Std :: fs :: File + documentation + try it! = E0308
I want to create an empty file, so I took an example and I used nightly build today
use std::fs::File;
use std::io::prelude::*;
fn main() {
let mut f = try!(File::create("foo"));
}
Startup rustc
has errors:
<std macros>:5:8: 6:42 error: mismatched types:
expected `()`,
found `core::result::Result<_, _>`
(expected (),
found enum `core::result::Result`) [E0308]
<std macros>:5 return $ crate:: result:: Result:: Err (
<std macros>:6 $ crate:: convert:: From:: from ( err ) ) } } )
<std macros>:1:1: 6:48 note: in expansion of try!
file_io.rs:5:17: 5:42 note: expansion site
<std macros>:5:8: 6:42 help: pass `--explain E0308` to see a detailed explanation
error: aborting due to previous error
If I uninstall try!
it compiles, but how should I handle errors? And why isn't the example compiled like?
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try!
is a macro to be used in functions that return Result
. Therefore it cannot be used in a function main
, because it returns one (empty tuple).
See how it expands:
fn main() {
let mut f = match File::create("foo") {
Ok(val) => val,
Err(err) => return Err(From::from(err)),
};
}
http://blog.burntsushi.net/rust-error-handling/ - good article on error handling in Rust for simple scripts like yours, using Result::unwrap
( File::create("foo").unwrap()
) is probably sane.
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Let's look at the macro try!()
:
macro_rules! try {
($expr:expr) => (match $expr {
$crate::result::Result::Ok(val) => val,
$crate::result::Result::Err(err) => {
return $crate::result::Result::Err($crate::convert::From::from(err))
}
})
}
As you can see, either try!()
gives val
as an expression or returns Err
from a function. Therefore, after processing the macro, the expanded code looks like this:
fn main() {
let mut f = (match File::create("foo") {
Ok(val) => val,
Err(err) => {
return Err(...)
}
})
}
Hopefully the error is now obvious: main()
should return ()
, but you return Err
(which is of type Result<File>
).
The moral of the story is that you are using try!()
the wrong script. It should only be used in a function that is already intended to return Result
, just as you would throw (obscure) an exception in C ++ or Java. In your case, however, you need to explicitly refer to the error - there are no bubbles there. A possible solution, although not very elegant, is to use .unwrap()
the program to crash in case Err
.
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