Should tap allow the closure to return an ignorable value?

[hzqd]

I code this:

use std::io::stdin;
String::new().tap_mut(|s| stdin().read_line(s).unwrap())

Getting error message:

xx |     String::new().tap_mut(|s| stdin().read_line(s).unwrap())
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `usize`

It can be solved by adding brace and semicolon to the closure, but that doesn't seem ergonomic enough.

Do you think tap should allow the closure to return an ignorable value?

[tgnottingham]

I don't think it should, because it introduces a new class of potential bugs. The current design prevents the user from falling into the trap of thinking that tap returns the value that the closure returns.

Here's a contrived example where the user might believe that this would print "HELLO HELLO", but it would only print "HELLO".

use tap::prelude::*;

fn echoed(s: &str) -> String {
    s.to_string() + " " + s
}

fn main() {
    let echo = "hello".to_ascii_uppercase().tap(|s| echoed(s));
    println!("{echo}");
}