 Looking Up and Expanding Abbreviations Abbrevs are usually expanded by certain interactive commands, including self-insert-command. This section describes the subroutines used in writing such commands, as well as the variables they use for communication.   Function: abbrev-symbol abbrev &optional table
 This function returns the symbol representing the abbrev named abbrev. It returns nil if that abbrev is not defined. The optional second argument table is the abbrev table in which to look it up. If table is nil, this function tries first the current buffer’s local abbrev table, and second the global abbrev table. 
  Function: abbrev-expansion abbrev &optional table
 This function returns the string that abbrev would expand into (as defined by the abbrev tables used for the current buffer). It returns nil if abbrev is not a valid abbrev. The optional argument table specifies the abbrev table to use, as in abbrev-symbol. 
  Command: expand-abbrev
 
This command expands the abbrev before point, if any. If point does not follow an abbrev, this command does nothing. To do the expansion, it calls the function that is the value of the abbrev-expand-function variable, with no arguments, and returns whatever that function does.  The default expansion function returns the abbrev symbol if it did expansion, and nil otherwise. If the abbrev symbol has a hook function that is a symbol whose no-self-insert property is non-nil, and if the hook function returns nil as its value, then the default expansion function returns nil, even though expansion did occur. 

  Function: abbrev-insert abbrev &optional name start end
 This function inserts the abbrev expansion of abbrev, replacing the text between start and end. If start is omitted, it defaults to point. name, if non-nil, should be the name by which this abbrev was found (a string); it is used to figure out whether to adjust the capitalization of the expansion. The function returns abbrev if the abbrev was successfully inserted, otherwise it returns nil. 
  Command: abbrev-prefix-mark &optional arg
 
This command marks the current location of point as the beginning of an abbrev. The next call to expand-abbrev will use the text from here to point (where it is then) as the abbrev to expand, rather than using the previous word as usual.  First, this command expands any abbrev before point, unless arg is non-nil. (Interactively, arg is the prefix argument.) Then it inserts a hyphen before point, to indicate the start of the next abbrev to be expanded. The actual expansion removes the hyphen. 

  User Option: abbrev-all-caps
 When this is set non-nil, an abbrev entered entirely in upper case is expanded using all upper case. Otherwise, an abbrev entered entirely in upper case is expanded by capitalizing each word of the expansion. 
  Variable: abbrev-start-location
 The value of this variable is a buffer position (an integer or a marker) for expand-abbrev to use as the start of the next abbrev to be expanded. The value can also be nil, which means to use the word before point instead. abbrev-start-location is set to nil each time expand-abbrev is called. This variable is also set by abbrev-prefix-mark. 
  Variable: abbrev-start-location-buffer
 The value of this variable is the buffer for which abbrev-start-location has been set. Trying to expand an abbrev in any other buffer clears abbrev-start-location. This variable is set by abbrev-prefix-mark. 
  Variable: last-abbrev
 This is the abbrev-symbol of the most recent abbrev expanded. This information is left by expand-abbrev for the sake of the unexpand-abbrev command (see Expanding Abbrevs in The GNU Emacs Manual). 
  Variable: last-abbrev-location
 This is the location of the most recent abbrev expanded. This contains information left by expand-abbrev for the sake of the unexpand-abbrev command. 
  Variable: last-abbrev-text
 This is the exact expansion text of the most recent abbrev expanded, after case conversion (if any). Its value is nil if the abbrev has already been unexpanded. This contains information left by expand-abbrev for the sake of the unexpand-abbrev command. 
  Variable: abbrev-expand-function
 The value of this variable is a function that expand-abbrev will call with no arguments to do the expansion. The function can do anything it wants before and after performing the expansion. It should return the abbrev symbol if expansion took place. 
 The following sample code shows a simple use of abbrev-expand-function. It assumes that foo-mode is a mode for editing certain files in which lines that start with ‘#’ are comments. You want to use Text mode abbrevs for those lines. The regular local abbrev table, foo-mode-abbrev-table is appropriate for all other lines. See Standard Abbrev Tables, for the definitions of local-abbrev-table and text-mode-abbrev-table. See Advising Functions, for details of add-function.   (defun foo-mode-abbrev-expand-function (expand)
  (if (not (save-excursion (forward-line 0) (eq (char-after) ?#)))
      ;; Performs normal expansion.
      (funcall expand)
    ;; We're inside a comment: use the text-mode abbrevs.
    (let ((local-abbrev-table text-mode-abbrev-table))
      (funcall expand))))

(add-hook 'foo-mode-hook
          (lambda ()
            (add-function :around (local 'abbrev-expand-function)
                          #'foo-mode-abbrev-expand-function)))



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