Emacs Lisp: How to Write Keyword Completion Command

By Xah Lee. Date: . Last updated: .

This page shows you how to implement keyword completion in emacs.

xlsl-keyword completion
Keyword completion in emacs.

Problem

You are writing a major mode for your own language. You want keyword completion feature.

When user calls complete-symbol or completion-at-point Ctrl+Alt+i in your major mode, completion should be done with your language's keywords.

Solution

Two things you have to do:

The function xyz-completion-at-point will not take any argument. It will return a list of this form:

(START END COLLECTION . PROPS)

Here's the complete code of a major mode with keyword completion feature.

;; sample major mode with keyword completion feature

;; this is your lang's keywords
(setq xyz-keywords
      '("touch"
       "touch_start"
       "touch_end"
       "for"
       "foreach"
       "forall"
       ))

(defun xyz-completion-at-point ()
  "This is the function to be used for the hook `completion-at-point-functions'."
  (interactive)
  (let* (
         (bds (bounds-of-thing-at-point 'symbol))
         (start (car bds))
         (end (cdr bds)))
    (list start end xyz-keywords . nil )))

(define-derived-mode xyz-mode c-mode "xyz"
  "Major mode for editing xyz lang code …"
  (add-hook 'completion-at-point-functions 'xyz-completion-at-point nil 'local))
  1. Copy and paste the above into a new buffer.
  2. Alt+x eval-buffer.
  3. Open a new buffer.
  4. M-x xyz-mode.
  5. Type f, then press Ctrl+Alt+i. Emacs will complete it to become “for”. Press again to see choices.

How does this work?

The command complete-symbol or completion-at-point is emacs standard command to complete word at point. The default key is Ctrl+Alt+i.

complete-symbol is a wrapper to. completion-at-point. You can see in the source code.

;; emacs 25.1.1 source code for complete-symbol. 2016-10-25
(defun complete-symbol (arg)
  "Perform completion on the text around point.
The completion method is determined by `completion-at-point-functions'.

With a prefix argument, this command does completion within
the collection of symbols listed in the index of the manual for the
language you are using."
  (interactive "P")
  (if arg (info-complete-symbol) (completion-at-point)))

The command completion-at-point will just call functions in the variable list completion-at-point-functions.

So, all you have to do is add your own completion function to the list completion-at-point-functions.

Our completion function is xyz-completion-at-point, and we added to the hook completion-at-point-functions by this line: 

(add-hook 'completion-at-point-functions 'xyz-completion-at-point nil 'local)

(info "(elisp) Completion in Buffers")

Solution 2: Using ido for Completion

2022-06-20 note: ido is becoming obsolete in emacs 28, because of fido-vertical-mode [see Emacs: icomplete mode, fido mode] In the following code, replace ido-completing-read by completing-read and turn on fido-vertical-mode

Here's another way to do completion, using ido-mode's interface.

emacs keyword completion ido
emacs keyword completion with ido

ido mode's User Interface is much more convenient. User does not need to press Tab. Completion is done automatically as one types. [see Emacs: icomplete vs ido mode]

Here is the function that does the completion, using ido interface.

(require 'ido) ; part of emacs

;; this is your lang's keywords
(setq abc-keywords
      '("touch"
       "touch_start"
       "touch_end"
       "for"
       "foreach"
       "forall"
       ))

(defun abc-complete-symbol ()
  "Perform keyword completion on current symbol.
This uses `ido-mode' user interface for completion."
  (interactive)
  (let* (
         ($bds (bounds-of-thing-at-point 'symbol))
         ($p1 (car $bds))
         ($p2 (cdr $bds))
         ($current-sym
          (if  (or (null $p1) (null $p2) (equal $p1 $p2))
              ""
            (buffer-substring-no-properties $p1 $p2)))
         $result-sym)
    (when (not $current-sym) (setq $current-sym ""))
    (setq $result-sym
          (ido-completing-read "" abc-keywords nil nil $current-sym ))
    (delete-region $p1 $p2)
    (insert $result-sym)))

You'll need to give it a key in your major mode.

[see Emacs Lisp: How to Create Keymap for Major Mode]

For temp testing, you can do:

(global-set-key (kbd "TAB") 'abc-complete-symbol)

Solution 3: Write Your Own Completion Function

Here's alternative way to do completion.

Suppose your language xyz has the following list of keywords.

;; this is your lang's keywords
(setq xyz-keywords
      '("touch"
       "touch_start"
       "touch_end"
       "for"
       "foreach"
       "forall"
       ))

The following is a standalone function that does the completion.

(defun xyz-complete-symbol ()
  "Perform keyword completion on word before cursor."
  (interactive)
  (let ((posEnd (point))
        (meat (thing-at-point 'symbol))
        maxMatchResult)

    ;; when nil, set it to empty string, so user can see all lang's keywords.
    ;; if not done, try-completion on nil result lisp error.
    (when (not meat) (setq meat ""))
    (setq maxMatchResult (try-completion meat xyz-keywords))

    (cond ((eq maxMatchResult t))
          ((null maxMatchResult)
           (message "Can't find completion for “%s”" meat)
           (ding))
          ((not (string-equal meat maxMatchResult))
           (delete-region (- posEnd (length meat)) posEnd)
           (insert maxMatchResult))
          (t (message "Making completion list…")
             (with-output-to-temp-buffer "*Completions*"
               (display-completion-list
                (all-completions meat xyz-keywords)
                meat))
             (message "Making completion list…%s" "done")))))

You'll need to give it a key in your major mode.

[see Emacs Lisp: How to Create Keymap for Major Mode]

For temp testing, you can just do:

(global-set-key (kbd "TAB") 'xyz-complete-symbol)

Then, open a new buffer, type any letter, say “t”, then press Tab, type some more letter, press Tab again.

The above code is very easy to understand. First, you grab the word before cursor, save it as “meat”. Then, you find the maximal match, save it as maxMatchResult. Then, we have a few cases:

  1. If the max match is the same as the word under cursor, then do nothing, because the word is already complete.
  2. If the max match is empty, then tell user there is no completion.
  3. If not the above two cases, then expand the current word to max match.
  4. Otherwise, pop up a dialog to list possible completions.

Lucky for us, emacs does most of the tedious job. The core functions that do the job are:

try-completion
Return the maximal match.
all-completions
Return all possible completions.
display-completion-list
Takes care of the user interface for displaying the possible completions, and making them clickable.

In the above, we used a simple list for our keywords, and fed them to emacs's completion functions. Emacs's completion functions can also take keyword argument in the form of a alist or hashtable. A alist looks like this:

(setq xyz-keywords
 '(("touch" . nil)
   ("touch_start" . nil)
   ("touch_end" . nil)))

The keyword list can also be a hash table. See: Emacs Lisp: Hash Table.

(info "(elisp) Completion")

Here's detailed comparison of the two most fundamental functions try-completion and all-completions.

try-completion

try-completion
(try-completion STRING COLLECTION &optional PREDICATE)
Return t, nil or a string of longest completion. COLLECTION is a list of strings (it can be list of cons pairs or hashtable and other. See elisp manual for detail). (info "(elisp) Basic Completion")
  • Return t if there's a exact match.
  • Return nil if there is no match.
  • Else, return a string of longest match. That is, in COLLECTION, find all strings that starts with STRING, then, of these, find the longest string that they all share at the start of string.
;; found no match. returns nil
(try-completion
 "c"
 '(
   "amc"
   "amb"
   ))
;; nil

;; only 1 possible match, and is same as input
;; returns t
(try-completion
 "amb"
 '(
   "amc"
   "amb"
   ))
;; t

;; only 1 possible match, and is longer than input
;; returns the matched string
(try-completion
 "ah"
 '(
   "amc"
   "amb"
   "ahu"
   ))
;; "ahu"

;; found more than 1 match. returns the longest start string shared by all matched strings
(try-completion
 "a"
 '(
   "amc"
   "amb"
   ))
;; "am"

all-completions

all-completions
Like try-completion, but it returns a list of all matches. 
;; no match
(all-completions
 "c"
 '(
   "amc"
   "amb"
   ))
;; nil

;; only only 1 possible match, same as input
(all-completions
 "amb"
 '(
   "amc"
   "amb"
   ))
;; ("amb")

;; only only 1 possible match, longer than input
(all-completions
 "ah"
 '(
   "amc"
   "amb"
   "ahu"
   ))
;; ("ahu")

;; found more than 1 match. returns them all
(all-completions
 "a"
 '(
   "amc"
   "amb"
   ))
;; ("amc" "amb")

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