8 Input and Output
A Racket port corresponds to the Unix notion of a stream (not to be confused with racket/stream’s streams).
A Racket port represents a source or sink of data, such as a file, a terminal, a TCP connection, or an in-memory string. Ports provide sequential access in which data can be read or written a piece of a time, without requiring the data to be consumed or produced all at once. More specifically, an input port represents a source from which a program can read data, and an output port represents a sink to which a program can write data.