peek_char
result = peek_char(f)
Input: f · Output: result
What it is
Looks at the next character of f (file handle, opened readable) without consuming it.
Overloads
One call shape — this function is not overloaded.
| Code | Input | Output | Description |
result = peek_char(f) | f stringnot documented yet | result string | Returns exactly what read_char(f) would return (a one-character string, or Nothing at EOF), cursor restored. |
Examples
From the documentation, and run by the build -- the output and the figure below are what the engine actually produced. Simplest first.
f = fopen("greeting.txt", "w")
write_char(f, "Hello,World")
close(f)
g = fopen("greeting.txt", "r")
first = read_char(g) # "H"
peeked = peek_char(g) # "e" -- cursor not moved
next = read_char(g) # "e" again, same char peeked returned
chunk = read_chars(g, 3) # "llo"
next_byte = peek_byte(g) # 44, the byte for ',' -- still not consumed
before_comma = read_until(g, ",") # "" -- the comma was right there, now consumed
w_byte = read_byte(g) # 87, the byte for 'W'
rest = read_all(g) # "orld"
close(g)
print("{first}{next}{chunk} peeked={peeked} next_byte={next_byte} before_comma=\"{before_comma}\" w_byte={w_byte} rest={rest}")
# Hello peeked=e next_byte=44 before_comma="" w_byte=87 rest=orld
output
Hello peeked=e next_byte=44 before_comma="" w_byte=87 rest=orld
from file-io
How it works
peek_line/peek_char/peek_byte (§ file I/O extensions, wrappers around read_line/read_char/read_byte; see fs_ops::call's own doc comment for the full design (why three distinct names rather than one generic peek(), and why this is implemented via a real seek round-trip rather than a buffered "un-read").
From the note above this builtin in the interpreter, where the reasoning was written down as it was built.