When a synonym-stream is useful
Imagine your project has a special variable whose value is a stream to which some output is directed, e.g. my-project:*log-output*. You can change where the output goes by setting the value of the special variable, but by default, output should go to *standard-output*.
The naive way to do this is a direct reference:
(defvar *log-output* *standard-output*)
However, this can run into trouble if *standard-output* is bound to something unexpected when that form is evaluated. For example, if file loading output is temporarily suppressed by binding *standard-output* to (make-broadcast-stream), output to *log-output* is then also discarded.
One way to work around it is with make-synonym-stream:
(defvar *log-output* (make-synonym-stream '*standard-output*))
With that setup, any output sent to *log-output* is sent to the stream that is the dynamic value of the symbol '*standard-output*. And since the dynamic value is looked up for each output, the output to *log-output* will go to *standard-output* even if *standard-output* is assigned some other stream value later on.
