listen

Function

Syntax

listen &optional input-stream → generalized-boolean

Arguments and Values

input-stream — an input stream designator. The default is standard input.

generalized-boolean — a generalized boolean.

Description

Returns true if there is a character immediately available from input-stream; otherwise, returns false. On a non-interactive input-stream, listen returns true except when at end of file₁. If an end of file is encountered, listen returns false. listen is intended to be used when input-stream obtains characters from an interactive device such as a keyboard.

Examples

 (progn (unread-char (read-char)) (list (listen) (read-char))) 
⊳ 1 
→ (T #\1) 
 (progn (clear-input) (listen)) 
→ NIL ;Unless you're a very fast typist!

Affected By

*standard-input*