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unget(+Stream)

Back up one character on Stream
+Stream
Integer (stream number) or Atom (reserved or user-defined symbolic stream name).

Description

Go back one character on the given Stream. This can be used to implement lookaheads. The number of characters that can be backed up is guaranteed to be at least 4.

Fail Conditions

None

Resatisfiable

No.

Exceptions

(4) instantiation fault
Stream is not instantiated.
(5) type error
Stream is neither an integer nor an atom.
(6) out of range
No further backup possible on this stream.

Examples

% look ahead one character in Stream:

    peek(Stream, X) :-
    	get(Stream, X),
	unget(Stream).

See Also

get / 2, get_char / 2