12.1.8 Structures as Ports
The prop:input-port and prop:output-port structure type properties identify structure types whose instances can serve as input and output ports, respectively.
Each property value can be either of the following:
An input port (for prop:input-port) or output port (for prop:output-port): In this case, using the structure as port is equivalent to using the given input or output port.
An exact, non-negative integer between 0 (inclusive) and the number of non-automatic fields in the structure type (exclusive, not counting supertype fields): The integer identifies a field in the structure, and the field must be designated as immutable. If the field contains an input port (for prop:input-port) or output port (for prop:output-port), the port is used. Otherwise, an empty string input port is used for prop:input-port, and a port that discards all data is used for prop:output-port.
Some procedures, such as file-position, work on both input and output ports. When given an instance of a structure type with both the prop:input-port and prop:output-port properties, the instance is used as an input port.