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1 ;;; backquote.el --- Full backquote support for elisp. Reverse compatible too.
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2
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3 ;; Copyright (C) 1997 Free Software Foundation, Inc.
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4
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5 ;; Maintainer: XEmacs Development Team
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6 ;; Keywords: extensions, dumped
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7
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8 ;; This file is part of XEmacs.
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9
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10 ;; XEmacs is free software; you can redistribute it and/or modify it
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11 ;; under the terms of the GNU General Public License as published by
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12 ;; the Free Software Foundation; either version 2, or (at your option)
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13 ;; any later version.
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14
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15 ;; XEmacs is distributed in the hope that it will be useful, but
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16 ;; WITHOUT ANY WARRANTY; without even the implied warranty of
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17 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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18 ;; General Public License for more details.
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19
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20 ;; You should have received a copy of the GNU General Public License
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21 ;; along with XEmacs; see the file COPYING. If not, write to the Free
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22 ;; Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
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23 ;; 02111-1307, USA.
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24
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25 ;;; Synched up with: Not synched with FSF.
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26
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27 ;;; Commentary:
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28
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29 ;; This file is dumped with XEmacs.
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30
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31 ;; The bulk of the code is originally from CMU Common Lisp (original notice
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32 ;; below).
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33
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34 ;; It correctly supports nested backquotes and backquoted vectors.
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35
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36 ;; Converted to work with elisp by Miles Bader <miles@cogsci.ed.ac.uk>
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37
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38 ;; Changes by Jonathan Stigelman <Stig@hackvan.com>:
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39 ;; - Documentation added
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40 ;; - support for old-backquote-compatibility-hook nixed because the
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41 ;; old-backquote compatibility is now done in the reader...
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42 ;; - nixed support for |,.| because
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43 ;; (a) it's not in CLtl2
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44 ;; (b) ",.foo" is the same as ". ,foo"
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45 ;; (c) because RMS isn't interested in using this version of backquote.el
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46 ;;
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47 ;; ben@xemacs.org added ,. support back in:
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48 ;; (a) yes, it is in CLtl2. Read closely on page 529.
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49 ;; (b) RMS in 19.30 adds C support for ,. even if it's not really
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50 ;; handled.
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51 ;;
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52 ;; **********************************************************************
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53 ;; This code was written as part of the CMU Common Lisp project at
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54 ;; Carnegie Mellon University, and has been placed in the public domain.
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55 ;; If you want to use this code or any part of CMU Common Lisp, please contact
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56 ;; Scott Fahlman or slisp-group@cs.cmu.edu.
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57 ;;
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58 ;; **********************************************************************
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59 ;;
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60 ;; BACKQUOTE: Code Spice Lispified by Lee Schumacher.
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61 ;;
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62 ;; The flags passed back by BQ-PROCESS-2 can be interpreted as follows:
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63 ;;
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64 ;; |`,|: [a] => a
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65 ;; NIL: [a] => a ;the NIL flag is used only when a is NIL
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66 ;; T: [a] => a ;the T flag is used when a is self-evaluating
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67 ;; QUOTE: [a] => (QUOTE a)
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68 ;; APPEND: [a] => (APPEND . a)
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69 ;; NCONC: [a] => (NCONC . a)
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70 ;; LIST: [a] => (LIST . a)
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71 ;; LIST*: [a] => (LIST* . a)
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72 ;;
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73 ;; The flags are combined according to the following set of rules:
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74 ;; ([a] means that a should be converted according to the previous table)
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75 ;;
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76 ;; \ car || otherwise | QUOTE or | |`,@| | |`,.|
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77 ;;cdr \ || | T or NIL | |
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78 ;;============================================================================
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79 ;; |`,| ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a [d]) |NCONC (a [d])
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80 ;; NIL ||LIST ([a]) |QUOTE (a) |<hair> a |<hair> a
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81 ;;QUOTE or T||LIST* ([a] [d]) |QUOTE (a . d) |APPEND (a [d]) |NCONC (a [d])
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82 ;; APPEND ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a . d) |NCONC (a [d])
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83 ;; NCONC ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a [d]) |NCONC (a . d)
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84 ;; LIST ||LIST ([a] . d) |LIST ([a] . d) |APPEND (a [d]) |NCONC (a [d])
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85 ;; LIST* ||LIST* ([a] . d) |LIST* ([a] . d) |APPEND (a [d]) |NCONC (a [d])
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86 ;;
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87 ;;<hair> involves starting over again pretending you had read ".,a)" instead
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88 ;; of ",@a)"
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89 ;;
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90
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91 ;; These are the forms it expects: |backquote| |`| |,| |,@| and |,.|.
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92
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93 ;;; Code:
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94
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95 (defconst bq-backquote-marker 'backquote)
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96 (defconst bq-backtick-marker '\`) ; remnant of the old lossage
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97 (defconst bq-comma-marker '\,)
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98 (defconst bq-at-marker '\,@)
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99 (defconst bq-dot-marker '\,\.)
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100
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101 ;;; ----------------------------------------------------------------
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102
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103 (fset '\` 'backquote)
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104
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105 (defmacro backquote (template)
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106 "Expand the internal representation of a backquoted TEMPLATE into a lisp form.
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107
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108 The backquote character is like the quote character in that it prevents the
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109 template which follows it from being evaluated, except that backquote
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110 permits you to evaluate portions of the quoted template. A comma character
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111 inside TEMPLATE indicates that the following item should be evaluated. A
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112 comma character may be followed by an at-sign, which indicates that the form
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113 which follows should be evaluated and inserted and \"spliced\" into the
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114 template. Forms following ,@ must evaluate to lists.
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115
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116 Here is how to use backquotes:
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117 (setq p 'b
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118 q '(c d e))
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119 `(a ,p ,@q) -> (a b c d e)
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120 `(a . b) -> (a . b)
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121 `(a . ,p) -> (a . b)
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122
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123 The XEmacs lisp reader expands lisp backquotes as it reads them.
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124 Examples:
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125 `atom is read as (backquote atom)
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126 `(a ,b ,@(c d e)) is read as (backquote (a (\\, b) (\\,\\@ (c d e))))
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127 `(a . ,p) is read as (backquote (a \\, p))
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128
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129 \(backquote TEMPLATE) is a macro that produces code to construct TEMPLATE.
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130 Note that this is very slow in interpreted code, but fast if you compile.
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131 TEMPLATE is one or more nested lists or vectors, which are `almost quoted'.
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132 They are copied recursively, with elements preceded by comma evaluated.
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133 (backquote (a b)) == (list 'a 'b)
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134 (backquote (a [b c])) == (list 'a (vector 'b 'c))
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135
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136 However, certain special lists are not copied. They specify substitution.
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137 Lists that look like (\\, EXP) are evaluated and the result is substituted.
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138 (backquote (a (\\, (+ x 5)))) == (list 'a (+ x 5))
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139
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140 Elements of the form (\\,\\@ EXP) are evaluated and then all the elements
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141 of the result are substituted. This result must be a list; it may
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142 be `nil'.
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143
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144 Elements of the form (\\,\\. EXP) are evaluated and then all the elements
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145 of the result are concatenated to the list of preceding elements in the list.
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146 They must occur as the last element of a list (not a vector).
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147 EXP may evaluate to nil.
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148
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149 As an example, a simple macro `push' could be written:
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150 (defmacro push (v l)
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151 `(setq ,l (cons ,@(list v l))))
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152 or as
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153 (defmacro push (v l)
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154 `(setq ,l (cons ,v ,l)))
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155
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156 For backwards compatibility, old-style emacs-lisp backquotes are still read.
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157 OLD STYLE NEW STYLE
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158 (` (foo (, bar) (,@ bing))) `(foo ,bar ,@bing)
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159
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160 Because of the old-style backquote support, you cannot use a new-style
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161 backquoted form as the first element of a list. Perhaps some day this
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162 restriction will go away, but for now you should be wary of it:
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163 (`(this ,will ,@fail))
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164 ((` (but (, this) will (,@ work))))
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165 This is an extremely rare thing to need to do in lisp."
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166 (bq-process template))
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167
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168 ;;; ----------------------------------------------------------------
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169
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170 (defconst bq-comma-flag 'unquote)
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171 (defconst bq-at-flag 'unquote-splicing)
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172 (defconst bq-dot-flag 'unquote-nconc-splicing)
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173
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174 (defun bq-process (form)
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175 (let* ((flag-result (bq-process-2 form))
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176 (flag (car flag-result))
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177 (result (cdr flag-result)))
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178 (cond ((eq flag bq-at-flag)
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179 (error ",@ after ` in form: %s" form))
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180 ((eq flag bq-dot-flag)
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181 (error ",. after ` in form: %s" form))
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182 (t
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183 (bq-process-1 flag result)))))
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184
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185 ;;; ----------------------------------------------------------------
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186
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187 (defun bq-vector-contents (vec)
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188 (let ((contents nil)
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189 (n (length vec)))
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190 (while (> n 0)
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191 (setq n (1- n))
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192 (setq contents (cons (aref vec n) contents)))
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193 contents))
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194
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195 ;;; This does the expansion from table 2.
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196 (defun bq-process-2 (code)
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197 (cond ((vectorp code)
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198 (let* ((dflag-d
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199 (bq-process-2 (bq-vector-contents code))))
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200 (cons 'vector (bq-process-1 (car dflag-d) (cdr dflag-d)))))
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201 ((atom code)
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202 (cond ((null code) (cons nil nil))
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203 ((or (numberp code) (eq code t))
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204 (cons t code))
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205 (t (cons 'quote code))))
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206 ((eq (car code) bq-at-marker)
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207 (cons bq-at-flag (nth 1 code)))
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208 ((eq (car code) bq-dot-marker)
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209 (cons bq-dot-flag (nth 1 code)))
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210 ((eq (car code) bq-comma-marker)
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211 (bq-comma (nth 1 code)))
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212 ((or (eq (car code) bq-backquote-marker)
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213 (eq (car code) bq-backtick-marker)) ; old lossage
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214 (bq-process-2 (bq-process (nth 1 code))))
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215 (t (let* ((aflag-a (bq-process-2 (car code)))
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216 (aflag (car aflag-a))
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217 (a (cdr aflag-a)))
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218 (let* ((dflag-d (bq-process-2 (cdr code)))
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219 (dflag (car dflag-d))
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220 (d (cdr dflag-d)))
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221 (if (eq dflag bq-at-flag)
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222 ;; get the errors later.
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223 (error ",@ after dot in %s" code))
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224 (if (eq dflag bq-dot-flag)
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225 (error ",. after dot in %s" code))
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226 (cond
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227 ((eq aflag bq-at-flag)
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228 (if (null dflag)
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229 (bq-comma a)
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230 (cons 'append
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231 (cond ((eq dflag 'append)
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232 (cons a d ))
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233 (t (list a (bq-process-1 dflag d)))))))
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234 ((eq aflag bq-dot-flag)
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235 (if (null dflag)
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236 (bq-comma a)
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237 (cons 'nconc
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238 (cond ((eq dflag 'nconc)
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239 (cons a d))
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240 (t (list a (bq-process-1 dflag d)))))))
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241 ((null dflag)
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242 (if (memq aflag '(quote t nil))
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243 (cons 'quote (list a))
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244 (cons 'list (list (bq-process-1 aflag a)))))
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245 ((memq dflag '(quote t))
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246 (if (memq aflag '(quote t nil))
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247 (cons 'quote (cons a d ))
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248 (cons 'list* (list (bq-process-1 aflag a)
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249 (bq-process-1 dflag d)))))
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250 (t (setq a (bq-process-1 aflag a))
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251 (if (memq dflag '(list list*))
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252 (cons dflag (cons a d))
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253 (cons 'list*
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254 (list a (bq-process-1 dflag d)))))))))))
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255
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256 ;;; This handles the <hair> cases
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257 (defun bq-comma (code)
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258 (cond ((atom code)
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259 (cond ((null code)
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260 (cons nil nil))
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261 ((or (numberp code) (eq code 't))
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262 (cons t code))
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263 (t (cons bq-comma-flag code))))
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264 ((eq (car code) 'quote)
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265 (cons (car code) (car (cdr code))))
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266 ((memq (car code) '(append list list* nconc))
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267 (cons (car code) (cdr code)))
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268 ((eq (car code) 'cons)
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269 (cons 'list* (cdr code)))
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270 (t (cons bq-comma-flag code))))
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271
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272 ;;; This handles table 1.
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273 (defun bq-process-1 (flag thing)
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274 (cond ((or (eq flag bq-comma-flag)
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275 (memq flag '(t nil)))
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276 thing)
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277 ((eq flag 'quote)
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278 (list 'quote thing))
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279 ((eq flag 'vector)
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280 (list 'apply '(function vector) thing))
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281 (t (cons (cdr
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282 (assq flag
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283 '((cons . cons)
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284 (list* . bq-list*)
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285 (list . list)
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286 (append . append)
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287 (nconc . nconc))))
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288 thing))))
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289
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290 ;;; ----------------------------------------------------------------
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291
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292 (defmacro bq-list* (&rest args)
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293 "Return a list of its arguments with last cons a dotted pair."
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294 (setq args (reverse args))
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295 (let ((result (car args)))
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296 (setq args (cdr args))
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297 (while args
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298 (setq result (list 'cons (car args) result))
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299 (setq args (cdr args)))
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300 result))
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301
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302 (provide 'backquote)
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303
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304 ;;; backquote.el ends here
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