Vfrdtyky

I am refactoring a small stack based language I wrote and decided I would have a parser phase separated from the execution phase and as a result I would parse strings to a Token enum. I tried using strum to do this, here is the smallest example I could manage:

extern crate strum; // 0.11.0

#[macro_use]

extern crate strum_macros; // 0.11.0



#[derive(EnumString)]

enum Token {

    #[strum(default="true")]

    Number(f64)

}



fn main() {

}

Basically, what this should do is if nothing is matched, default to parsing a float. So, convert anything it is given to float in this case. Which gave:

error[E0277]: the trait bound `f64: std::convert::From<&str>` is not satisfied

 --> src/main.rs:5:10

  |

5 | #[derive(EnumString)]

  |          ^^^^^^^^^^ the trait `std::convert::From<&str>` is not implemented for `f64`

  |

  = help: the following implementations were found:

            <f64 as std::convert::From<u32>>

            <f64 as std::convert::From<i32>>

            <f64 as std::convert::From<f32>>

            <f64 as std::convert::From<i8>>

          and 3 others

  = note: required because of the requirements on the impl of `std::convert::Into<f64>` for `&str`

I also tried adding a impl for the conversion:

extern crate strum;

#[macro_use]

extern crate strum_macros;



impl<'a> From<&'a str> for f64 {

    #[inline]

    fn from(s: &'a str) -> Self {

        // See footnote [1] for why conversion is done this way.

        s.parse::<f64>().unwrap_or(0.0).to_owned();

    }

}



#[derive(EnumString)]

enum Token {

    #[strum(default="true")]

    Number(f64)

}



fn main() {

}

However, this gives:

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types

 --> src/main.rs:5:1

  |

5 | impl<'a> From<&'a str> for f64 {

  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate

  |

  = note: the impl does not reference any types defined in this crate

  = note: define and implement a trait or new type instead

Because I use strum I could not put this on Rust Playground, but the Cargo.toml is:

[package]

name = "strum_test"

version = "0.1.0"



[dependencies]

strum = "0.11.0"

strum_macros = "0.11.0"

And the file I edited was in strum_test/src/main.rs. Is there a way to handle this behavior in strum, or is there at least another graceful way of handling this conversion from &str to enum Token?

Based off of Sven Marnach's comment I added a struct Float(f64). Here is all the code for reference:

extern crate strum;

#[macro_use]

extern crate strum_macros;



struct Float(f64);



impl<'a> From<&'a str> for Float {

    #[inline]

    fn from(s: &'a str) -> Self {

        Float(s.parse::<f64>().unwrap_or(0.0).to_owned())

    }

}



#[derive(EnumString)]

enum Token {

    #[strum(default="true")]

    Number(Float)

}



fn main() {

}