19.6.7 Programmed Completion

Sometimes it is not possible or convenient to create an alist or an obarray containing all the intended possible completions ahead of time. In such a case, you can supply your own function to compute the completion of a given string. This is called programmed completion. Emacs uses programmed completion when completing file names (see File Name Completion), among many other cases.

To use this feature, pass a function as the collection argument to completing-read. The function completing-read arranges to pass your completion function along to try-completion, all-completions, and other basic completion functions, which will then let your function do all the work.

The completion function should accept three arguments:

• The string to be completed.
• A predicate function with which to filter possible matches, or nil if none. The function should call the predicate for each possible match, and ignore the match if the predicate returns nil.
• A flag specifying the type of completion operation to perform. This is one of the following four values:

  nil

  This specifies a try-completion operation. The function should return t if the specified string is a unique and exact match; if there is more than one match, it should return the common substring of all matches (if the string is an exact match for one completion alternative but also matches other longer alternatives, the return value is the string); if there are no matches, it should return nil.

  t

  This specifies an all-completions operation. The function should return a list of all possible completions of the specified string.

  lambda

  This specifies a test-completion operation. The function should return t if the specified string is an exact match for some completion alternative; nil otherwise.

  (boundaries . suffix)

  This specifies a completion-boundaries operation. The function should return (boundaries start . end), where start is the position of the beginning boundary in the specified string, and end is the position of the end boundary in suffix.

  metadata

  This specifies a request for information about the state of the current completion. The return value should have the form (metadata . alist), where alist is an alist whose elements are described below.

  If the flag has any other value, the completion function should return nil.

The following is a list of metadata entries that a completion function may return in response to a metadata flag argument:

category

The value should be a symbol describing what kind of text the completion function is trying to complete. If the symbol matches one of the keys in completion-category-overrides, the usual completion behavior is overridden. See Completion Variables.

annotation-function

The value should be a function for annotating completions. The function should take one argument, string, which is a possible completion. It should return a string, which is displayed after the completion string in the *Completions* buffer.

display-sort-function

The value should be a function for sorting completions. The function should take one argument, a list of completion strings, and return a sorted list of completion strings. It is allowed to alter the input list destructively.

cycle-sort-function

The value should be a function for sorting completions, when completion-cycle-threshold is non-nil and the user is cycling through completion alternatives. See Completion Options in The GNU Emacs Manual. Its argument list and return value are the same as for display-sort-function.

Function: completion-table-dynamic function

This function is a convenient way to write a function that can act as a programmed completion function. The argument function should be a function that takes one argument, a string, and returns an alist of possible completions of it. You can think of completion-table-dynamic as a transducer between that interface and the interface for programmed completion functions.

Function: completion-table-with-cache function &optional ignore-case

This is a wrapper for completion-table-dynamic that saves the last argument-result pair. This means that multiple lookups with the same argument only need to call function once. This can be useful when a slow operation is involved, such as calling an external process.