This is a proposal for the WG2 "port operations" work item. It provides useful iterators over generic ports, and ways to combine and alias ports. It is mostly based on Chicken's ports unit and CL's streams facilities.
These procedures do not check that source and sink are actually ports, so reader and writer may perform arbitrary operations as long as they can be invoked as (reader source) and (writer value sink), respectively.
(port-for-each iterator reader source)
Applies iterator to the successive results of calling reader on source until it returns an EOF object (not including the EOF object), discarding the results. Returns an undefined value.
(port-map iterator reader source)
Applies iterator to the successive results of calling reader on source until it returns an EOF object (not including the EOF object), returning a list of the collected results.
(port-fold iterator reader source seed ...)
Applies iterator to the successive results of calling reader on source, passing the seed values as additional arguments to iterator. The results returned by iterator become the new seed values. When reader returns an EOF object, the last results of iterator are returned.
(port-unfold terminate? iterator writer sink seed ...)
Uses iterator to generate the sequence of values seed, (iterator seed), (iterator (iterator seed)), and so on. Uses writer to output the values to sink. Terminate? is invoked before calling iterator each time; if it returns #f, the unfolding terminates. Returns an undefined value.
(copy-port source sink reader writer)
Reads all remaining data from source using the procedure reader and writes it to sink using the procedure writer. Reader defaults to read-char and writer to write-char. Returns an undefined value.
Broadcast ports, concatenated ports, and alias ports are mutually disjoint types, and are disjoint from other types of Scheme objects.
(make-broadcast-port output-port ...)
Returns a custom output port that emits everything written into it to the output-ports. An operation can only be performed on a broadcast port if it can be performed on all the output-ports. Closing the broadcast port does not close any of the output-ports. If no ports are specified, the result serves as a sink that discards all its output.
Returns #t if obj is a broadcast port, and #f otherwise.
(make-concatenated-port input-port ...)
Returns a custom input port that reads its input from each input-port until it returns an EOF object, then returns just one EOF object itself. An operation can only be performed on a concatenated port if it can be performed on all the input-ports. Closing the concatenated port does not close any of the input-ports. If no input-ports are specified, the result serves as an empty source.
Returns #t if obj is a concatenated port, and #f otherwise.
Returns a port which passes any port operations invoked on it to the current value of the dynamic variable parameter.
Returns #t if obj is an alias port, and #f otherwise.