rust : convert a writer function to an io::Read

piper.rs

extern crate pipe;

/**
Some libraries (such as handlebars or serde) offer functions that can generate data when given
an object that implements io::Write.

Some other libraries (such as Rocket) can consume data only from objects implementing io::Read.

Here is an example `piper` function that can be used to make these two kinds of libraries together.

Ultimately, the second kind of libraries should be changed to accept writable objects directly.
**/

use pipe::{pipe, PipeReader, PipeWriter};
use std::io::Result;
use std::thread;

/// Starts the given function in a thread and return a reader of the generated data
pub fn piper<F: Send + 'static>(writefn: F) -> PipeReader
    where F: FnOnce(PipeWriter) -> Result<()> {
    let (reader, writer) = pipe();
    thread::spawn(move || writefn(writer));
    reader
}

#[cfg(test)]
mod tests {
    use std::io::Read;
    use std::io::Write;
    use super::piper;

    #[test]
    fn it_works() {
        let mut reader = piper(|mut w| w.write_all(b"hello world"));
        let mut res = String::new();
        reader.read_to_string(&mut res).unwrap();
        assert_eq!("hello world", res);
    }

    #[test]
    fn infinite() {
        let reader = piper(|mut w| {
            loop { w.write_all(b"A")?; }
        });
        let mut res = String::new();
        reader.take(4).read_to_string(&mut res).unwrap();
        assert_eq!("AAAA", res);
    }
}