looks ahead for next input character on the current input stream or on the input stream Stream.
A valid input text stream.
The resulting next input character available on the stream.
peek_char/[1,2] looks ahead of the next input character of the
specified input stream and unifies the character with Char.
The peeked character is still available for subsequent input on the
It is safe to call
times without actually inputting any character. For example:
| ?- peek_char(X), peek_char(X), get_char(X). |: a X = a
Stream errors (see ref-iou-sfh-est), plus: