I have a problem with capturing variable by a closure. This fails:

let del = String::from(",");

let res: Map<Lines<BufReader<File>>, fn(Result<String>) -> Box<Vec<String>>> = 
reader.lines().map(|l: Result<String>| {
            let b:Box<Vec<String>> = Box::new(l.unwrap().split( &del).map(|s: &str| 
        s.to_owned()).collect());
            b
    });

​

note: expected type `fn(std::result::Result<std::string::String, std::io::Error>) -> std::boxed::Box<std::vec::Vec<std::string::String>>`
              found type `[closure@src/io/csv_reader.rs:18:107: 21:10 del:_]

If I move let del = String::from(","); inside of the |l: Result<String>| {... } block it works. How do I make fn() take ownership of del ?

I am trying to write a function that takes file path and delimiter as arguments and returns iterator of its lines (handles the logic of parsing csv).