elisp: Compiling Macros
13.3 Macros and Byte Compilation
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You might ask why we take the trouble to compute an expansion for a
macro and then evaluate the expansion. Why not have the macro body
produce the desired results directly? The reason has to do with
compilation.
When a macro call appears in a Lisp program being compiled, the Lisp
compiler calls the macro definition just as the interpreter would, and
receives an expansion. But instead of evaluating this expansion, it
compiles the expansion as if it had appeared directly in the program.
As a result, the compiled code produces the value and side effects
intended for the macro, but executes at full compiled speed. This would
not work if the macro body computed the value and side effects
itself—they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled. The
compiler has a special feature to help you do this: if a file being
compiled contains a ‘defmacro’ form, the macro is defined temporarily
for the rest of the compilation of that file.
Byte-compiling a file also executes any ‘require’ calls at top-level
in the file, so you can ensure that necessary macro definitions are
available during compilation by requiring the files that define them
(
Named Features). To avoid loading the macro definition files
when someone _runs_ the compiled program, write ‘eval-when-compile’
around the ‘require’ calls (Eval During Compile).
Info Catalog
elisp: Expansion
elisp: Macros
elisp: Defining Macros