Compiling Macros

Although it may seem odd that macros return a form to produce a result and not simply the result this is their most important feature. It allows the expansion and the evaluation of the expansion to happen at different times.

The Lisp compiler makes use of this; when it comes across a macro call in a form it is compiling it uses the macroexpand function to produce the expansion of that form which it then compiles straight into the object code. Obviously this is good for performance (why evaluate the expansion every time it is needed when once will do?).

Some rules do need to be observed to make this work properly:

• When the compiler compiles a file it remembers the macros which have been defined by that file; it can only expand a macro call if the definition of the macro appears before the macro call itself (it can't read your mind).
• The macro expansion function (i.e. the definition of the macro) should not have any side effects or evaluate its arguments (the value of a symbol at compile-time probably won't be the same as its value at run-time).
• Macros which are defined by another file must be loaded so they can be recognised. Use the require function, the compiler will evaluate any top-level require forms it sees to bring in any macro definitions used.

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