条件编译
在编译时选择代码路径
条件编译 是 CoddyKit 上的免费 Mojo Academy 课时。 这是第 3 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Mojo Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Mojo Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
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. 🎯
常见问题解答
「条件编译」课时是免费的吗？
是的 — 「条件编译」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Mojo Academy 课程的其余内容，请升级到 CoddyKit PRO。 Mojo Academy 课程共包含 4 节课。
「条件编译」这节课中我会学到什么？
在编译时选择代码路径 你通过在浏览器中直接运行的动手代码来练习 Mojo Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 Mojo Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Mojo Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 3 节课，共 4 节。
「条件编译」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 Mojo Academy 课中编写并运行代码吗？
能。每节 Mojo Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。