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1
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2 @node Major Modes, Indentation, Mule, Top
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3 @chapter Major Modes
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4 @cindex major modes
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5 @kindex TAB
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6 @kindex DEL
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7 @kindex LFD
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8
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9 Emacs has many different @dfn{major modes}, each of which customizes
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10 Emacs for editing text of a particular sort. The major modes are mutually
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11 exclusive; at any time, each buffer has one major mode. The mode line
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12 normally contains the name of the current major mode in parentheses.
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13 @xref{Mode Line}.
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14
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15 The least specialized major mode is called @dfn{Fundamental mode}. This
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16 mode has no mode-specific redefinitions or variable settings. Each
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17 Emacs command behaves in its most generic manner, and each option is in its
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18 default state. For editing any specific type of text, such as Lisp code or
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19 English text, you should switch to the appropriate major mode, such as Lisp
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20 mode or Text mode.
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21
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22 Selecting a major mode changes the meanings of a few keys to become
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23 more specifically adapted to the language being edited. @key{TAB},
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24 @key{DEL}, and @key{LFD} are changed frequently. In addition, commands
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25 which handle comments use the mode to determine how to delimit comments.
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26 Many major modes redefine the syntactical properties of characters
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27 appearing in the buffer. @xref{Syntax}.
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28
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29 The major modes fall into three major groups. Programming modes
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30 (@pxref{Programs}) are for specific programming languages. They tend to
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31 be line-oriented, often enforcing indentation. They emphasize
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32 facilities for creating and displaying structure. Text modes
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33 (like Nroff mode, @TeX{} mode, Outline mode, XML mode, etc.@:) are for
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34 editing human readable text. The remaining major modes are not intended
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35 for direct use in editing user files; they are used in buffers created
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36 by Emacs for specific purposes. Examples of such modes include Dired
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37 mode which is used for buffers made by Dired (@pxref{Dired}), Mail mode
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38 for buffers made by @kbd{C-x m} (@pxref{Sending Mail}), and Shell mode
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39 for buffers used for communicating with an inferior shell process
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40 (@pxref{Interactive Shell}).
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41
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42 Most programming language major modes specify that only blank lines
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43 separate paragraphs. This is so that the paragraph commands remain useful.
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44 @xref{Paragraphs}. They also cause Auto Fill minor mode to use the
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45 definition of
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46 @key{TAB} to indent the new lines it creates. This is because most lines
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47 in a program are usually indented. @xref{Indentation}.
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48
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49 @menu
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50 * Choosing Modes:: How major modes are specified or chosen.
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51 * Mode Hooks:: Customizing a major mode
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52 @end menu
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53
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54 @node Choosing Modes, Mode Hooks, Major Modes, Major Modes
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428
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55 @section Choosing Major Modes
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56
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57 You can select a major mode explicitly for the current buffer, but
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58 most of the time Emacs determines which mode to use based on the file
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59 name or some text in the file.
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60
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61 Use a @kbd{M-x} command to explicitly select a new major mode. Add
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62 @code{-mode} to the name of a major mode to get the name of a command to
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63 select that mode. For example, to enter Lisp mode, execute @kbd{M-x
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64 lisp-mode}.
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65
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66 @vindex auto-mode-alist
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67 When you visit a file, Emacs usually chooses the right major mode
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68 based on the file's name. For example, files whose names end in
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69 @code{.c} are edited in C mode. The variable @code{auto-mode-alist}
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70 controls the correspondence between file names and major mode. Its value
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71 is a list in which each element has the form:
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72
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73 @example
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74 (@var{regexp} . @var{mode-function})
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75 @end example
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76
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77 @noindent
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78 For example, one element normally found in the list has the form
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79 @code{(@t{"\\.c$"} . c-mode)}. It is responsible for selecting C mode
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80 for files whose names end in @file{.c}. (Note that @samp{\\} is needed in
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81 Lisp syntax to include a @samp{\} in the string, which is needed to
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82 suppress the special meaning of @samp{.} in regexps.) The only practical
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83 way to change this variable is with Lisp code.
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84
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85 You can specify which major mode should be used for editing a certain
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86 file by a special sort of text in the first non-blank line of the file.
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87 The mode name should appear in this line both preceded and followed by
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88 @samp{-*-}. Other text may appear on the line as well. For example,
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89
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90 @example
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91 ;-*-Lisp-*-
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92 @end example
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93
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94 @noindent
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95 tells Emacs to use Lisp mode. Note how the semicolon is used to make Lisp
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96 treat this line as a comment. Such an explicit specification overrides any
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97 default mode based on the file name.
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98
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99 Another format of mode specification is:
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100
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101 @example
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102 -*-Mode: @var{modename};-*-
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103 @end example
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104
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105 @noindent
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106 which allows other things besides the major mode name to be specified.
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107 However, Emacs does not look for anything except the mode name.
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108
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109 The major mode can also be specified in a local variables list.
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110 @xref{File Variables}.
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111
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112 @vindex default-major-mode
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113 When you visit a file that does not specify a major mode to use, or
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114 when you create a new buffer with @kbd{C-x b}, Emacs uses the major mode
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115 specified by the variable @code{default-major-mode}. Normally this
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116 value is the symbol @code{fundamental-mode}, which specifies Fundamental
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117 mode. If @code{default-major-mode} is @code{nil}, the major mode is
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118 taken from the previously selected buffer.
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119
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120 @node Mode Hooks, , Choosing Modes, Major Modes
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121 @section Mode Hook Variables
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122
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123 @cindex Hook variables
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124 @cindex mode hook
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125 @findex add-hook
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126 @findex remove-hook
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127 @vindex lisp-mode-hook
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128 @vindex emacs-lisp-mode-hook
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129 @vindex lisp-interaction-mode-hook
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130 @vindex scheme-mode-hook
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131
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132 The last step taken by a major mode, by convention, is to invoke a
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133 list of user supplied functions that are stored in a @dfn{hook}
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134 variable. This allows a user to further customize the major mode, and
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135 is particularly convenient for associating minor modes with major modes.
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136 @xref{Minor Modes}. For example, to automatically turn on the Auto Fill
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137 minor mode when Text mode is invoked, add the following code to the
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138 initialization file (@pxref{Init File})
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139
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140 @example
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141 (add-hook 'text-mode-hook 'turn-on-auto-fill)
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142 @end example
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143
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144 Derived modes often @dfn{inherit} the parent mode's hooks. For
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145 example, Texinfo Mode is derived from Text Mode, so customizing
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146 @code{text-mode-hook} as above will also enable Auto Fill minor mode in
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147 buffers containing Texinfo files.
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148
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149 Hooks are also commonly used to set up buffer local variables
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150 (@pxref{Locals}).
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151
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152 The name of the hook variable is created by appending the string
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153 @code{-hook} to the name of the function used to invoke the major mode.
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154 For example, as seen above, the hook variable used by Text Mode would be
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155 named @code{text-mode-hook}. By convention the mode hook function
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156 receives no arguments. If a hook variable does not exist, or it has the
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157 value @code{nil}, the major mode simply ignores it.
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158
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159 The recommended way to add functions to a hook variable is with the
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160 @code{add-hook} function.
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161 @code{add-hook} will check that the function is not already
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162 listed in the hook variable before adding it. It will also create a hook
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163 variable with the value @code{nil} if one does not exist before adding
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164 the function. @code{add-hook} adds functions to the front of the hook
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165 variable list. This means that the last hook added is run first by the
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166 major mode. It is considered very poor style to write hook functions
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167 that depend on the order that hooks are executed.
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168
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169 Hooks can be removed from hook variables with @code{remove-hook}.
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170
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171
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