4.6.2 About Streams

A Prolog stream can refer to a file or to the user’s terminal⁵. Each stream is used either for input or for output, but typically not for both. A stream is either text, for character and term I/O, or binary, for byte I/O. At any one time there is a current input stream and a current output stream.

Input and output predicates fall into two categories:

1. those that use the current input or output stream;
2. those that take an explicit stream argument;

Initially, the current input and output streams both refer to the user’s terminal. Each input and output built-in predicate refers implicitly or explicitly to a stream. The predicates that perform byte, character and term I/O operations come in pairs such that (A) refers to the current stream, and (B) specifies a stream.

     predicate_name/n  (A)
     predicate_name/n+1  (B)