output-stream-p

(output-stream-p stream)

t when the stream can be written to. A string output stream, a file stream opened for output (:output, :append, :overwrite) or :io, a socket, the standard output stream and *error-output* answer t; a string input stream, a file stream opened :input and a file stream that has been closed answer nil. A synonym stream answers for the stream it currently forwards to. The designator t answers t for both directions, and anything that is not a stream answers nil. A Gray stream instance answers t when its class descends from rontolisp:fundamental-output-stream.