use std::error::Error; use std::fs::File; use std::io::BufReader; use std::io::prelude::*; use std::path::Path; fn is_aeiou(x: &char) -> bool { "aeiou".chars().any(|y| y == *x) } fn is_weird_auo(x: &char) -> bool { "äüö".chars().any(|y| y == *x) } fn valid(line: &str) -> bool { line.chars().any(|c| is_aeiou(&c)) && line.chars().filter(is_weird_auo).fuse().nth(1).is_some() } fn main() { // Create a path to the desired file let path = Path::new("foo.txt"); let display = path.display(); // Open the path in read-only mode, returns `io::Result<File>` let file = match File::open(&path) { // The `description` method of `io::Error` returns a string that describes the error Err(why) => panic!("couldn't open {}: {}", display, Error::description(&why)), Ok(file) => file, }; let reader = BufReader::new(file); let lines = reader.lines(); let bad_line = lines.map(|l| l.unwrap()).filter(|line| !valid(line)).next(); match bad_line { Some(line_n) => println!("Line {} doesn't pass the test", line_n), None => println!("All lines are good!"), } // Alternate way if you don't need the line number. More readable //let all_good = lines.map(|l| l.unwrap()).all(valid); }