make-echo-stream

(make-echo-stream input-stream output-stream)

A two-way stream that also writes everything it reads to the output component — the interactive-echo idiom.

The input side is a Gray stream, so read-line and read-char dispatch over it and each character read is echoed to the output component along the way; write-char / write-string reach the output side directly. Only what is read echoes: peek-char looks at the input component without writing. The components are recoverable with echo-stream-input-stream and echo-stream-output-stream.

Each component must stream in the right direction: an input stream that is not one, or an output stream that is not one, signals a type-error.