Peeking at the next character without consuming it is useful when the interpretation of “this character” depends on what the next one is.
peek_byte([Stream,] N) unifies N with the next unconsumed
byte from the current or given input stream, which must be binary.
peek_code([Stream,] N) unifies N with the next unconsumed
character code from the current or given input stream, which must be text.
peek_char([Stream,] A) unifies A with the next unconsumed
character atom from the current or given input stream, which must be text.