19.6.4 High-Level Completion Functions

This section describes the higher-level convenience functions for reading certain sorts of names with completion.

In most cases, you should not call these functions in the middle of a Lisp function. When possible, do all minibuffer input as part of reading the arguments for a command, in the interactive specification. See Defining Commands.

Function: read-buffer prompt &optional default require-match predicate

This function reads the name of a buffer and returns it as a string. It prompts with prompt. The argument default is the default name to use, the value to return if the user exits with an empty minibuffer. If non-nil, it should be a string, a list of strings, or a buffer. If it is a list, the default value is the first element of this list. It is mentioned in the prompt, but is not inserted in the minibuffer as initial input.

The argument prompt should be a string ending with a colon and a space. If default is non-nil, the function inserts it in prompt before the colon to follow the convention for reading from the minibuffer with a default value (see Programming Tips).

The optional argument require-match has the same meaning as in completing-read. See Minibuffer Completion.

The optional argument predicate, if non-nil, specifies a function to filter the buffers that should be considered: the function will be called with every potential candidate as its argument, and should return nil to reject the candidate, non-nil to accept it.

In the following example, the user enters ‘minibuffer.t’, and then types RET. The argument require-match is t, and the only buffer name starting with the given input is ‘minibuffer.texi’, so that name is the value.

(read-buffer "Buffer name: " "foo" t)

;; After evaluation of the preceding expression,
;;   the following prompt appears,
;;   with an empty minibuffer:

---------- Buffer: Minibuffer ----------
Buffer name (default foo): ∗
---------- Buffer: Minibuffer ----------

;; The user types minibuffer.t RET.
     ⇒ "minibuffer.texi"

User Option: read-buffer-function

This variable, if non-nil, specifies a function for reading buffer names. read-buffer calls this function instead of doing its usual work, with the same arguments passed to read-buffer.

User Option: read-buffer-completion-ignore-case

If this variable is non-nil, read-buffer ignores case when performing completion while reading the buffer name.

Function: read-command prompt &optional default

This function reads the name of a command and returns it as a Lisp symbol. The argument prompt is used as in read-from-minibuffer. Recall that a command is anything for which commandp returns t, and a command name is a symbol for which commandp returns t. See Interactive Call.

The argument default specifies what to return if the user enters null input. It can be a symbol, a string or a list of strings. If it is a string, read-command interns it before returning it. If it is a list, read-command interns the first element of this list. If default is nil, that means no default has been specified; then if the user enters null input, the return value is (intern ""), that is, a symbol whose name is an empty string.

(read-command "Command name? ")

;; After evaluation of the preceding expression,
;;   the following prompt appears with an empty minibuffer:

---------- Buffer: Minibuffer ----------
Command name?
---------- Buffer: Minibuffer ----------

If the user types forward-c RET, then this function returns forward-char.

The read-command function is a simplified interface to completing-read. It uses the variable obarray so as to complete in the set of extant Lisp symbols, and it uses the commandp predicate so as to accept only command names:

(read-command prompt)
≡
(intern (completing-read prompt obarray
                         'commandp t nil))

Function: read-variable prompt &optional default

This function reads the name of a customizable variable and returns it as a symbol. Its arguments have the same form as those of read-command. It behaves just like read-command, except that it uses the predicate custom-variable-p instead of commandp.

Command: read-color &optional prompt convert allow-empty display

This function reads a string that is a color specification, either the color’s name or an RGB hex value such as #RRRGGGBBB. It prompts with prompt (default: "Color (name or #RGB triplet):") and provides completion for color names, but not for hex RGB values. In addition to names of standard colors, completion candidates include the foreground and background colors at point.

Valid RGB values are described in Color Names.

The function’s return value is the string typed by the user in the minibuffer. However, when called interactively or if the optional argument convert is non-nil, it converts any input color name into the corresponding RGB value string and instead returns that. This function requires a valid color specification to be input. Empty color names are allowed when allow-empty is non-nil and the user enters null input.

Interactively, or when display is non-nil, the return value is also displayed in the echo area.

See also the functions read-coding-system and read-non-nil-coding-system, in User-Chosen Coding Systems, and read-input-method-name, in Input Methods.