14.5 Common Problems Using Macros

Macro expansion can have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.

• Wrong Time		Do the work in the expansion, not in the macro.
• Argument Evaluation		The expansion should evaluate each macro arg once.
• Surprising Local Vars		Local variable bindings in the expansion require special care.
• Eval During Expansion		Don’t evaluate them; put them in the expansion.
• Repeated Expansion		Avoid depending on how many times expansion is done.