How to pass BufReader to another function in rust?

Question

new to rust, I'm asking this to hopefully understand how to work with the language better..

Heres me trying to read a file, pass the file handle to a different function and print the lines from there:

use std::fs::File;
use std::io::{self, BufReader};
use std::io::BufRead;
use async_std::task;

async fn scrape<R: BufRead>(reader: &mut R) {
    reader.lines().for_each(|line| println!(line));
}

fn main() -> io::Result<()> {
    let file = File::open("wlist.txt")?;
    let reader = BufReader::new(file);

    // print_type_of(&reader); // prints std::io::buffered::BufReader<std::fs::File>
    task::block_on(scrape(&mut reader));

    Ok(())
}

// helper functions
fn print_type_of<T>(_: &T) {
    println!("{}", std::any::type_name::<T>())
}

Would love some insights regarding what concepts i'm missing and how this should be done properly !

It's also frequently easier for optimizer to speed up an iterator consumption in comparison to `for` loops, so you better avoid loops unless it greatly eases your code. You can `reader.lines().for_each(|line| println!(line));` — Alexey S. Larionov, May 30 '20 at 17:25
also `reader.lines()` returns an iterator over `io::Result`, not just strings. So you have to catch IO errors in your function — Alexey S. Larionov, May 30 '20 at 17:29
thank you for the tips! still waiting for more help with this since i can't get it to work — toti, May 30 '20 at 17:44
What's the problem? The error I get is ``error: format argument must be a string literal`` which doesn't seem to have anything to do with `async` or `BufReader`. — trent, May 30 '20 at 18:05
[I found this question by searching for the error message.](https://stackoverflow.com/questions/27734708/println-error-expected-a-literal-format-argument-must-be-a-string-literal) — trent, May 30 '20 at 18:07

score 3 · Accepted Answer · answered May 30 '20 at 18:24

This version atleast compiles, but there's no async in the playground so I coudn't add async.

You have to catch errors in scrape by using try_for_each function, inside you parse io::Result<String> via and_then. In your main you have to catch all errors explicitly, not allowed to let io::Error slip through ? operator

use std::fs::File;
use std::io::{self, BufReader};
use std::io::BufRead;
//use async_std::task;

fn scrape<R: BufRead>(reader: &mut R) -> io::Result<()> {
    reader.lines()
        .try_for_each(|lineResult| lineResult.and_then(|line| Ok(println!("{}", line))))?;
    Ok(())
}

fn main() {
    //let fileResult = File::open("wlist.txt");
    //let mut file;
    //match fileResult {
    //    Ok(f) => file = f,
    //    Err(e) => println!("File open error! {}", e),
    //}
    let mut file = b"hello\nworld\nhow\nare\nyou\ndoing" as &[u8]; // no file in playground, so just a buffer
    let mut reader = BufReader::new(file);

    // print_type_of(&reader); // prints std::io::buffered::BufReader<std::fs::File>
    match scrape(&mut reader) {
        Ok(_) => println!("Scraping is done!"),
        Err(e) => println!("File read error! {}", e),
    }
}

// helper functions
fn print_type_of<T>(_: &T) {
    println!("{}", std::any::type_name::<T>())
}

This is super informative and helpful, currently messing around with the code and already learning so THANKS. only error left is when I use your uncommented version of the file from text file instead of binary - it errors on the `BufReader::new(file);` line with this `error: use of possibly-uninitialized file` — toti, May 30 '20 at 18:44
You can change to `Err(e) => { println!("File open error! {}", e); return; },` in first `match` — Alexey S. Larionov, May 30 '20 at 19:02

How to pass BufReader to another function in rust?

1 Answers1