Here is a minimal working example. As @Stefan wrote, "I'd recommend splitting streamed input through some other means into chunks you can handle".
What somewhat works (and I'd be glad for suggestions on how to improve it), is to combine a File::bytes() method and then only take as many bytes as necessary and pass them to nom::streaming::take.
let reader = file.bytes();
let buf = reader.take(length).collect::<B>()?;
let (_input, chunk) = take(length)(&*buf)...;
The complete function can look like this:
/// Parse the first handful of bytes and return the bytes interpreted as UTF8
fn parse_first_bytes(file: std::fs::File, length: usize) -> Result<String> {
type B = std::result::Result<Vec<u8>, std::io::Error>;
let reader = file.bytes();
let buf = reader.take(length).collect::<B>()?;
let (_input, chunk) = take(length)(&*buf)
.finish()
.map_err(|nom::error::Error { input: _, code: _ }| eyre!("..."))?;
let s = String::from_utf8_lossy(chunk);
Ok(s.to_string())
}
Here is the rest of main for an implementation similar to Unix' head command.
use color_eyre::Result;
use eyre::eyre;
use nom::{bytes::streaming::take, Finish};
use std::{fs::File, io::Read, path::PathBuf};
use structopt::StructOpt;
#[derive(Debug, StructOpt)]
#[structopt(about = "A minimal example of parsing a file only partially.
This implements the POSIX 'head' utility.")]
struct Args {
/// Input File
#[structopt(parse(from_os_str))]
input: PathBuf,
/// Number of bytes to consume
#[structopt(short = "c", default_value = "32")]
num_bytes: usize,
}
fn main() -> Result<()> {
let args = Args::from_args();
let file = File::open(args.input)?;
let head = parse_first_bytes(file, args.num_bytes)?;
println!("{}", head);
Ok(())
}
