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4.3 Auto-Completion Mode

When the auto-completion minor mode is enabled by calling the auto-completion-mode command, predictive mode will automatically look for completions of words as you type. This is especially useful when used in conjunction with dynamic completion (see Dynamic Completion). If you would like auto-completion-mode to be enabled automatically whenever predictive mode is enabled, set the predictive-auto-complete variable (set by default). Otherwise, you will not only have to enable auto-completion-mode manually, but also manually set the auto-completion-source to the predictive setting.

Enable auto-completion Mode. In this minor mode, completions are found automatically as words are typed.
When non-nil (the default), enabling and disabling predictive mode will also automatically enable or disable auto-completion-mode, without needing to run the auto-completion-mode command manually.
If predictive-auto-complete is not enabled, then you must set this variable manually to the predictive setting in order to use predictive auto-completion.

Word-constituent characters are added to the current word before finding new completions. The current syntax table (see Syntax) determines which characters are word-constituents and which are not (although the behaviour of individual characters can be overridden).

By customizing auto-completion-syntax-alist, you can make punctuation and white-space characters automatically accept the current completion. If you want to exceptionally insert a punctuation character as though it were a word-constituent (such as the ‘.’ in ‘e.g.’), you can often prefix the character key with M-. The bindings M-. and M-- are already set up by default, but you can easily add more using the completion-define-word-constituent-binding convenience function in your .emacs file. See Character Syntax and Key Bindings, for more details about the syntax and key binding features. The following variables affect the behaviour of auto-completion-mode:

Whether auto-completion-mode allows you to type normally, ignoring the provisional completions until you want to use one (select ‘type normally’ in the customization buffer), or whether punctuation characters automatically accept the current provisional completion before the character is inserted (select ‘punctuation accepts’ in the customization buffer). For full details, see Character Syntax and Key Bindings.
Minimum number of characters that must be typed before the various completion mechanisms are activated when using auto-completion-mode. Note that you can still explicitly invoke completion on a shorter prefix using the complete-predictive command (see Inserting Completions).
Number of seconds Emacs must be idle before the various completion mechanisms are activated when using auto-completion-mode (see Auto-Completion Mode).
Number of seconds Emacs must be idle after a backwards deletion (see Deleting Characters) before the various completion mechanisms are activated when using auto-completion-mode (see Auto-Completion Mode). Having a small delay is useful when the delete key is held down continuously. Default is 0.1 seconds.