README

to compile:
valac -o sum --pkg vala-1.0 stdin_reader.vala sum_application.vala

stdin_reader.vala

using GLib;

namespace snuxoll {
    class StdinReader : Gee.Iterable<string>,
      Gee.CollectionObject<string> {

        public StdinReader () {

        }

        public bool eof() {
            return stdin.eof();
        }

        public Type get_element_type() {
            return typeof(string);
        }

        public Gee.Iterator<string> iterator() {
            return new Iterator<string>(this);
        }

        public string? read_line() {
            var line = new StringBuilder();
            var buffer = new char[1024];
            while ( ( !line.str.has_suffix ("\n") ) ) {
                string chunk = stdin.gets(buffer);
                if (chunk != null) {
                    line.append(chunk);
                } else {
                  return null;
                }
            }
            return line.str;
        }

        private class Iterator<T> : Gee.Iterator<T>,
          Gee.CollectionObject<T> {

            private StdinReader reader { get; set; }

            public Iterator(StdinReader _reader) {
                reader = _reader;
            }

            public bool next() {
                return reader.eof();
            }

            public T? get() {
                return reader.read_line();
            }

        }
    }
}

sum_application.vala

using GLib;
using snuxoll;

class SumApplication : Object {

    private uint64 total { get; set; }

    public SumApplication() {
        this.total = 0;
    }

    public void run() {
        var reader = new StdinReader();
        foreach(string line in reader) {
            var nums = line.strip().split(" ");
            foreach(string num in nums) {
                this.total += num.to_int();
            }
        }
        stdout.printf("Grand Total: %llu\n", total);
    }

    static int main(string[] args) {
        var app = new SumApplication();
        app.run();
        return 0;
    }

}