The test has to use parameters or aliases, values the compiler already knows, not data that only appears while the program runs.

The branch that fails the condition is simply not compiled, so it adds no size and costs nothing at runtime.

Use it to switch logic based on a type parameter, choosing the best implementation for floats versus integers, for example.

Pair conditional compilation with platform checks to emit the right kernel for the CPU or accelerator you are targeting.

Gate optional features behind an alias flag; when it is False the whole feature compiles away, leaving a lean binary.

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.

Conditional compilation mixes well with parameterized functions and unrolled loops, letting each specialization take its own optimal path.

From a single definition you generate tailored builds for each platform or type, with no runtime branching tax to pay. 🚀

Each branch must still compile on its own when chosen, so write every path to be correct for the parameter values that select it.

Compare @parameter if with a normal if.

@parameter if chooses code paths at compile time: dead branches vanish, you specialize by type or platform, and the binary stays lean. 🎯