io

• readLine() — the next line of stdin without \n or \r\n, or nil at the end.
• readText() — the rest of stdin.
• write(v) — writes v to stdout as print does, without a newline.
• eprint(v), ewrite(v) — the same to stderr, with and without a newline.

Reading stdin or writing stderr first flushes what was printed.

A filter that sums the numbers it is given, one per line, and warns about the lines that are not numbers:

// sum.rn
let mut total = 0;
let mut line = io.readLine();
while line != nil {
    try {
        total += number(line);
    } catch e is ValueError {
        io.eprint("skipping '{line}'");
    }
    line = io.readLine();
}
io.write("total: ");
print(total);

$ printf '10\n20\nabc\n12\n' | rune run sum.rn
skipping 'abc'
total: 42

readText() takes all of stdin at once, for input that is not line by line:

// count.rn
let words = regex.split(r"\s+", io.readText().trim());
print("{words.len()} words");

$ echo "the quick brown fox" | rune run count.rn
4 words