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1 @c -*-texinfo-*-
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2 @c This is part of the XEmacs Lisp Reference Manual.
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3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
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4 @c See the file lispref.texi for copying conditions.
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5 @setfilename ../../info/macros.info
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6 @node Macros, Loading, Functions, Top
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7 @chapter Macros
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8 @cindex macros
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9
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10 @dfn{Macros} enable you to define new control constructs and other
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11 language features. A macro is defined much like a function, but instead
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12 of telling how to compute a value, it tells how to compute another Lisp
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13 expression which will in turn compute the value. We call this
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14 expression the @dfn{expansion} of the macro.
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15
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16 Macros can do this because they operate on the unevaluated expressions
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17 for the arguments, not on the argument values as functions do. They can
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18 therefore construct an expansion containing these argument expressions
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19 or parts of them.
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20
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21 If you are using a macro to do something an ordinary function could
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22 do, just for the sake of speed, consider using an inline function
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23 instead. @xref{Inline Functions}.
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24
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25 @menu
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26 * Simple Macro:: A basic example.
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27 * Expansion:: How, when and why macros are expanded.
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28 * Compiling Macros:: How macros are expanded by the compiler.
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29 * Defining Macros:: How to write a macro definition.
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30 * Backquote:: Easier construction of list structure.
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31 * Problems with Macros:: Don't evaluate the macro arguments too many times.
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32 Don't hide the user's variables.
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33 @end menu
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34
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35 @node Simple Macro
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36 @section A Simple Example of a Macro
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37
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38 Suppose we would like to define a Lisp construct to increment a
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39 variable value, much like the @code{++} operator in C. We would like to
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40 write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
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41 Here's a macro definition that does the job:
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42
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43 @findex inc
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44 @example
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45 @group
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46 (defmacro inc (var)
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47 (list 'setq var (list '1+ var)))
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48 @end group
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49 @end example
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50
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51 When this is called with @code{(inc x)}, the argument @code{var} has
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52 the value @code{x}---@emph{not} the @emph{value} of @code{x}. The body
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53 of the macro uses this to construct the expansion, which is @code{(setq
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54 x (1+ x))}. Once the macro definition returns this expansion, Lisp
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55 proceeds to evaluate it, thus incrementing @code{x}.
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56
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57 @node Expansion
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58 @section Expansion of a Macro Call
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59 @cindex expansion of macros
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60 @cindex macro call
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61
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62 A macro call looks just like a function call in that it is a list which
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63 starts with the name of the macro. The rest of the elements of the list
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64 are the arguments of the macro.
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65
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66 Evaluation of the macro call begins like evaluation of a function call
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67 except for one crucial difference: the macro arguments are the actual
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68 expressions appearing in the macro call. They are not evaluated before
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69 they are given to the macro definition. By contrast, the arguments of a
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70 function are results of evaluating the elements of the function call
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71 list.
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72
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73 Having obtained the arguments, Lisp invokes the macro definition just
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74 as a function is invoked. The argument variables of the macro are bound
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75 to the argument values from the macro call, or to a list of them in the
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76 case of a @code{&rest} argument. And the macro body executes and
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77 returns its value just as a function body does.
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78
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79 The second crucial difference between macros and functions is that the
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80 value returned by the macro body is not the value of the macro call.
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81 Instead, it is an alternate expression for computing that value, also
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82 known as the @dfn{expansion} of the macro. The Lisp interpreter
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83 proceeds to evaluate the expansion as soon as it comes back from the
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84 macro.
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85
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86 Since the expansion is evaluated in the normal manner, it may contain
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87 calls to other macros. It may even be a call to the same macro, though
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88 this is unusual.
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89
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90 You can see the expansion of a given macro call by calling
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91 @code{macroexpand}.
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92
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93 @defun macroexpand form &optional environment
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94 @cindex macro expansion
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95 This function expands @var{form}, if it is a macro call. If the result
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96 is another macro call, it is expanded in turn, until something which is
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97 not a macro call results. That is the value returned by
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98 @code{macroexpand}. If @var{form} is not a macro call to begin with, it
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99 is returned as given.
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100
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101 Note that @code{macroexpand} does not look at the subexpressions of
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102 @var{form} (although some macro definitions may do so). Even if they
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103 are macro calls themselves, @code{macroexpand} does not expand them.
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104
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105 The function @code{macroexpand} does not expand calls to inline functions.
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106 Normally there is no need for that, since a call to an inline function is
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107 no harder to understand than a call to an ordinary function.
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108
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109 If @var{environment} is provided, it specifies an alist of macro
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110 definitions that shadow the currently defined macros. Byte compilation
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111 uses this feature.
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112
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113 @smallexample
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114 @group
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115 (defmacro inc (var)
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116 (list 'setq var (list '1+ var)))
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117 @result{} inc
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118 @end group
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119
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120 @group
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121 (macroexpand '(inc r))
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122 @result{} (setq r (1+ r))
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123 @end group
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124
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125 @group
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126 (defmacro inc2 (var1 var2)
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127 (list 'progn (list 'inc var1) (list 'inc var2)))
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128 @result{} inc2
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129 @end group
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130
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131 @group
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132 (macroexpand '(inc2 r s))
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133 @result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.}
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134 @end group
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135 @end smallexample
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136 @end defun
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137
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138 @node Compiling Macros
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139 @section Macros and Byte Compilation
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140 @cindex byte-compiling macros
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141
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142 You might ask why we take the trouble to compute an expansion for a
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143 macro and then evaluate the expansion. Why not have the macro body
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144 produce the desired results directly? The reason has to do with
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145 compilation.
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146
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147 When a macro call appears in a Lisp program being compiled, the Lisp
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148 compiler calls the macro definition just as the interpreter would, and
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149 receives an expansion. But instead of evaluating this expansion, it
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150 compiles the expansion as if it had appeared directly in the program.
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151 As a result, the compiled code produces the value and side effects
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152 intended for the macro, but executes at full compiled speed. This would
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153 not work if the macro body computed the value and side effects
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154 itself---they would be computed at compile time, which is not useful.
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155
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156 In order for compilation of macro calls to work, the macros must be
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157 defined in Lisp when the calls to them are compiled. The compiler has a
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158 special feature to help you do this: if a file being compiled contains a
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159 @code{defmacro} form, the macro is defined temporarily for the rest of
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160 the compilation of that file. To use this feature, you must define the
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161 macro in the same file where it is used and before its first use.
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162
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163 Byte-compiling a file executes any @code{require} calls at top-level
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164 in the file. This is in case the file needs the required packages for
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165 proper compilation. One way to ensure that necessary macro definitions
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166 are available during compilation is to require the files that define
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167 them (@pxref{Named Features}). To avoid loading the macro definition files
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168 when someone @emph{runs} the compiled program, write
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169 @code{eval-when-compile} around the @code{require} calls (@pxref{Eval
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170 During Compile}).
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171
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172 @node Defining Macros
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173 @section Defining Macros
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174
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175 A Lisp macro is a list whose @sc{car} is @code{macro}. Its @sc{cdr} should
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176 be a function; expansion of the macro works by applying the function
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177 (with @code{apply}) to the list of unevaluated argument-expressions
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178 from the macro call.
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179
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180 It is possible to use an anonymous Lisp macro just like an anonymous
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181 function, but this is never done, because it does not make sense to pass
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182 an anonymous macro to functionals such as @code{mapcar}. In practice,
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183 all Lisp macros have names, and they are usually defined with the
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184 special form @code{defmacro}.
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185
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186 @defspec defmacro name argument-list body-forms@dots{}
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187 @code{defmacro} defines the symbol @var{name} as a macro that looks
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188 like this:
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189
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190 @example
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191 (macro lambda @var{argument-list} . @var{body-forms})
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192 @end example
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193
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194 This macro object is stored in the function cell of @var{name}. The
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195 value returned by evaluating the @code{defmacro} form is @var{name}, but
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196 usually we ignore this value.
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197
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198 The shape and meaning of @var{argument-list} is the same as in a
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199 function, and the keywords @code{&rest} and @code{&optional} may be used
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200 (@pxref{Argument List}). Macros may have a documentation string, but
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201 any @code{interactive} declaration is ignored since macros cannot be
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202 called interactively.
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203 @end defspec
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204
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205 @node Backquote
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206 @section Backquote
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207 @cindex backquote (list substitution)
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208 @cindex ` (list substitution)
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209 @findex `
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210
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211 Macros often need to construct large list structures from a mixture of
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212 constants and nonconstant parts. To make this easier, use the macro
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213 @samp{`} (often called @dfn{backquote}).
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214
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215 Backquote allows you to quote a list, but selectively evaluate
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216 elements of that list. In the simplest case, it is identical to the
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217 special form @code{quote} (@pxref{Quoting}). For example, these
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218 two forms yield identical results:
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219
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220 @example
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221 @group
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222 `(a list of (+ 2 3) elements)
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223 @result{} (a list of (+ 2 3) elements)
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224 @end group
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225 @group
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226 '(a list of (+ 2 3) elements)
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227 @result{} (a list of (+ 2 3) elements)
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228 @end group
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229 @end example
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230
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231 @findex , @r{(with Backquote)}
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232 The special marker @samp{,} inside of the argument to backquote
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233 indicates a value that isn't constant. Backquote evaluates the
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234 argument of @samp{,} and puts the value in the list structure:
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235
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236 @example
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237 @group
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238 (list 'a 'list 'of (+ 2 3) 'elements)
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239 @result{} (a list of 5 elements)
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240 @end group
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241 @group
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242 `(a list of ,(+ 2 3) elements)
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243 @result{} (a list of 5 elements)
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244 @end group
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245 @end example
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246
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247 @findex ,@@ @r{(with Backquote)}
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248 @cindex splicing (with backquote)
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249 You can also @dfn{splice} an evaluated value into the resulting list,
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250 using the special marker @samp{,@@}. The elements of the spliced list
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251 become elements at the same level as the other elements of the resulting
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252 list. The equivalent code without using @samp{`} is often unreadable.
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253 Here are some examples:
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254
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255 @example
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256 @group
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257 (setq some-list '(2 3))
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258 @result{} (2 3)
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259 @end group
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260 @group
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261 (cons 1 (append some-list '(4) some-list))
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262 @result{} (1 2 3 4 2 3)
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263 @end group
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264 @group
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265 `(1 ,@@some-list 4 ,@@some-list)
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266 @result{} (1 2 3 4 2 3)
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267 @end group
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268
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269 @group
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270 (setq list '(hack foo bar))
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271 @result{} (hack foo bar)
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272 @end group
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273 @group
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274 (cons 'use
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275 (cons 'the
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276 (cons 'words (append (cdr list) '(as elements)))))
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277 @result{} (use the words foo bar as elements)
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278 @end group
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279 @group
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280 `(use the words ,@@(cdr list) as elements)
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281 @result{} (use the words foo bar as elements)
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282 @end group
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283 @end example
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284
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285 @quotation
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286 In older versions of Emacs (before XEmacs 19.12 or FSF Emacs version
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287 19.29), @samp{`} used a different syntax which required an extra level
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288 of parentheses around the entire backquote construct. Likewise, each
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289 @samp{,} or @samp{,@@} substitution required an extra level of
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290 parentheses surrounding both the @samp{,} or @samp{,@@} and the
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291 following expression. The old syntax required whitespace between the
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292 @samp{`}, @samp{,} or @samp{,@@} and the following expression.
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293
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294 This syntax is still accepted, but no longer recommended except for
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295 compatibility with old Emacs versions.
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296 @end quotation
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297
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298 @node Problems with Macros
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299 @section Common Problems Using Macros
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300
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301 The basic facts of macro expansion have counterintuitive consequences.
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302 This section describes some important consequences that can lead to
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303 trouble, and rules to follow to avoid trouble.
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304
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305 @menu
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306 * Argument Evaluation:: The expansion should evaluate each macro arg once.
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307 * Surprising Local Vars:: Local variable bindings in the expansion
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308 require special care.
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309 * Eval During Expansion:: Don't evaluate them; put them in the expansion.
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310 * Repeated Expansion:: Avoid depending on how many times expansion is done.
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311 @end menu
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312
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313 @node Argument Evaluation
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314 @subsection Evaluating Macro Arguments Repeatedly
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315
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316 When defining a macro you must pay attention to the number of times
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317 the arguments will be evaluated when the expansion is executed. The
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318 following macro (used to facilitate iteration) illustrates the problem.
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319 This macro allows us to write a simple ``for'' loop such as one might
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320 find in Pascal.
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321
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322 @findex for
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323 @smallexample
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324 @group
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325 (defmacro for (var from init to final do &rest body)
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326 "Execute a simple \"for\" loop.
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327 For example, (for i from 1 to 10 do (print i))."
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328 (list 'let (list (list var init))
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329 (cons 'while (cons (list '<= var final)
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330 (append body (list (list 'inc var)))))))
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331 @end group
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332 @result{} for
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333
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334 @group
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335 (for i from 1 to 3 do
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336 (setq square (* i i))
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337 (princ (format "\n%d %d" i square)))
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338 @expansion{}
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339 @end group
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340 @group
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341 (let ((i 1))
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342 (while (<= i 3)
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343 (setq square (* i i))
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344 (princ (format "%d %d" i square))
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345 (inc i)))
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346 @end group
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347 @group
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348
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349 @print{}1 1
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350 @print{}2 4
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351 @print{}3 9
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352 @result{} nil
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353 @end group
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354 @end smallexample
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355
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356 @noindent
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357 (The arguments @code{from}, @code{to}, and @code{do} in this macro are
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358 ``syntactic sugar''; they are entirely ignored. The idea is that you
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359 will write noise words (such as @code{from}, @code{to}, and @code{do})
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360 in those positions in the macro call.)
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361
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362 Here's an equivalent definition simplified through use of backquote:
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363
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364 @smallexample
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365 @group
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366 (defmacro for (var from init to final do &rest body)
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367 "Execute a simple \"for\" loop.
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368 For example, (for i from 1 to 10 do (print i))."
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369 `(let ((,var ,init))
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370 (while (<= ,var ,final)
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371 ,@@body
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372 (inc ,var))))
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373 @end group
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374 @end smallexample
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375
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376 Both forms of this definition (with backquote and without) suffer from
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377 the defect that @var{final} is evaluated on every iteration. If
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378 @var{final} is a constant, this is not a problem. If it is a more
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379 complex form, say @code{(long-complex-calculation x)}, this can slow
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380 down the execution significantly. If @var{final} has side effects,
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381 executing it more than once is probably incorrect.
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382
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383 @cindex macro argument evaluation
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384 A well-designed macro definition takes steps to avoid this problem by
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385 producing an expansion that evaluates the argument expressions exactly
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386 once unless repeated evaluation is part of the intended purpose of the
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387 macro. Here is a correct expansion for the @code{for} macro:
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388
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389 @smallexample
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390 @group
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391 (let ((i 1)
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392 (max 3))
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393 (while (<= i max)
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394 (setq square (* i i))
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395 (princ (format "%d %d" i square))
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396 (inc i)))
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397 @end group
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398 @end smallexample
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399
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400 Here is a macro definition that creates this expansion:
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401
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402 @smallexample
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403 @group
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404 (defmacro for (var from init to final do &rest body)
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405 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
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406 `(let ((,var ,init)
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407 (max ,final))
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408 (while (<= ,var max)
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409 ,@@body
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410 (inc ,var))))
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411 @end group
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412 @end smallexample
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413
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414 Unfortunately, this introduces another problem.
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415 @ifinfo
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416 Proceed to the following node.
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417 @end ifinfo
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418
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419 @node Surprising Local Vars
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420 @subsection Local Variables in Macro Expansions
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421
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422 @ifinfo
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423 In the previous section, the definition of @code{for} was fixed as
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424 follows to make the expansion evaluate the macro arguments the proper
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425 number of times:
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426
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427 @smallexample
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428 @group
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429 (defmacro for (var from init to final do &rest body)
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430 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
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431 @end group
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432 @group
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433 `(let ((,var ,init)
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434 (max ,final))
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435 (while (<= ,var max)
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436 ,@@body
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437 (inc ,var))))
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438 @end group
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439 @end smallexample
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440 @end ifinfo
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441
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442 The new definition of @code{for} has a new problem: it introduces a
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443 local variable named @code{max} which the user does not expect. This
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444 causes trouble in examples such as the following:
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445
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446 @smallexample
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447 @group
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448 (let ((max 0))
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449 (for x from 0 to 10 do
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450 (let ((this (frob x)))
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451 (if (< max this)
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452 (setq max this)))))
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453 @end group
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454 @end smallexample
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455
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456 @noindent
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457 The references to @code{max} inside the body of the @code{for}, which
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458 are supposed to refer to the user's binding of @code{max}, really access
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459 the binding made by @code{for}.
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460
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461 The way to correct this is to use an uninterned symbol instead of
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462 @code{max} (@pxref{Creating Symbols}). The uninterned symbol can be
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463 bound and referred to just like any other symbol, but since it is
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464 created by @code{for}, we know that it cannot already appear in the
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465 user's program. Since it is not interned, there is no way the user can
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466 put it into the program later. It will never appear anywhere except
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467 where put by @code{for}. Here is a definition of @code{for} that works
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468 this way:
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469
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470 @smallexample
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471 @group
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472 (defmacro for (var from init to final do &rest body)
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473 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
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474 (let ((tempvar (make-symbol "max")))
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475 `(let ((,var ,init)
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476 (,tempvar ,final))
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477 (while (<= ,var ,tempvar)
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478 ,@@body
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479 (inc ,var)))))
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480 @end group
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481 @end smallexample
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482
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483 @noindent
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484 This creates an uninterned symbol named @code{max} and puts it in the
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485 expansion instead of the usual interned symbol @code{max} that appears
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486 in expressions ordinarily.
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487
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488 @node Eval During Expansion
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489 @subsection Evaluating Macro Arguments in Expansion
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490
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491 Another problem can happen if you evaluate any of the macro argument
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492 expressions during the computation of the expansion, such as by calling
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493 @code{eval} (@pxref{Eval}). If the argument is supposed to refer to the
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494 user's variables, you may have trouble if the user happens to use a
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495 variable with the same name as one of the macro arguments. Inside the
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496 macro body, the macro argument binding is the most local binding of this
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497 variable, so any references inside the form being evaluated do refer
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498 to it. Here is an example:
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499
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500 @example
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501 @group
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502 (defmacro foo (a)
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503 (list 'setq (eval a) t))
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504 @result{} foo
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505 @end group
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506 @group
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507 (setq x 'b)
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508 (foo x) @expansion{} (setq b t)
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509 @result{} t ; @r{and @code{b} has been set.}
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510 ;; @r{but}
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511 (setq a 'c)
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512 (foo a) @expansion{} (setq a t)
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513 @result{} t ; @r{but this set @code{a}, not @code{c}.}
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514
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515 @end group
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516 @end example
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517
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518 It makes a difference whether the user's variable is named @code{a} or
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519 @code{x}, because @code{a} conflicts with the macro argument variable
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520 @code{a}.
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521
|
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522 Another reason not to call @code{eval} in a macro definition is that
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523 it probably won't do what you intend in a compiled program. The
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|
524 byte-compiler runs macro definitions while compiling the program, when
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525 the program's own computations (which you might have wished to access
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526 with @code{eval}) don't occur and its local variable bindings don't
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527 exist.
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528
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529 The safe way to work with the run-time value of an expression is to
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530 put the expression into the macro expansion, so that its value is
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531 computed as part of executing the expansion.
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532
|
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533 @node Repeated Expansion
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|
|
534 @subsection How Many Times is the Macro Expanded?
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|
|
535
|
|
|
536 Occasionally problems result from the fact that a macro call is
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537 expanded each time it is evaluated in an interpreted function, but is
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538 expanded only once (during compilation) for a compiled function. If the
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539 macro definition has side effects, they will work differently depending
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540 on how many times the macro is expanded.
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541
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542 In particular, constructing objects is a kind of side effect. If the
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543 macro is called once, then the objects are constructed only once. In
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544 other words, the same structure of objects is used each time the macro
|
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545 call is executed. In interpreted operation, the macro is reexpanded
|
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546 each time, producing a fresh collection of objects each time. Usually
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547 this does not matter---the objects have the same contents whether they
|
|
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548 are shared or not. But if the surrounding program does side effects
|
|
|
549 on the objects, it makes a difference whether they are shared. Here is
|
|
|
550 an example:
|
|
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551
|
|
|
552 @lisp
|
|
|
553 @group
|
|
|
554 (defmacro empty-object ()
|
|
|
555 (list 'quote (cons nil nil)))
|
|
|
556 @end group
|
|
|
557
|
|
|
558 @group
|
|
|
559 (defun initialize (condition)
|
|
|
560 (let ((object (empty-object)))
|
|
|
561 (if condition
|
|
|
562 (setcar object condition))
|
|
|
563 object))
|
|
|
564 @end group
|
|
|
565 @end lisp
|
|
|
566
|
|
|
567 @noindent
|
|
|
568 If @code{initialize} is interpreted, a new list @code{(nil)} is
|
|
|
569 constructed each time @code{initialize} is called. Thus, no side effect
|
|
|
570 survives between calls. If @code{initialize} is compiled, then the
|
|
|
571 macro @code{empty-object} is expanded during compilation, producing a
|
|
|
572 single ``constant'' @code{(nil)} that is reused and altered each time
|
|
|
573 @code{initialize} is called.
|
|
|
574
|
|
|
575 One way to avoid pathological cases like this is to think of
|
|
|
576 @code{empty-object} as a funny kind of constant, not as a memory
|
|
|
577 allocation construct. You wouldn't use @code{setcar} on a constant such
|
|
|
578 as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)}
|
|
|
579 either.
|
