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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/control.info
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6 @node Control Structures, Variables, Evaluation, Top
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7 @chapter Control Structures
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8 @cindex special forms for control structures
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9 @cindex control structures
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10
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11 A Lisp program consists of expressions or @dfn{forms} (@pxref{Forms}).
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12 We control the order of execution of the forms by enclosing them in
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13 @dfn{control structures}. Control structures are special forms which
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14 control when, whether, or how many times to execute the forms they
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15 contain.
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16
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17 The simplest order of execution is sequential execution: first form
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18 @var{a}, then form @var{b}, and so on. This is what happens when you
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19 write several forms in succession in the body of a function, or at top
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20 level in a file of Lisp code---the forms are executed in the order
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21 written. We call this @dfn{textual order}. For example, if a function
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22 body consists of two forms @var{a} and @var{b}, evaluation of the
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23 function evaluates first @var{a} and then @var{b}, and the function's
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24 value is the value of @var{b}.
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25
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26 Explicit control structures make possible an order of execution other
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27 than sequential.
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28
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29 XEmacs Lisp provides several kinds of control structure, including
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30 other varieties of sequencing, conditionals, iteration, and (controlled)
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31 jumps---all discussed below. The built-in control structures are
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32 special forms since their subforms are not necessarily evaluated or not
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33 evaluated sequentially. You can use macros to define your own control
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34 structure constructs (@pxref{Macros}).
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35
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36 @menu
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37 * Sequencing:: Evaluation in textual order.
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38 * Conditionals:: @code{if}, @code{cond}.
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39 * Combining Conditions:: @code{and}, @code{or}, @code{not}.
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40 * Iteration:: @code{while} loops.
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41 * Nonlocal Exits:: Jumping out of a sequence.
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42 @end menu
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43
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44 @node Sequencing
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45 @section Sequencing
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46
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47 Evaluating forms in the order they appear is the most common way
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48 control passes from one form to another. In some contexts, such as in a
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49 function body, this happens automatically. Elsewhere you must use a
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50 control structure construct to do this: @code{progn}, the simplest
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51 control construct of Lisp.
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52
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53 A @code{progn} special form looks like this:
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54
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55 @example
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56 @group
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57 (progn @var{a} @var{b} @var{c} @dots{})
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58 @end group
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59 @end example
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60
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61 @noindent
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62 and it says to execute the forms @var{a}, @var{b}, @var{c} and so on, in
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63 that order. These forms are called the body of the @code{progn} form.
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64 The value of the last form in the body becomes the value of the entire
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65 @code{progn}.
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66
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67 @cindex implicit @code{progn}
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68 In the early days of Lisp, @code{progn} was the only way to execute
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69 two or more forms in succession and use the value of the last of them.
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70 But programmers found they often needed to use a @code{progn} in the
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71 body of a function, where (at that time) only one form was allowed. So
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72 the body of a function was made into an ``implicit @code{progn}'':
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73 several forms are allowed just as in the body of an actual @code{progn}.
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74 Many other control structures likewise contain an implicit @code{progn}.
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75 As a result, @code{progn} is not used as often as it used to be. It is
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76 needed now most often inside an @code{unwind-protect}, @code{and},
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77 @code{or}, or in the @var{then}-part of an @code{if}.
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78
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79 @defspec progn forms@dots{}
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80 This special form evaluates all of the @var{forms}, in textual
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81 order, returning the result of the final form.
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82
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83 @example
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84 @group
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85 (progn (print "The first form")
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86 (print "The second form")
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87 (print "The third form"))
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88 @print{} "The first form"
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89 @print{} "The second form"
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90 @print{} "The third form"
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91 @result{} "The third form"
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92 @end group
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93 @end example
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94 @end defspec
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95
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96 Two other control constructs likewise evaluate a series of forms but return
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97 a different value:
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98
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99 @defspec prog1 form1 forms@dots{}
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100 This special form evaluates @var{form1} and all of the @var{forms}, in
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101 textual order, returning the result of @var{form1}.
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102
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103 @example
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104 @group
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105 (prog1 (print "The first form")
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106 (print "The second form")
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107 (print "The third form"))
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108 @print{} "The first form"
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109 @print{} "The second form"
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110 @print{} "The third form"
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111 @result{} "The first form"
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112 @end group
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113 @end example
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114
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115 Here is a way to remove the first element from a list in the variable
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116 @code{x}, then return the value of that former element:
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117
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118 @example
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119 (prog1 (car x) (setq x (cdr x)))
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120 @end example
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121 @end defspec
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122
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123 @defspec prog2 form1 form2 forms@dots{}
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124 This special form evaluates @var{form1}, @var{form2}, and all of the
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125 following @var{forms}, in textual order, returning the result of
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126 @var{form2}.
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127
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128 @example
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129 @group
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130 (prog2 (print "The first form")
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131 (print "The second form")
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132 (print "The third form"))
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133 @print{} "The first form"
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134 @print{} "The second form"
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135 @print{} "The third form"
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136 @result{} "The second form"
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137 @end group
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138 @end example
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139 @end defspec
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140
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141 @node Conditionals
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142 @section Conditionals
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143 @cindex conditional evaluation
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144
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145 Conditional control structures choose among alternatives. XEmacs Lisp
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146 has two conditional forms: @code{if}, which is much the same as in other
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147 languages, and @code{cond}, which is a generalized case statement.
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148
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149 @defspec if condition then-form else-forms@dots{}
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150 @code{if} chooses between the @var{then-form} and the @var{else-forms}
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151 based on the value of @var{condition}. If the evaluated @var{condition} is
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152 non-@code{nil}, @var{then-form} is evaluated and the result returned.
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153 Otherwise, the @var{else-forms} are evaluated in textual order, and the
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154 value of the last one is returned. (The @var{else} part of @code{if} is
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155 an example of an implicit @code{progn}. @xref{Sequencing}.)
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156
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157 If @var{condition} has the value @code{nil}, and no @var{else-forms} are
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158 given, @code{if} returns @code{nil}.
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159
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160 @code{if} is a special form because the branch that is not selected is
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161 never evaluated---it is ignored. Thus, in the example below,
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162 @code{true} is not printed because @code{print} is never called.
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163
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164 @example
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165 @group
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166 (if nil
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167 (print 'true)
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168 'very-false)
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169 @result{} very-false
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170 @end group
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171 @end example
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172 @end defspec
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173
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174 @defspec cond clause@dots{}
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175 @code{cond} chooses among an arbitrary number of alternatives. Each
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176 @var{clause} in the @code{cond} must be a list. The @sc{car} of this
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177 list is the @var{condition}; the remaining elements, if any, the
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178 @var{body-forms}. Thus, a clause looks like this:
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179
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180 @example
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181 (@var{condition} @var{body-forms}@dots{})
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182 @end example
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183
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184 @code{cond} tries the clauses in textual order, by evaluating the
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185 @var{condition} of each clause. If the value of @var{condition} is
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186 non-@code{nil}, the clause ``succeeds''; then @code{cond} evaluates its
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187 @var{body-forms}, and the value of the last of @var{body-forms} becomes
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188 the value of the @code{cond}. The remaining clauses are ignored.
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189
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190 If the value of @var{condition} is @code{nil}, the clause ``fails'', so
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191 the @code{cond} moves on to the following clause, trying its
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192 @var{condition}.
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193
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194 If every @var{condition} evaluates to @code{nil}, so that every clause
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195 fails, @code{cond} returns @code{nil}.
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196
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197 A clause may also look like this:
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198
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199 @example
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200 (@var{condition})
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201 @end example
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202
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203 @noindent
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204 Then, if @var{condition} is non-@code{nil} when tested, the value of
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205 @var{condition} becomes the value of the @code{cond} form.
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206
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207 The following example has four clauses, which test for the cases where
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208 the value of @code{x} is a number, string, buffer and symbol,
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209 respectively:
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210
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211 @example
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212 @group
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213 (cond ((numberp x) x)
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214 ((stringp x) x)
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215 ((bufferp x)
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216 (setq temporary-hack x) ; @r{multiple body-forms}
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217 (buffer-name x)) ; @r{in one clause}
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218 ((symbolp x) (symbol-value x)))
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219 @end group
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220 @end example
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221
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222 Often we want to execute the last clause whenever none of the previous
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223 clauses was successful. To do this, we use @code{t} as the
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224 @var{condition} of the last clause, like this: @code{(t
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225 @var{body-forms})}. The form @code{t} evaluates to @code{t}, which is
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226 never @code{nil}, so this clause never fails, provided the @code{cond}
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227 gets to it at all.
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228
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229 For example,
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230
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231 @example
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232 @group
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233 (cond ((eq a 'hack) 'foo)
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234 (t "default"))
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235 @result{} "default"
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236 @end group
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237 @end example
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238
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239 @noindent
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240 This expression is a @code{cond} which returns @code{foo} if the value
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241 of @code{a} is 1, and returns the string @code{"default"} otherwise.
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242 @end defspec
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243
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244 Any conditional construct can be expressed with @code{cond} or with
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245 @code{if}. Therefore, the choice between them is a matter of style.
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246 For example:
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247
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248 @example
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249 @group
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250 (if @var{a} @var{b} @var{c})
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251 @equiv{}
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252 (cond (@var{a} @var{b}) (t @var{c}))
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253 @end group
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254 @end example
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255
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256 @node Combining Conditions
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257 @section Constructs for Combining Conditions
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258
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259 This section describes three constructs that are often used together
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260 with @code{if} and @code{cond} to express complicated conditions. The
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261 constructs @code{and} and @code{or} can also be used individually as
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262 kinds of multiple conditional constructs.
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263
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264 @defun not condition
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265 This function tests for the falsehood of @var{condition}. It returns
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266 @code{t} if @var{condition} is @code{nil}, and @code{nil} otherwise.
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267 The function @code{not} is identical to @code{null}, and we recommend
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268 using the name @code{null} if you are testing for an empty list.
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269 @end defun
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270
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271 @defspec and conditions@dots{}
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272 The @code{and} special form tests whether all the @var{conditions} are
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273 true. It works by evaluating the @var{conditions} one by one in the
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274 order written.
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275
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276 If any of the @var{conditions} evaluates to @code{nil}, then the result
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277 of the @code{and} must be @code{nil} regardless of the remaining
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278 @var{conditions}; so @code{and} returns right away, ignoring the
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279 remaining @var{conditions}.
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280
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281 If all the @var{conditions} turn out non-@code{nil}, then the value of
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282 the last of them becomes the value of the @code{and} form.
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283
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284 Here is an example. The first condition returns the integer 1, which is
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285 not @code{nil}. Similarly, the second condition returns the integer 2,
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286 which is not @code{nil}. The third condition is @code{nil}, so the
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287 remaining condition is never evaluated.
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288
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289 @example
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290 @group
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291 (and (print 1) (print 2) nil (print 3))
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292 @print{} 1
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293 @print{} 2
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294 @result{} nil
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295 @end group
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296 @end example
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297
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298 Here is a more realistic example of using @code{and}:
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299
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300 @example
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301 @group
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302 (if (and (consp foo) (eq (car foo) 'x))
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303 (message "foo is a list starting with x"))
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304 @end group
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305 @end example
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306
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307 @noindent
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308 Note that @code{(car foo)} is not executed if @code{(consp foo)} returns
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309 @code{nil}, thus avoiding an error.
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310
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311 @code{and} can be expressed in terms of either @code{if} or @code{cond}.
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312 For example:
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313
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314 @example
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315 @group
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316 (and @var{arg1} @var{arg2} @var{arg3})
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317 @equiv{}
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318 (if @var{arg1} (if @var{arg2} @var{arg3}))
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319 @equiv{}
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320 (cond (@var{arg1} (cond (@var{arg2} @var{arg3}))))
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321 @end group
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322 @end example
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323 @end defspec
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324
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325 @defspec or conditions@dots{}
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326 The @code{or} special form tests whether at least one of the
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327 @var{conditions} is true. It works by evaluating all the
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328 @var{conditions} one by one in the order written.
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329
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330 If any of the @var{conditions} evaluates to a non-@code{nil} value, then
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331 the result of the @code{or} must be non-@code{nil}; so @code{or} returns
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332 right away, ignoring the remaining @var{conditions}. The value it
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333 returns is the non-@code{nil} value of the condition just evaluated.
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334
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335 If all the @var{conditions} turn out @code{nil}, then the @code{or}
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336 expression returns @code{nil}.
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337
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338 For example, this expression tests whether @code{x} is either 0 or
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339 @code{nil}:
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340
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341 @example
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342 (or (eq x nil) (eq x 0))
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343 @end example
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344
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345 Like the @code{and} construct, @code{or} can be written in terms of
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346 @code{cond}. For example:
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347
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348 @example
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349 @group
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350 (or @var{arg1} @var{arg2} @var{arg3})
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351 @equiv{}
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352 (cond (@var{arg1})
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353 (@var{arg2})
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354 (@var{arg3}))
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355 @end group
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356 @end example
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357
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358 You could almost write @code{or} in terms of @code{if}, but not quite:
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359
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360 @example
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361 @group
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362 (if @var{arg1} @var{arg1}
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363 (if @var{arg2} @var{arg2}
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364 @var{arg3}))
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365 @end group
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366 @end example
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367
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368 @noindent
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369 This is not completely equivalent because it can evaluate @var{arg1} or
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370 @var{arg2} twice. By contrast, @code{(or @var{arg1} @var{arg2}
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371 @var{arg3})} never evaluates any argument more than once.
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372 @end defspec
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373
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374 @node Iteration
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375 @section Iteration
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376 @cindex iteration
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377 @cindex recursion
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378
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379 Iteration means executing part of a program repetitively. For
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380 example, you might want to repeat some computation once for each element
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381 of a list, or once for each integer from 0 to @var{n}. You can do this
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382 in XEmacs Lisp with the special form @code{while}:
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383
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384 @defspec while condition forms@dots{}
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385 @code{while} first evaluates @var{condition}. If the result is
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386 non-@code{nil}, it evaluates @var{forms} in textual order. Then it
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387 reevaluates @var{condition}, and if the result is non-@code{nil}, it
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388 evaluates @var{forms} again. This process repeats until @var{condition}
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389 evaluates to @code{nil}.
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390
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391 There is no limit on the number of iterations that may occur. The loop
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392 will continue until either @var{condition} evaluates to @code{nil} or
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393 until an error or @code{throw} jumps out of it (@pxref{Nonlocal Exits}).
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394
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395 The value of a @code{while} form is always @code{nil}.
|
|
|
396
|
|
|
397 @example
|
|
|
398 @group
|
|
|
399 (setq num 0)
|
|
|
400 @result{} 0
|
|
|
401 @end group
|
|
|
402 @group
|
|
|
403 (while (< num 4)
|
|
|
404 (princ (format "Iteration %d." num))
|
|
|
405 (setq num (1+ num)))
|
|
|
406 @print{} Iteration 0.
|
|
|
407 @print{} Iteration 1.
|
|
|
408 @print{} Iteration 2.
|
|
|
409 @print{} Iteration 3.
|
|
|
410 @result{} nil
|
|
|
411 @end group
|
|
|
412 @end example
|
|
|
413
|
|
|
414 If you would like to execute something on each iteration before the
|
|
|
415 end-test, put it together with the end-test in a @code{progn} as the
|
|
|
416 first argument of @code{while}, as shown here:
|
|
|
417
|
|
|
418 @example
|
|
|
419 @group
|
|
|
420 (while (progn
|
|
|
421 (forward-line 1)
|
|
|
422 (not (looking-at "^$"))))
|
|
|
423 @end group
|
|
|
424 @end example
|
|
|
425
|
|
|
426 @noindent
|
|
|
427 This moves forward one line and continues moving by lines until it
|
|
|
428 reaches an empty. It is unusual in that the @code{while} has no body,
|
|
|
429 just the end test (which also does the real work of moving point).
|
|
|
430 @end defspec
|
|
|
431
|
|
|
432 @node Nonlocal Exits
|
|
|
433 @section Nonlocal Exits
|
|
|
434 @cindex nonlocal exits
|
|
|
435
|
|
|
436 A @dfn{nonlocal exit} is a transfer of control from one point in a
|
|
|
437 program to another remote point. Nonlocal exits can occur in XEmacs Lisp
|
|
|
438 as a result of errors; you can also use them under explicit control.
|
|
|
439 Nonlocal exits unbind all variable bindings made by the constructs being
|
|
|
440 exited.
|
|
|
441
|
|
|
442 @menu
|
|
|
443 * Catch and Throw:: Nonlocal exits for the program's own purposes.
|
|
|
444 * Examples of Catch:: Showing how such nonlocal exits can be written.
|
|
|
445 * Errors:: How errors are signaled and handled.
|
|
|
446 * Cleanups:: Arranging to run a cleanup form if an error happens.
|
|
|
447 @end menu
|
|
|
448
|
|
|
449 @node Catch and Throw
|
|
|
450 @subsection Explicit Nonlocal Exits: @code{catch} and @code{throw}
|
|
|
451
|
|
|
452 Most control constructs affect only the flow of control within the
|
|
|
453 construct itself. The function @code{throw} is the exception to this
|
|
|
454 rule of normal program execution: it performs a nonlocal exit on
|
|
|
455 request. (There are other exceptions, but they are for error handling
|
|
|
456 only.) @code{throw} is used inside a @code{catch}, and jumps back to
|
|
|
457 that @code{catch}. For example:
|
|
|
458
|
|
|
459 @example
|
|
|
460 @group
|
|
|
461 (catch 'foo
|
|
|
462 (progn
|
|
|
463 @dots{}
|
|
|
464 (throw 'foo t)
|
|
|
465 @dots{}))
|
|
|
466 @end group
|
|
|
467 @end example
|
|
|
468
|
|
|
469 @noindent
|
|
|
470 The @code{throw} transfers control straight back to the corresponding
|
|
|
471 @code{catch}, which returns immediately. The code following the
|
|
|
472 @code{throw} is not executed. The second argument of @code{throw} is used
|
|
|
473 as the return value of the @code{catch}.
|
|
|
474
|
|
|
475 The @code{throw} and the @code{catch} are matched through the first
|
|
|
476 argument: @code{throw} searches for a @code{catch} whose first argument
|
|
|
477 is @code{eq} to the one specified. Thus, in the above example, the
|
|
|
478 @code{throw} specifies @code{foo}, and the @code{catch} specifies the
|
|
|
479 same symbol, so that @code{catch} is applicable. If there is more than
|
|
|
480 one applicable @code{catch}, the innermost one takes precedence.
|
|
|
481
|
|
|
482 Executing @code{throw} exits all Lisp constructs up to the matching
|
|
|
483 @code{catch}, including function calls. When binding constructs such as
|
|
|
484 @code{let} or function calls are exited in this way, the bindings are
|
|
|
485 unbound, just as they are when these constructs exit normally
|
|
|
486 (@pxref{Local Variables}). Likewise, @code{throw} restores the buffer
|
|
|
487 and position saved by @code{save-excursion} (@pxref{Excursions}), and
|
|
|
488 the narrowing status saved by @code{save-restriction} and the window
|
|
|
489 selection saved by @code{save-window-excursion} (@pxref{Window
|
|
|
490 Configurations}). It also runs any cleanups established with the
|
|
|
491 @code{unwind-protect} special form when it exits that form
|
|
|
492 (@pxref{Cleanups}).
|
|
|
493
|
|
|
494 The @code{throw} need not appear lexically within the @code{catch}
|
|
|
495 that it jumps to. It can equally well be called from another function
|
|
|
496 called within the @code{catch}. As long as the @code{throw} takes place
|
|
|
497 chronologically after entry to the @code{catch}, and chronologically
|
|
|
498 before exit from it, it has access to that @code{catch}. This is why
|
|
|
499 @code{throw} can be used in commands such as @code{exit-recursive-edit}
|
|
|
500 that throw back to the editor command loop (@pxref{Recursive Editing}).
|
|
|
501
|
|
|
502 @cindex CL note---only @code{throw} in Emacs
|
|
|
503 @quotation
|
|
|
504 @b{Common Lisp note:} Most other versions of Lisp, including Common Lisp,
|
|
|
505 have several ways of transferring control nonsequentially: @code{return},
|
|
|
506 @code{return-from}, and @code{go}, for example. XEmacs Lisp has only
|
|
|
507 @code{throw}.
|
|
|
508 @end quotation
|
|
|
509
|
|
|
510 @defspec catch tag body@dots{}
|
|
|
511 @cindex tag on run time stack
|
|
|
512 @code{catch} establishes a return point for the @code{throw} function. The
|
|
|
513 return point is distinguished from other such return points by @var{tag},
|
|
|
514 which may be any Lisp object. The argument @var{tag} is evaluated normally
|
|
|
515 before the return point is established.
|
|
|
516
|
|
|
517 With the return point in effect, @code{catch} evaluates the forms of the
|
|
|
518 @var{body} in textual order. If the forms execute normally, without
|
|
|
519 error or nonlocal exit, the value of the last body form is returned from
|
|
|
520 the @code{catch}.
|
|
|
521
|
|
|
522 If a @code{throw} is done within @var{body} specifying the same value
|
|
|
523 @var{tag}, the @code{catch} exits immediately; the value it returns is
|
|
|
524 whatever was specified as the second argument of @code{throw}.
|
|
|
525 @end defspec
|
|
|
526
|
|
|
527 @defun throw tag value
|
|
|
528 The purpose of @code{throw} is to return from a return point previously
|
|
|
529 established with @code{catch}. The argument @var{tag} is used to choose
|
|
|
530 among the various existing return points; it must be @code{eq} to the value
|
|
|
531 specified in the @code{catch}. If multiple return points match @var{tag},
|
|
|
532 the innermost one is used.
|
|
|
533
|
|
|
534 The argument @var{value} is used as the value to return from that
|
|
|
535 @code{catch}.
|
|
|
536
|
|
|
537 @kindex no-catch
|
|
|
538 If no return point is in effect with tag @var{tag}, then a @code{no-catch}
|
|
|
539 error is signaled with data @code{(@var{tag} @var{value})}.
|
|
|
540 @end defun
|
|
|
541
|
|
|
542 @node Examples of Catch
|
|
|
543 @subsection Examples of @code{catch} and @code{throw}
|
|
|
544
|
|
|
545 One way to use @code{catch} and @code{throw} is to exit from a doubly
|
|
|
546 nested loop. (In most languages, this would be done with a ``go to''.)
|
|
|
547 Here we compute @code{(foo @var{i} @var{j})} for @var{i} and @var{j}
|
|
|
548 varying from 0 to 9:
|
|
|
549
|
|
|
550 @example
|
|
|
551 @group
|
|
|
552 (defun search-foo ()
|
|
|
553 (catch 'loop
|
|
|
554 (let ((i 0))
|
|
|
555 (while (< i 10)
|
|
|
556 (let ((j 0))
|
|
|
557 (while (< j 10)
|
|
|
558 (if (foo i j)
|
|
|
559 (throw 'loop (list i j)))
|
|
|
560 (setq j (1+ j))))
|
|
|
561 (setq i (1+ i))))))
|
|
|
562 @end group
|
|
|
563 @end example
|
|
|
564
|
|
|
565 @noindent
|
|
|
566 If @code{foo} ever returns non-@code{nil}, we stop immediately and return a
|
|
|
567 list of @var{i} and @var{j}. If @code{foo} always returns @code{nil}, the
|
|
|
568 @code{catch} returns normally, and the value is @code{nil}, since that
|
|
|
569 is the result of the @code{while}.
|
|
|
570
|
|
|
571 Here are two tricky examples, slightly different, showing two
|
|
|
572 return points at once. First, two return points with the same tag,
|
|
|
573 @code{hack}:
|
|
|
574
|
|
|
575 @example
|
|
|
576 @group
|
|
|
577 (defun catch2 (tag)
|
|
|
578 (catch tag
|
|
|
579 (throw 'hack 'yes)))
|
|
|
580 @result{} catch2
|
|
|
581 @end group
|
|
|
582
|
|
|
583 @group
|
|
|
584 (catch 'hack
|
|
|
585 (print (catch2 'hack))
|
|
|
586 'no)
|
|
|
587 @print{} yes
|
|
|
588 @result{} no
|
|
|
589 @end group
|
|
|
590 @end example
|
|
|
591
|
|
|
592 @noindent
|
|
|
593 Since both return points have tags that match the @code{throw}, it goes to
|
|
|
594 the inner one, the one established in @code{catch2}. Therefore,
|
|
|
595 @code{catch2} returns normally with value @code{yes}, and this value is
|
|
|
596 printed. Finally the second body form in the outer @code{catch}, which is
|
|
|
597 @code{'no}, is evaluated and returned from the outer @code{catch}.
|
|
|
598
|
|
|
599 Now let's change the argument given to @code{catch2}:
|
|
|
600
|
|
|
601 @example
|
|
|
602 @group
|
|
|
603 (defun catch2 (tag)
|
|
|
604 (catch tag
|
|
|
605 (throw 'hack 'yes)))
|
|
|
606 @result{} catch2
|
|
|
607 @end group
|
|
|
608
|
|
|
609 @group
|
|
|
610 (catch 'hack
|
|
|
611 (print (catch2 'quux))
|
|
|
612 'no)
|
|
|
613 @result{} yes
|
|
|
614 @end group
|
|
|
615 @end example
|
|
|
616
|
|
|
617 @noindent
|
|
|
618 We still have two return points, but this time only the outer one has
|
|
|
619 the tag @code{hack}; the inner one has the tag @code{quux} instead.
|
|
|
620 Therefore, @code{throw} makes the outer @code{catch} return the value
|
|
|
621 @code{yes}. The function @code{print} is never called, and the
|
|
|
622 body-form @code{'no} is never evaluated.
|
|
|
623
|
|
|
624 @node Errors
|
|
|
625 @subsection Errors
|
|
|
626 @cindex errors
|
|
|
627
|
|
|
628 When XEmacs Lisp attempts to evaluate a form that, for some reason,
|
|
|
629 cannot be evaluated, it @dfn{signals} an @dfn{error}.
|
|
|
630
|
|
|
631 When an error is signaled, XEmacs's default reaction is to print an
|
|
|
632 error message and terminate execution of the current command. This is
|
|
|
633 the right thing to do in most cases, such as if you type @kbd{C-f} at
|
|
|
634 the end of the buffer.
|
|
|
635
|
|
|
636 In complicated programs, simple termination may not be what you want.
|
|
|
637 For example, the program may have made temporary changes in data
|
|
|
638 structures, or created temporary buffers that should be deleted before
|
|
|
639 the program is finished. In such cases, you would use
|
|
|
640 @code{unwind-protect} to establish @dfn{cleanup expressions} to be
|
|
|
641 evaluated in case of error. (@xref{Cleanups}.) Occasionally, you may
|
|
|
642 wish the program to continue execution despite an error in a subroutine.
|
|
|
643 In these cases, you would use @code{condition-case} to establish
|
|
|
644 @dfn{error handlers} to recover control in case of error.
|
|
|
645
|
|
|
646 Resist the temptation to use error handling to transfer control from
|
|
|
647 one part of the program to another; use @code{catch} and @code{throw}
|
|
|
648 instead. @xref{Catch and Throw}.
|
|
|
649
|
|
|
650 @menu
|
|
|
651 * Signaling Errors:: How to report an error.
|
|
|
652 * Processing of Errors:: What XEmacs does when you report an error.
|
|
|
653 * Handling Errors:: How you can trap errors and continue execution.
|
|
|
654 * Error Symbols:: How errors are classified for trapping them.
|
|
|
655 @end menu
|
|
|
656
|
|
|
657 @node Signaling Errors
|
|
|
658 @subsubsection How to Signal an Error
|
|
|
659 @cindex signaling errors
|
|
|
660
|
|
|
661 Most errors are signaled ``automatically'' within Lisp primitives
|
|
|
662 which you call for other purposes, such as if you try to take the
|
|
|
663 @sc{car} of an integer or move forward a character at the end of the
|
|
|
664 buffer; you can also signal errors explicitly with the functions
|
|
|
665 @code{error} and @code{signal}.
|
|
|
666
|
|
|
667 Quitting, which happens when the user types @kbd{C-g}, is not
|
|
|
668 considered an error, but it is handled almost like an error.
|
|
|
669 @xref{Quitting}.
|
|
|
670
|
|
|
671 @defun error format-string &rest args
|
|
|
672 This function signals an error with an error message constructed by
|
|
|
673 applying @code{format} (@pxref{String Conversion}) to
|
|
|
674 @var{format-string} and @var{args}.
|
|
|
675
|
|
|
676 These examples show typical uses of @code{error}:
|
|
|
677
|
|
|
678 @example
|
|
|
679 @group
|
|
|
680 (error "You have committed an error.
|
|
|
681 Try something else.")
|
|
|
682 @error{} You have committed an error.
|
|
|
683 Try something else.
|
|
|
684 @end group
|
|
|
685
|
|
|
686 @group
|
|
|
687 (error "You have committed %d errors." 10)
|
|
|
688 @error{} You have committed 10 errors.
|
|
|
689 @end group
|
|
|
690 @end example
|
|
|
691
|
|
|
692 @code{error} works by calling @code{signal} with two arguments: the
|
|
|
693 error symbol @code{error}, and a list containing the string returned by
|
|
|
694 @code{format}.
|
|
|
695
|
|
|
696 If you want to use your own string as an error message verbatim, don't
|
|
|
697 just write @code{(error @var{string})}. If @var{string} contains
|
|
|
698 @samp{%}, it will be interpreted as a format specifier, with undesirable
|
|
|
699 results. Instead, use @code{(error "%s" @var{string})}.
|
|
|
700 @end defun
|
|
|
701
|
|
|
702 @defun signal error-symbol data
|
|
|
703 This function signals an error named by @var{error-symbol}. The
|
|
|
704 argument @var{data} is a list of additional Lisp objects relevant to the
|
|
|
705 circumstances of the error.
|
|
|
706
|
|
|
707 The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol
|
|
|
708 bearing a property @code{error-conditions} whose value is a list of
|
|
|
709 condition names. This is how XEmacs Lisp classifies different sorts of
|
|
|
710 errors.
|
|
|
711
|
|
|
712 The number and significance of the objects in @var{data} depends on
|
|
|
713 @var{error-symbol}. For example, with a @code{wrong-type-arg} error,
|
|
|
714 there are two objects in the list: a predicate that describes the type
|
|
|
715 that was expected, and the object that failed to fit that type.
|
|
|
716 @xref{Error Symbols}, for a description of error symbols.
|
|
|
717
|
|
|
718 Both @var{error-symbol} and @var{data} are available to any error
|
|
|
719 handlers that handle the error: @code{condition-case} binds a local
|
|
|
720 variable to a list of the form @code{(@var{error-symbol} .@:
|
|
|
721 @var{data})} (@pxref{Handling Errors}). If the error is not handled,
|
|
|
722 these two values are used in printing the error message.
|
|
|
723
|
|
|
724 The function @code{signal} never returns (though in older Emacs versions
|
|
|
725 it could sometimes return).
|
|
|
726
|
|
|
727 @smallexample
|
|
|
728 @group
|
|
|
729 (signal 'wrong-number-of-arguments '(x y))
|
|
|
730 @error{} Wrong number of arguments: x, y
|
|
|
731 @end group
|
|
|
732
|
|
|
733 @group
|
|
|
734 (signal 'no-such-error '("My unknown error condition."))
|
|
|
735 @error{} peculiar error: "My unknown error condition."
|
|
|
736 @end group
|
|
|
737 @end smallexample
|
|
|
738 @end defun
|
|
|
739
|
|
|
740 @cindex CL note---no continuable errors
|
|
|
741 @quotation
|
|
|
742 @b{Common Lisp note:} XEmacs Lisp has nothing like the Common Lisp
|
|
|
743 concept of continuable errors.
|
|
|
744 @end quotation
|
|
|
745
|
|
|
746 @node Processing of Errors
|
|
|
747 @subsubsection How XEmacs Processes Errors
|
|
|
748
|
|
|
749 When an error is signaled, @code{signal} searches for an active
|
|
|
750 @dfn{handler} for the error. A handler is a sequence of Lisp
|
|
|
751 expressions designated to be executed if an error happens in part of the
|
|
|
752 Lisp program. If the error has an applicable handler, the handler is
|
|
|
753 executed, and control resumes following the handler. The handler
|
|
|
754 executes in the environment of the @code{condition-case} that
|
|
|
755 established it; all functions called within that @code{condition-case}
|
|
|
756 have already been exited, and the handler cannot return to them.
|
|
|
757
|
|
|
758 If there is no applicable handler for the error, the current command is
|
|
|
759 terminated and control returns to the editor command loop, because the
|
|
|
760 command loop has an implicit handler for all kinds of errors. The
|
|
|
761 command loop's handler uses the error symbol and associated data to
|
|
|
762 print an error message.
|
|
|
763
|
|
|
764 @cindex @code{debug-on-error} use
|
|
|
765 An error that has no explicit handler may call the Lisp debugger. The
|
|
|
766 debugger is enabled if the variable @code{debug-on-error} (@pxref{Error
|
|
|
767 Debugging}) is non-@code{nil}. Unlike error handlers, the debugger runs
|
|
|
768 in the environment of the error, so that you can examine values of
|
|
|
769 variables precisely as they were at the time of the error.
|
|
|
770
|
|
|
771 @node Handling Errors
|
|
|
772 @subsubsection Writing Code to Handle Errors
|
|
|
773 @cindex error handler
|
|
|
774 @cindex handling errors
|
|
|
775
|
|
|
776 The usual effect of signaling an error is to terminate the command
|
|
|
777 that is running and return immediately to the XEmacs editor command loop.
|
|
|
778 You can arrange to trap errors occurring in a part of your program by
|
|
|
779 establishing an error handler, with the special form
|
|
|
780 @code{condition-case}. A simple example looks like this:
|
|
|
781
|
|
|
782 @example
|
|
|
783 @group
|
|
|
784 (condition-case nil
|
|
|
785 (delete-file filename)
|
|
|
786 (error nil))
|
|
|
787 @end group
|
|
|
788 @end example
|
|
|
789
|
|
|
790 @noindent
|
|
|
791 This deletes the file named @var{filename}, catching any error and
|
|
|
792 returning @code{nil} if an error occurs.
|
|
|
793
|
|
|
794 The second argument of @code{condition-case} is called the
|
|
|
795 @dfn{protected form}. (In the example above, the protected form is a
|
|
|
796 call to @code{delete-file}.) The error handlers go into effect when
|
|
|
797 this form begins execution and are deactivated when this form returns.
|
|
|
798 They remain in effect for all the intervening time. In particular, they
|
|
|
799 are in effect during the execution of functions called by this form, in
|
|
|
800 their subroutines, and so on. This is a good thing, since, strictly
|
|
|
801 speaking, errors can be signaled only by Lisp primitives (including
|
|
|
802 @code{signal} and @code{error}) called by the protected form, not by the
|
|
|
803 protected form itself.
|
|
|
804
|
|
|
805 The arguments after the protected form are handlers. Each handler
|
|
|
806 lists one or more @dfn{condition names} (which are symbols) to specify
|
|
|
807 which errors it will handle. The error symbol specified when an error
|
|
|
808 is signaled also defines a list of condition names. A handler applies
|
|
|
809 to an error if they have any condition names in common. In the example
|
|
|
810 above, there is one handler, and it specifies one condition name,
|
|
|
811 @code{error}, which covers all errors.
|
|
|
812
|
|
|
813 The search for an applicable handler checks all the established handlers
|
|
|
814 starting with the most recently established one. Thus, if two nested
|
|
|
815 @code{condition-case} forms offer to handle the same error, the inner of
|
|
|
816 the two will actually handle it.
|
|
|
817
|
|
|
818 When an error is handled, control returns to the handler. Before this
|
|
|
819 happens, XEmacs unbinds all variable bindings made by binding constructs
|
|
|
820 that are being exited and executes the cleanups of all
|
|
|
821 @code{unwind-protect} forms that are exited. Once control arrives at
|
|
|
822 the handler, the body of the handler is executed.
|
|
|
823
|
|
|
824 After execution of the handler body, execution continues by returning
|
|
|
825 from the @code{condition-case} form. Because the protected form is
|
|
|
826 exited completely before execution of the handler, the handler cannot
|
|
|
827 resume execution at the point of the error, nor can it examine variable
|
|
|
828 bindings that were made within the protected form. All it can do is
|
|
|
829 clean up and proceed.
|
|
|
830
|
|
|
831 @code{condition-case} is often used to trap errors that are
|
|
|
832 predictable, such as failure to open a file in a call to
|
|
|
833 @code{insert-file-contents}. It is also used to trap errors that are
|
|
|
834 totally unpredictable, such as when the program evaluates an expression
|
|
|
835 read from the user.
|
|
|
836
|
|
|
837 Error signaling and handling have some resemblance to @code{throw} and
|
|
|
838 @code{catch}, but they are entirely separate facilities. An error
|
|
|
839 cannot be caught by a @code{catch}, and a @code{throw} cannot be handled
|
|
|
840 by an error handler (though using @code{throw} when there is no suitable
|
|
|
841 @code{catch} signals an error that can be handled).
|
|
|
842
|
|
|
843 @defspec condition-case var protected-form handlers@dots{}
|
|
|
844 This special form establishes the error handlers @var{handlers} around
|
|
|
845 the execution of @var{protected-form}. If @var{protected-form} executes
|
|
|
846 without error, the value it returns becomes the value of the
|
|
|
847 @code{condition-case} form; in this case, the @code{condition-case} has
|
|
|
848 no effect. The @code{condition-case} form makes a difference when an
|
|
|
849 error occurs during @var{protected-form}.
|
|
|
850
|
|
|
851 Each of the @var{handlers} is a list of the form @code{(@var{conditions}
|
|
|
852 @var{body}@dots{})}. Here @var{conditions} is an error condition name
|
|
|
853 to be handled, or a list of condition names; @var{body} is one or more
|
|
|
854 Lisp expressions to be executed when this handler handles an error.
|
|
|
855 Here are examples of handlers:
|
|
|
856
|
|
|
857 @smallexample
|
|
|
858 @group
|
|
|
859 (error nil)
|
|
|
860
|
|
|
861 (arith-error (message "Division by zero"))
|
|
|
862
|
|
|
863 ((arith-error file-error)
|
|
|
864 (message
|
|
|
865 "Either division by zero or failure to open a file"))
|
|
|
866 @end group
|
|
|
867 @end smallexample
|
|
|
868
|
|
|
869 Each error that occurs has an @dfn{error symbol} that describes what
|
|
|
870 kind of error it is. The @code{error-conditions} property of this
|
|
|
871 symbol is a list of condition names (@pxref{Error Symbols}). Emacs
|
|
|
872 searches all the active @code{condition-case} forms for a handler that
|
|
|
873 specifies one or more of these condition names; the innermost matching
|
|
|
874 @code{condition-case} handles the error. Within this
|
|
|
875 @code{condition-case}, the first applicable handler handles the error.
|
|
|
876
|
|
|
877 After executing the body of the handler, the @code{condition-case}
|
|
|
878 returns normally, using the value of the last form in the handler body
|
|
|
879 as the overall value.
|
|
|
880
|
|
|
881 The argument @var{var} is a variable. @code{condition-case} does not
|
|
|
882 bind this variable when executing the @var{protected-form}, only when it
|
|
|
883 handles an error. At that time, it binds @var{var} locally to a list of
|
|
|
884 the form @code{(@var{error-symbol} . @var{data})}, giving the
|
|
|
885 particulars of the error. The handler can refer to this list to decide
|
|
|
886 what to do. For example, if the error is for failure opening a file,
|
|
|
887 the file name is the second element of @var{data}---the third element of
|
|
|
888 @var{var}.
|
|
|
889
|
|
|
890 If @var{var} is @code{nil}, that means no variable is bound. Then the
|
|
|
891 error symbol and associated data are not available to the handler.
|
|
|
892 @end defspec
|
|
|
893
|
|
|
894 @cindex @code{arith-error} example
|
|
|
895 Here is an example of using @code{condition-case} to handle the error
|
|
|
896 that results from dividing by zero. The handler prints out a warning
|
|
|
897 message and returns a very large number.
|
|
|
898
|
|
|
899 @smallexample
|
|
|
900 @group
|
|
|
901 (defun safe-divide (dividend divisor)
|
|
|
902 (condition-case err
|
|
|
903 ;; @r{Protected form.}
|
|
|
904 (/ dividend divisor)
|
|
|
905 ;; @r{The handler.}
|
|
|
906 (arith-error ; @r{Condition.}
|
|
|
907 (princ (format "Arithmetic error: %s" err))
|
|
|
908 1000000)))
|
|
|
909 @result{} safe-divide
|
|
|
910 @end group
|
|
|
911
|
|
|
912 @group
|
|
|
913 (safe-divide 5 0)
|
|
|
914 @print{} Arithmetic error: (arith-error)
|
|
|
915 @result{} 1000000
|
|
|
916 @end group
|
|
|
917 @end smallexample
|
|
|
918
|
|
|
919 @noindent
|
|
|
920 The handler specifies condition name @code{arith-error} so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this @code{condition-case}. Thus,
|
|
|
921
|
|
|
922 @smallexample
|
|
|
923 @group
|
|
|
924 (safe-divide nil 3)
|
|
|
925 @error{} Wrong type argument: integer-or-marker-p, nil
|
|
|
926 @end group
|
|
|
927 @end smallexample
|
|
|
928
|
|
|
929 Here is a @code{condition-case} that catches all kinds of errors,
|
|
|
930 including those signaled with @code{error}:
|
|
|
931
|
|
|
932 @smallexample
|
|
|
933 @group
|
|
|
934 (setq baz 34)
|
|
|
935 @result{} 34
|
|
|
936 @end group
|
|
|
937
|
|
|
938 @group
|
|
|
939 (condition-case err
|
|
|
940 (if (eq baz 35)
|
|
|
941 t
|
|
|
942 ;; @r{This is a call to the function @code{error}.}
|
|
|
943 (error "Rats! The variable %s was %s, not 35" 'baz baz))
|
|
|
944 ;; @r{This is the handler; it is not a form.}
|
|
|
945 (error (princ (format "The error was: %s" err))
|
|
|
946 2))
|
|
|
947 @print{} The error was: (error "Rats! The variable baz was 34, not 35")
|
|
|
948 @result{} 2
|
|
|
949 @end group
|
|
|
950 @end smallexample
|
|
|
951
|
|
|
952 @node Error Symbols
|
|
|
953 @subsubsection Error Symbols and Condition Names
|
|
|
954 @cindex error symbol
|
|
|
955 @cindex error name
|
|
|
956 @cindex condition name
|
|
|
957 @cindex user-defined error
|
|
|
958 @kindex error-conditions
|
|
|
959
|
|
|
960 When you signal an error, you specify an @dfn{error symbol} to specify
|
|
|
961 the kind of error you have in mind. Each error has one and only one
|
|
|
962 error symbol to categorize it. This is the finest classification of
|
|
|
963 errors defined by the XEmacs Lisp language.
|
|
|
964
|
|
|
965 These narrow classifications are grouped into a hierarchy of wider
|
|
|
966 classes called @dfn{error conditions}, identified by @dfn{condition
|
|
|
967 names}. The narrowest such classes belong to the error symbols
|
|
|
968 themselves: each error symbol is also a condition name. There are also
|
|
|
969 condition names for more extensive classes, up to the condition name
|
|
|
970 @code{error} which takes in all kinds of errors. Thus, each error has
|
|
|
971 one or more condition names: @code{error}, the error symbol if that
|
|
|
972 is distinct from @code{error}, and perhaps some intermediate
|
|
|
973 classifications.
|
|
|
974
|
|
|
975 In order for a symbol to be an error symbol, it must have an
|
|
|
976 @code{error-conditions} property which gives a list of condition names.
|
|
|
977 This list defines the conditions that this kind of error belongs to.
|
|
|
978 (The error symbol itself, and the symbol @code{error}, should always be
|
|
|
979 members of this list.) Thus, the hierarchy of condition names is
|
|
|
980 defined by the @code{error-conditions} properties of the error symbols.
|
|
|
981
|
|
|
982 In addition to the @code{error-conditions} list, the error symbol
|
|
|
983 should have an @code{error-message} property whose value is a string to
|
|
|
984 be printed when that error is signaled but not handled. If the
|
|
|
985 @code{error-message} property exists, but is not a string, the error
|
|
|
986 message @samp{peculiar error} is used.
|
|
|
987 @cindex peculiar error
|
|
|
988
|
|
|
989 Here is how we define a new error symbol, @code{new-error}:
|
|
|
990
|
|
|
991 @example
|
|
|
992 @group
|
|
|
993 (put 'new-error
|
|
|
994 'error-conditions
|
|
|
995 '(error my-own-errors new-error))
|
|
|
996 @result{} (error my-own-errors new-error)
|
|
|
997 @end group
|
|
|
998 @group
|
|
|
999 (put 'new-error 'error-message "A new error")
|
|
|
1000 @result{} "A new error"
|
|
|
1001 @end group
|
|
|
1002 @end example
|
|
|
1003
|
|
|
1004 @noindent
|
|
|
1005 This error has three condition names: @code{new-error}, the narrowest
|
|
|
1006 classification; @code{my-own-errors}, which we imagine is a wider
|
|
|
1007 classification; and @code{error}, which is the widest of all.
|
|
|
1008
|
|
|
1009 The error string should start with a capital letter but it should
|
|
|
1010 not end with a period. This is for consistency with the rest of Emacs.
|
|
|
1011
|
|
|
1012 Naturally, XEmacs will never signal @code{new-error} on its own; only
|
|
|
1013 an explicit call to @code{signal} (@pxref{Signaling Errors}) in your
|
|
|
1014 code can do this:
|
|
|
1015
|
|
|
1016 @example
|
|
|
1017 @group
|
|
|
1018 (signal 'new-error '(x y))
|
|
|
1019 @error{} A new error: x, y
|
|
|
1020 @end group
|
|
|
1021 @end example
|
|
|
1022
|
|
|
1023 This error can be handled through any of the three condition names.
|
|
|
1024 This example handles @code{new-error} and any other errors in the class
|
|
|
1025 @code{my-own-errors}:
|
|
|
1026
|
|
|
1027 @example
|
|
|
1028 @group
|
|
|
1029 (condition-case foo
|
|
|
1030 (bar nil t)
|
|
|
1031 (my-own-errors nil))
|
|
|
1032 @end group
|
|
|
1033 @end example
|
|
|
1034
|
|
|
1035 The significant way that errors are classified is by their condition
|
|
|
1036 names---the names used to match errors with handlers. An error symbol
|
|
|
1037 serves only as a convenient way to specify the intended error message
|
|
|
1038 and list of condition names. It would be cumbersome to give
|
|
|
1039 @code{signal} a list of condition names rather than one error symbol.
|
|
|
1040
|
|
|
1041 By contrast, using only error symbols without condition names would
|
|
|
1042 seriously decrease the power of @code{condition-case}. Condition names
|
|
|
1043 make it possible to categorize errors at various levels of generality
|
|
|
1044 when you write an error handler. Using error symbols alone would
|
|
|
1045 eliminate all but the narrowest level of classification.
|
|
|
1046
|
|
|
1047 @xref{Standard Errors}, for a list of all the standard error symbols
|
|
|
1048 and their conditions.
|
|
|
1049
|
|
|
1050 @node Cleanups
|
|
|
1051 @subsection Cleaning Up from Nonlocal Exits
|
|
|
1052
|
|
|
1053 The @code{unwind-protect} construct is essential whenever you
|
|
|
1054 temporarily put a data structure in an inconsistent state; it permits
|
|
|
1055 you to ensure the data are consistent in the event of an error or throw.
|
|
|
1056
|
|
|
1057 @defspec unwind-protect body cleanup-forms@dots{}
|
|
|
1058 @cindex cleanup forms
|
|
|
1059 @cindex protected forms
|
|
|
1060 @cindex error cleanup
|
|
|
1061 @cindex unwinding
|
|
|
1062 @code{unwind-protect} executes the @var{body} with a guarantee that the
|
|
|
1063 @var{cleanup-forms} will be evaluated if control leaves @var{body}, no
|
|
|
1064 matter how that happens. The @var{body} may complete normally, or
|
|
|
1065 execute a @code{throw} out of the @code{unwind-protect}, or cause an
|
|
|
1066 error; in all cases, the @var{cleanup-forms} will be evaluated.
|
|
|
1067
|
|
|
1068 If the @var{body} forms finish normally, @code{unwind-protect} returns
|
|
|
1069 the value of the last @var{body} form, after it evaluates the
|
|
|
1070 @var{cleanup-forms}. If the @var{body} forms do not finish,
|
|
|
1071 @code{unwind-protect} does not return any value in the normal sense.
|
|
|
1072
|
|
|
1073 Only the @var{body} is actually protected by the @code{unwind-protect}.
|
|
|
1074 If any of the @var{cleanup-forms} themselves exits nonlocally (e.g., via
|
|
|
1075 a @code{throw} or an error), @code{unwind-protect} is @emph{not}
|
|
|
1076 guaranteed to evaluate the rest of them. If the failure of one of the
|
|
|
1077 @var{cleanup-forms} has the potential to cause trouble, then protect it
|
|
|
1078 with another @code{unwind-protect} around that form.
|
|
|
1079
|
|
|
1080 The number of currently active @code{unwind-protect} forms counts,
|
|
|
1081 together with the number of local variable bindings, against the limit
|
|
|
1082 @code{max-specpdl-size} (@pxref{Local Variables}).
|
|
|
1083 @end defspec
|
|
|
1084
|
|
|
1085 For example, here we make an invisible buffer for temporary use, and
|
|
|
1086 make sure to kill it before finishing:
|
|
|
1087
|
|
|
1088 @smallexample
|
|
|
1089 @group
|
|
|
1090 (save-excursion
|
|
|
1091 (let ((buffer (get-buffer-create " *temp*")))
|
|
|
1092 (set-buffer buffer)
|
|
|
1093 (unwind-protect
|
|
|
1094 @var{body}
|
|
|
1095 (kill-buffer buffer))))
|
|
|
1096 @end group
|
|
|
1097 @end smallexample
|
|
|
1098
|
|
|
1099 @noindent
|
|
|
1100 You might think that we could just as well write @code{(kill-buffer
|
|
|
1101 (current-buffer))} and dispense with the variable @code{buffer}.
|
|
|
1102 However, the way shown above is safer, if @var{body} happens to get an
|
|
|
1103 error after switching to a different buffer! (Alternatively, you could
|
|
|
1104 write another @code{save-excursion} around the body, to ensure that the
|
|
|
1105 temporary buffer becomes current in time to kill it.)
|
|
|
1106
|
|
|
1107 @findex ftp-login
|
|
|
1108 Here is an actual example taken from the file @file{ftp.el}. It
|
|
|
1109 creates a process (@pxref{Processes}) to try to establish a connection
|
|
|
1110 to a remote machine. As the function @code{ftp-login} is highly
|
|
|
1111 susceptible to numerous problems that the writer of the function cannot
|
|
|
1112 anticipate, it is protected with a form that guarantees deletion of the
|
|
|
1113 process in the event of failure. Otherwise, XEmacs might fill up with
|
|
|
1114 useless subprocesses.
|
|
|
1115
|
|
|
1116 @smallexample
|
|
|
1117 @group
|
|
|
1118 (let ((win nil))
|
|
|
1119 (unwind-protect
|
|
|
1120 (progn
|
|
|
1121 (setq process (ftp-setup-buffer host file))
|
|
|
1122 (if (setq win (ftp-login process host user password))
|
|
|
1123 (message "Logged in")
|
|
|
1124 (error "Ftp login failed")))
|
|
|
1125 (or win (and process (delete-process process)))))
|
|
|
1126 @end group
|
|
|
1127 @end smallexample
|
|
|
1128
|
|
|
1129 This example actually has a small bug: if the user types @kbd{C-g} to
|
|
|
1130 quit, and the quit happens immediately after the function
|
|
|
1131 @code{ftp-setup-buffer} returns but before the variable @code{process} is
|
|
|
1132 set, the process will not be killed. There is no easy way to fix this bug,
|
|
|
1133 but at least it is very unlikely.
|
|
|
1134
|
|
|
1135 Here is another example which uses @code{unwind-protect} to make sure
|
|
|
1136 to kill a temporary buffer. In this example, the value returned by
|
|
|
1137 @code{unwind-protect} is used.
|
|
|
1138
|
|
|
1139 @smallexample
|
|
|
1140 (defun shell-command-string (cmd)
|
|
|
1141 "Return the output of the shell command CMD, as a string."
|
|
|
1142 (save-excursion
|
|
|
1143 (set-buffer (generate-new-buffer " OS*cmd"))
|
|
|
1144 (shell-command cmd t)
|
|
|
1145 (unwind-protect
|
|
|
1146 (buffer-string)
|
|
|
1147 (kill-buffer (current-buffer)))))
|
|
|
1148 @end smallexample
|
