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17.5 Evaluation During Compilation

These features permit you to write code to be evaluated during compilation of a program.

special form eval-and-compile body…​

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 require is automatically 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).

special form eval-when-compile body…​

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 defsubst functions 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 Lisp idiom (eval-when (compile eval) …). Elsewhere, the Common Lisp ‘#.’ reader macro (but not when interpreting) is closer to what eval-when-compile does.