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21.6.2 Completion and the Minibuffer

This section describes the basic interface for reading from the minibuffer with completion.

Function: completing-read prompt collection &optional predicate require-match initial history default inherit-input-method ¶

This function reads a string in the minibuffer, assisting the user by providing completion. It activates the minibuffer with prompt prompt, which must be a string.

The actual completion is done by passing the completion table collection and the completion predicate predicate to the function try-completion (see Basic Completion Functions). This happens in certain commands bound in the local keymaps used for completion. Some of these commands also call test-completion. Thus, if predicate is non-nil, it should be compatible with collection and completion-ignore-case. See Definition of test-completion.

See Programmed Completion, for detailed requirements when collection is a function.

The value of the optional argument require-match determines how the user may exit the minibuffer:

• If nil, the usual minibuffer exit commands work regardless of the input in the minibuffer.
• If t, the usual minibuffer exit commands won’t exit unless the input completes to an element of collection.
• If confirm, the user can exit with any input, but is asked for confirmation if the input is not an element of collection.
• If confirm-after-completion, the user can exit with any input, but is asked for confirmation if the preceding command was a completion command (i.e., one of the commands in minibuffer-confirm-exit-commands) and the resulting input is not an element of collection. See Minibuffer Commands that Do Completion.
• If a function, it is called with the input as the only argument. The function should return a non-nil value if the input is acceptable.
• Any other value of require-match behaves like t, except that the exit commands won’t exit if it performs completion.

However, empty input is always permitted, regardless of the value of require-match; in that case, completing-read returns the first element of default, if it is a list; "", if default is nil; or default. The string or strings in default are also available to the user through the history commands (see Minibuffer Commands). In addition, the completion candidates are added to the “future history” when the values in default are exhausted by M-n; see minibuffer-default-add-function.

The function completing-read uses minibuffer-local-completion-map as the keymap if require-match is nil, and uses minibuffer-local-must-match-map if require-match is non-nil. See Minibuffer Commands that Do Completion.

The argument history specifies which history list variable to use for saving the input and for minibuffer history commands. It defaults to minibuffer-history. If history is the symbol t, history is not recorded. See Minibuffer History.

The argument initial is mostly deprecated; we recommend using a non-nil value only in conjunction with specifying a cons cell for history. See Initial Input. For default input, use default instead.

If the argument inherit-input-method is non-nil, then the minibuffer inherits the current input method (see Input Methods) and the setting of enable-multibyte-characters (see Text Representations) from whichever buffer was current before entering the minibuffer.

If the variable completion-ignore-case is non-nil, completion ignores case when comparing the input against the possible matches. See Basic Completion Functions. In this mode of operation, predicate must also ignore case, or you will get surprising results.

Here’s an example of using completing-read:

(completing-read
 "Complete a foo: "
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
 nil t "fo")

;; After evaluation of the preceding expression,
;;   the following appears in the minibuffer:

---------- Buffer: Minibuffer ----------
Complete a foo: fo∗
---------- Buffer: Minibuffer ----------

If the user then types DEL DEL b RET, completing-read returns barfoo.

The completing-read function binds variables to pass information to the commands that actually do completion. They are described in the following section.

Variable: completing-read-function ¶

The value of this variable must be a function, which is called by completing-read to actually do its work. It should accept the same arguments as completing-read. This can be bound to a different function to completely override the normal behavior of completing-read.