Lesson 34 of 55
10 mins readPython Writing Macros: macro ... end & @macroexpand
In Plain English
Macros take unevaluated expressions as inputs and return modified expressions at parse time before code is compiled. Call them with the @ prefix.
Deep Dive: How It Works
Macro Definition: macro name(arg) ... end receives arguments as Expr or Symbol objects and returns a replacement Expr.
Macro Invocation: @name expr passes the unevaluated AST of expr to the macro.
@macroexpand: A built-in tool that shows the exact code generated by a macro without running it.
Core Rules to Remember
Compile-Time Transformation: Macros execute during parsing/lowering before LLVM compilation.
@macroexpand Inspection: Debug macro outputs by inspecting their expanded code.
Live Interactive Example
Hit Run Code to see it liveWriting and Expanding a Custom Macro
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: @log_run transformed the input code into a logging block at compile time.
Your Turn: Micro Challenge
No pressure! Edit the starter code below and test your solution with instant feedback.
Micro Exercise
Define a Doubling Macro
- Define `macro double_it(ex) return :( 2 * $(esc(ex)) ) end`.
- Call `val = @double_it 10 + 5` and print `"Doubled: $val"`.
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Sandbox Output
Click "Run & Check" to test your solution.
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