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      Michał "phoe" Herda · Monday, 3 January, 2022 - 18:55

    #CommonLisp #Lisp

    Someone noted that they'd like a stream that can append to an existing stream with a fill pointer, like the stream that with-output-to-string can produce, except with indefinite extent. A little bit of Lisp hackery produced something that seems to work, even if greatly untested (yet).

    ;;; A fill pointer output stream with indefinite extent
    (defclass fill-pointer-output-stream
      ((string :accessor fill-pointer-output-stream-string :initarg :string))
      (:default-initargs :string (a:required-argument :string)))
    (defmethod trivial-gray-streams:stream-line-column
        ((stream fill-pointer-output-stream)))
    (defmethod trivial-gray-streams:stream-start-line-p
        ((stream fill-pointer-output-stream)))
    (defmethod trivial-gray-streams:stream-write-char
        ((stream fill-pointer-output-stream) char)
      (vector-push-extend char (fill-pointer-output-stream-string stream)))
    CL-USER> (let* ((string (make-array 0 :element-type 'character :fill-pointer 0))
                    (stream (make-instance 'fill-pointer-output-stream :string string)))
               (write-string "asdf" stream)
               (close stream)