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Diffstat (limited to 'lispref/macros.texi')
| -rw-r--r-- | lispref/macros.texi | 50 |
1 files changed, 34 insertions, 16 deletions
diff --git a/lispref/macros.texi b/lispref/macros.texi index 22a07f14dbe..9ec72243562 100644 --- a/lispref/macros.texi +++ b/lispref/macros.texi @@ -1,9 +1,9 @@ @c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. -@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. +@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../info/macros -@node Macros, Loading, Functions, Top +@node Macros, Customization, Functions, Top @chapter Macros @cindex macros @@ -153,12 +153,13 @@ 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 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 -@code{defmacro} form, the macro is defined temporarily for the rest of -the compilation of that file. To use this feature, you must define the -macro in the same file where it is used and before its first use. + 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 @code{defmacro} form, the macro is defined +temporarily for the rest of the compilation of that file. To make this +feature work, you must put the @code{defmacro} in the same file where it +is used, and before its first use. Byte-compiling a file executes any @code{require} calls at top-level in the file. This is in case the file needs the required packages for @@ -244,9 +245,25 @@ argument of @samp{,} and puts the value in the list structure: @end group @end example + Substitution with @samp{,} is allowed at deeper levels of the list +structure also. For example: + +@example +@group +(defmacro t-becomes-nil (variable) + `(if (eq ,variable t) + (setq ,variable nil))) +@end group + +@group +(t-becomes-nil foo) + @equiv{} (if (eq foo t) (setq foo nil)) +@end group +@end example + @findex ,@@ @r{(with Backquote)} @cindex splicing (with backquote) -You can also @dfn{splice} an evaluated value into the resulting list, + You can also @dfn{splice} an evaluated value into the resulting list, using the special marker @samp{,@@}. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using @samp{`} is often unreadable. @@ -291,8 +308,8 @@ and the following expression. The old syntax required whitespace between the @samp{`}, @samp{,} or @samp{,@@} and the following expression. -This syntax is still accepted, but no longer recommended except for -compatibility with old Emacs versions. +This syntax is still accepted, for compatibility with old Emacs +versions, but we recommend not using it in new programs. @end quotation @node Problems with Macros @@ -488,14 +505,14 @@ in expressions ordinarily. @node Eval During Expansion @subsection Evaluating Macro Arguments in Expansion - Another problem can happen if you evaluate any of the macro argument -expressions during the computation of the expansion, such as by calling + Another problem can happen if you the macro definition itself +evaluates any of the macro argument expressions, such as by calling @code{eval} (@pxref{Eval}). If the argument is supposed to refer to the user's variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this -variable, so any references inside the form being evaluated do refer -to it. Here is an example: +variable, so any references inside the form being evaluated do refer to +it. Here is an example: @example @group @@ -528,7 +545,8 @@ exist. The safe way to work with the run-time value of an expression is to put the expression into the macro expansion, so that its value is -computed as part of executing the expansion. +computed as part of executing the expansion. This is what the other +examples in this chapter do. @node Repeated Expansion @subsection How Many Times is the Macro Expanded? |