17.2 Byte-Compilation Functions

You can byte-compile an individual function or macro definition with the byte-compile function. You can compile a whole file with byte-compile-file, or several files with byte-recompile-directory or batch-byte-compile.

Sometimes, the byte compiler produces warning and/or error messages (see Compiler Errors, for details). These messages are normally recorded in a buffer called *Compile-Log*, which uses Compilation mode. See Compilation Mode in The GNU Emacs Manual. However, if the variable byte-compile-debug is non-nil, error messages will be signaled as Lisp errors instead (see Errors).

Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with require forms which specify the files containing the needed macro definitions (see Features). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles require forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Evaluation During Compilation). For more details, See Macros and Byte Compilation.

Inline (defsubst) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower.

Function: byte-compile symbol

This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; byte-compile does not handle function indirection. The return value is the byte-code function object which is the compiled definition of symbol (see Byte-Code Function Objects).

(defun factorial (integer)
  "Compute factorial of INTEGER."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
⇒ factorial

(byte-compile 'factorial)
  [1 factorial] 4
  "Compute factorial of INTEGER.\n\n(fn INTEGER)"]

If symbol’s definition is a byte-code function object, byte-compile does nothing and returns nil. It does not compile the symbol’s definition again, since the original (non-compiled) code has already been replaced in the symbol’s function cell by the byte-compiled code.

The argument to byte-compile can also be a lambda expression. In that case, the function returns the corresponding compiled code but does not store it anywhere.

Command: compile-defun &optional arg

This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.

compile-defun normally displays the result of evaluation in the echo area, but if arg is non-nil, it inserts the result in the current buffer after the form it has compiled.

Command: byte-compile-file filename

This function compiles a file of Lisp code named filename into a file of byte-code. The output file’s name is made by changing the ‘.el’ suffix into ‘.elc’; if filename does not end in ‘.el’, it adds ‘.elc’ to the end of filename.

Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.

This command returns t if there were no errors and nil otherwise. When called interactively, it prompts for the file name.

$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el

(byte-compile-file "~/emacs/push.el")
     ⇒ t

$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
-rw-rw-rw- 1 lewis lewis 638 Oct  8 20:25 push.elc
Command: byte-recompile-directory directory &optional flag force follow-symlinks

This command recompiles every ‘.el’ file in directory (or its subdirectories) that needs recompilation. A file needs recompilation if a ‘.elc’ file exists but is older than the ‘.el’ file.

When a ‘.el’ file has no corresponding ‘.elc’ file, flag says what to do. If it is nil, this command ignores these files. If flag is 0, it compiles them. If it is neither nil nor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well.

Interactively, byte-recompile-directory prompts for directory and flag is the prefix argument.

If force is non-nil, this command recompiles every ‘.el’ file that has a ‘.elc’ file.

This command will normally not compile ‘.el’ files that are symlinked. If the optional follow-symlink parameter is non-nil, symlinked ‘.el’ will also be compiled.

The returned value is unpredictable.

Function: batch-byte-compile &optional noforce

This function runs byte-compile-file on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code.

If noforce is non-nil, this function does not recompile files that have an up-to-date ‘.elc’ file.

$ emacs -batch -f batch-byte-compile *.el