At first I wondered why he calls the monadic flatmap operation "for_each" but it makes sense in that he's actually creating a (lazy) list comprehension DSL here. The code within main() is mostly equivalent to the following Python code:

  triples = ((a, b, c) for c in itertools.count()
                       for a in range(1, c+1)
                       for b in range(a, c+1)
                       if a*a + b*b == c*c)

  for (a,b,c) in itertools.islice(triples, 10):
      print("{}, {}, {}".format(a,b,c))

The equivalent in Rust, just because I'm currently learning it:

    let triples = (1..)
        .flat_map(move |c| (1..=c)
            .flat_map(move |a| (a..=c)
                .flat_map(move |b| Some((a, b, c))
                    .filter(|_| a*a + b*b == c*c))));
    
    
    for (a,b,c) in triples.take(10) {
        println!("{}, {}, {}", a, b, c)
    }