rust - How do I overcome match arms with incompatible types for structs implementing same trait?

Question

I am attempting to write the cat command to learn Rust, but I can't seem to convert command line arguments into reader structs.

use std::{env, io};
use std::fs::File;

fn main() {
    for arg in env::args().skip(1) {
        let reader = match arg.as_str() {
            "-" => io::stdin(),
            path => File::open(&path).unwrap(),
        };
    }
}

Error:

error[E0308]: match arms have incompatible types
 --> src/main.rs:6:22
  |
6 |         let reader = match arg.as_str() {
  |                      ^ expected struct `std::io::Stdin`, found struct `std::fs::File`
  |
  = note: expected type `std::io::Stdin`
  = note:    found type `std::fs::File`
note: match arm with an incompatible type
 --> src/main.rs:8:21
  |
8 |             path => File::open(&path).unwrap(),
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^

It doesn't seem like it's possible to polymorphically match trait implementers (related). How can I use either a File or Stdin as a reader?

Answer 1

The problem is that stdin() returns an object of type Stdio and File::open(...).unwrap() returns an object of type File. In Rust, all arms of a match have to return values of the same type.

In this case you probably wanted to return a common Read object. Unfortunately Read is a trait so you cannot pass it by value. The easiest alternative is to resort to heap allocation:

use std::{env, io};
use std::io::prelude::*;
use std::fs::File;

fn main() {
    for arg in env::args().skip(1) {
        let reader = match arg.as_str() {
            "-" => Box::new(io::stdin()) as Box<Read>,
            path => Box::new(File::open(&path).unwrap()) as Box<Read>,
        };
    }
}