Spyke
rust·Rustbysilva

Allow argument in macro to be Option<T> or T

Hey,

Is there any way to create a macro that allows a Some<T> or T as input?

It's for creating a Span struct that I'm using:

struct Span {
    line: usize,
    column: usize,
    file_path: Option<String>,
}

...and I have the following macro:

macro_rules! span {
    ($line:expr, $column:expr) => {
        Span {
            line: $line,
            column: $column
            file_path: None,
        }
    };

    ($line:expr, $column:expr, $file_path: expr) => {
        Span {
            line: $line,
            column: $column
            file_path: Some($file_path.to_string()),
        }
    };
}

...which allows me to do this:

let foo = span!(1, 1);
let bar = span!(1, 1, "file.txt");

However, sometimes I don't want to pass in the file path directly but through a variable that is Option. To do this, I always have to match the variable:

let file_path = Some("file.txt");

let foo = match file_path {
    Some(file_path) => span!(1, 1, file_path),
    None => span!(1, 1),
}

Is there a way which allows me to directly use span!(1, 1, file_path) where file_path could be "file.txt", Some("file.txt") or None?

Thanks in advance!

View original on sopuli.xyz
28
Comments16
silvareply
sopuli.xyz

This does not work, as rust cannot infer the type of path

2
BB_Creply
programming.dev

A generic impl is impossible.

Imagine you want to turn a Into<String> to Some(val.into()) and Option<Into<String>> to val.map(Into::into).

Now, what if there is a type T where impl From <Option<T>> for String is implemented?
Then we would have a conflict.

If you only need this for &str and String, then you can add a wrapper type OptionStringWrapper(Option<String>) and implement From<T> for OptionStringWrapper for all concrete type cases you want to support, and go from there.

2
Miaoureply
jlai.lu

I think the point is that the variable itself is an Option. Your example only works for literal Option (although the value inside the optional itself might not be a literal).

One option to OP's problem is to use an auxiliary trait implemented on both string and Option

2
v9CYKjLeia10dZpz88iUreply
programming.dev

Two of your macro rules are not used 😉 (expand to see which ones).

The macro rules are all used. (Macros are matched from top to bottom by the declared match types. The ident/expressions can't match until after the more text based Option matching.)

let _foo = Span { line: 1, column: 1, file_path: None };
let _bar = Span { line: 1, column: 1, file_path: "file.txt".upgrade() };
let _baz = Span { line: 1, column: 1, file_path: Some("file.txt".to_string()) };
let _baz = Span { line: 1, column: 1, file_path: None };
let _baz = Span { line: 1, column: 1, file_path: borrowed.upgrade() };
let _baz = Span { line: 1, column: 1, file_path: owned.upgrade() };

This doesn’t support Option<&str>. If it did, we would lose literal None support 😉

I didn't make Option<&str> an option because the struct is for type Option<String>. It does support Option though.

impl OptionUpgrade for Option<String> {
    fn upgrade(self) -> Option<String> {
        self
    }
}

It looks like the following, and uses the last match case.

let opt: Option<String> = Some("text".into());
let opt = span!(1, 1, opt);

With macro expansion

let opt: Option<String> = Some("text".into());
let opt = Span { line: 1, column: 1, file_path: opt.upgrade() };

There's not anything stopping it from supporting Option<&str> though. This would be the implementation

impl OptionUpgrade for Option<&str> {
    fn upgrade(self) -> Option<String> {
        self.map(|v| v.into())
    }
}
1
BB_Creply
programming.dev

The macro rules are all used.

Oops. I was looking at it wrong.

I didn’t make Option<&str> an option because the struct is for type Option<String>.

Re-read the end of OP's requirements.

1
v9CYKjLeia10dZpz88iUreply
programming.dev

I made an edit.

There's not anything stopping it from supporting Option<&str> though. This would be the implementation

impl OptionUpgrade for Option<&str> {
    fn upgrade(self) -> Option<String> {
        self.map(|v| v.into())
    }
}

It's also possible to just make it generic over Option types

impl<A: ToString> OptionUpgrade for Option<A> {
    fn upgrade(self) -> Option<String> {
        self.map(|v| v.to_string())
    }
}
1
BB_Creply
programming.dev

Yes, but then the concrete type of None literals becomes unknown, which is what I was trying to point out.

1
tuna
discuss.tchncs.de

You might be okay with this:

macro_rules! span {
    ($line:expr, $column:expr) => {
        Span {
            line: $line,
            column: $column,
            file_path: None,
        }
    };
    ($line:expr, $column:expr, $file_path:literal) => {
        Span {
            line: $line,
            column: $column,
            file_path: Some($file_path.to_string()),
        }
    };
    ($line:expr, $column:expr, $file_path:expr) => {
        Span {
            line: $line,
            column: $column,
            file_path: $file_path,
        }
    };
}

Playground

However, sometimes I don't want to pass in the file path directly but through a variable that is Option<String>.

Essentially I took this to mean str literals will be auto wrapped in Some, but anything else is expected to be Option<String>

1

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