These features permit you to write code to be evaluated during compilation of a program.
This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).
You can get a similar result by putting body in a separate file
and referring to that file with
require. That method is
preferable when body is large. Effectively
eval-and-compile, the package is loaded both when
compiling and executing.
autoload is also effectively
eval-and-compile too. It’s
recognized when compiling, so uses of such a function don’t produce
“not known to be defined” warnings.
Most uses of
eval-and-compile are fairly sophisticated.
If a macro has a helper function to build its result, and that macro
is used both locally and outside the package, then
eval-and-compile should be used to get the helper both when
compiling and then later when running.
If functions are defined programmatically (with
fset say), then
eval-and-compile can be used to have that done at compile-time
as well as run-time, so calls to those functions are checked (and
warnings about “not known to be defined” suppressed).
This form marks body to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, body is evaluated normally.
If you have a constant that needs some calculation to produce,
eval-when-compile can do that at compile-time. For example,
(defvar my-regexp (eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))
If you’re using another package, but only need macros from it (the
byte compiler will expand those), then
eval-when-compile can be
used to load it for compiling, but not executing. For example,
(eval-when-compile (require 'my-macro-package))
The same sort of thing goes for macros and
defined locally and only for use within the file. They are needed for
compiling the file, but in most cases they are not needed for
execution of the compiled file. For example,
(eval-when-compile (unless (fboundp 'some-new-thing) (defmacro 'some-new-thing () (compatibility code))))
This is often good for code that’s only a fallback for compatibility with other versions of Emacs.
Common Lisp Note: At top level,
eval-when-compile is analogous to the Common
(eval-when (compile eval) …). Elsewhere, the
Common Lisp ‘#.’ reader macro (but not when interpreting) is closer