From https://thesquareplanet.com/blog/the-path-to-rust/

rust_functional.rs

// From https://thesquareplanet.com/blog/the-path-to-rust/
// Example 1: functional
let idx = args 
  // iterate over our arguments 
  .iter() 
  // open each file 
  .map(|fname| (fname.as_str(), fs::File::open(fname.as_str()))) 
  // check for errors 
  .map(|(fname, f)| { f.and_then(|f| Ok((fname, f))) 
    .expect(&format!("input file {} could not be opened", fname)) 
  }) 
// make a buffered reader 
.map(|(fname, f)| (fname, io::BufReader::new(f))) 
  // for each file 
  .flat_map(|(f, file)| { 
    file 
    // read the lines 
    .lines() 
    // split into words 
    .flat_map(|line| { 
      line.unwrap().split_whitespace() 
      .map(|w| w.to_string()).collect::<Vec<_>>().into_iter() 
      // NOTE: the collect+into_iter here is icky 
      // have a look at the flat_map entry 
      // in the Appendix for why it's here 
    }) 
    // prune duplicates 
    .collect::<HashSet<_>>() 
      .into_iter() 
      // and emit inverted index entry 
      .map(move |word| (word, f)) 
    }) 
  .fold(HashMap::new(), |mut idx, (word, f)| { 
    // absorb all entries into a vector of file names per word 
    idx.entry(word) 
      .or_insert(Vec::new()) 
      .push(f); 
    idx 
});