19.6.1 Basic Completion Functions

The following completion functions have nothing in themselves to do with minibuffers. We describe them here to keep them near the higher-level completion features that do use the minibuffer.

Function: try-completion string collection &optional predicate

This function returns the longest common substring of all possible completions of string in collection.

collection is called the completion table. Its value must be a list of strings or cons cells, an obarray, a hash table, or a completion function.

try-completion compares string against each of the permissible completions specified by the completion table. If no permissible completions match, it returns nil. If there is just one matching completion, and the match is exact, it returns t. Otherwise, it returns the longest initial sequence common to all possible matching completions.

If collection is a list, the permissible completions are specified by the elements of the list, each of which should be either a string, or a cons cell whose CAR is either a string or a symbol (a symbol is converted to a string using symbol-name). If the list contains elements of any other type, those are ignored.

If collection is an obarray (see Creating Symbols), the names of all symbols in the obarray form the set of permissible completions.

If collection is a hash table, then the keys that are strings are the possible completions. Other keys are ignored.

You can also use a function as collection. Then the function is solely responsible for performing completion; try-completion returns whatever this function returns. The function is called with three arguments: string, predicate and nil (the third argument is so that the same function can be used in all-completions and do the appropriate thing in either case). See Programmed Completion.

If the argument predicate is non-nil, then it must be a function of one argument, unless collection is a hash table, in which case it should be a function of two arguments. It is used to test each possible match, and the match is accepted only if predicate returns non-nil. The argument given to predicate is either a string or a cons cell (the CAR of which is a string) from the alist, or a symbol (not a symbol name) from the obarray. If collection is a hash table, predicate is called with two arguments, the string key and the associated value.

In addition, to be acceptable, a completion must also match all the regular expressions in completion-regexp-list. (Unless collection is a function, in which case that function has to handle completion-regexp-list itself.)

In the first of the following examples, the string ‘foo’ is matched by three of the alist CARs. All of the matches begin with the characters ‘fooba’, so that is the result. In the second example, there is only one possible match, and it is exact, so the return value is t.

(try-completion
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)))
     ⇒ "fooba"

(try-completion "foo" '(("barfoo" 2) ("foo" 3)))
     ⇒ t

In the following example, numerous symbols begin with the characters ‘forw’, and all of them begin with the word ‘forward’. In most of the symbols, this is followed with a ‘-’, but not in all, so no more than ‘forward’ can be completed.

(try-completion "forw" obarray)
     ⇒ "forward"

Finally, in the following example, only two of the three possible matches pass the predicate test (the string ‘foobaz’ is too short). Both of those begin with the string ‘foobar’.

(defun test (s)
  (> (length (car s)) 6))
     ⇒ test

(try-completion
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
 'test)
     ⇒ "foobar"

Function: all-completions string collection &optional predicate

This function returns a list of all possible completions of string. The arguments to this function are the same as those of try-completion, and it uses completion-regexp-list in the same way that try-completion does.

If collection is a function, it is called with three arguments: string, predicate and t; then all-completions returns whatever the function returns. See Programmed Completion.

Here is an example, using the function test shown in the example for try-completion:

(defun test (s)
  (> (length (car s)) 6))
     ⇒ test

(all-completions
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
 'test)
     ⇒ ("foobar1" "foobar2")

Function: test-completion string collection &optional predicate

This function returns non-nil if string is a valid completion alternative specified by collection and predicate. The arguments are the same as in try-completion. For instance, if collection is a list of strings, this is true if string appears in the list and predicate is satisfied.

This function uses completion-regexp-list in the same way that try-completion does.

If predicate is non-nil and if collection contains several strings that are equal to each other, as determined by compare-strings according to completion-ignore-case, then predicate should accept either all or none of them. Otherwise, the return value of test-completion is essentially unpredictable.

If collection is a function, it is called with three arguments, the values string, predicate and lambda; whatever it returns, test-completion returns in turn.

Function: completion-boundaries string collection predicate suffix

This function returns the boundaries of the field on which collection will operate, assuming that string holds the text before point and suffix holds the text after point.

Normally completion operates on the whole string, so for all normal collections, this will always return (0 . (length suffix)). But more complex completion such as completion on files is done one field at a time. For example, completion of "/usr/sh" will include "/usr/share/" but not "/usr/share/doc" even if "/usr/share/doc" exists. Also all-completions on "/usr/sh" will not include "/usr/share/" but only "share/". So if string is "/usr/sh" and suffix is "e/doc", completion-boundaries will return (5 . 1) which tells us that the collection will only return completion information that pertains to the area after "/usr/" and before "/doc".

If you store a completion alist in a variable, you should mark the variable as “risky” by giving it a non-nil risky-local-variable property. See File Local Variables.

Variable: completion-ignore-case

If the value of this variable is non-nil, case is not considered significant in completion. Within read-file-name, this variable is overridden by read-file-name-completion-ignore-case (see Reading File Names); within read-buffer, it is overridden by read-buffer-completion-ignore-case (see High-Level Completion).

Variable: completion-regexp-list

This is a list of regular expressions. The completion functions only consider a completion acceptable if it matches all regular expressions in this list, with case-fold-search (see Searching and Case) bound to the value of completion-ignore-case.

Macro: lazy-completion-table var fun

This macro provides a way to initialize the variable var as a collection for completion in a lazy way, not computing its actual contents until they are first needed. You use this macro to produce a value that you store in var. The actual computation of the proper value is done the first time you do completion using var. It is done by calling fun with no arguments. The value fun returns becomes the permanent value of var.

Here is an example:

(defvar foo (lazy-completion-table foo make-my-alist))

There are several functions that take an existing completion table and return a modified version. completion-table-case-fold returns a case-insensitive table. completion-table-in-turn and completion-table-merge combine multiple input tables in different ways. completion-table-subvert alters a table to use a different initial prefix. completion-table-with-quoting returns a table suitable for operating on quoted text. completion-table-with-predicate filters a table with a predicate function. completion-table-with-terminator adds a terminating string.