[rust-dev] Rust Guide 22 and 23

[rust-dev] Rust Guide 22 and 23

[Péter Mózes Merl]

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        Hi Everyone,

after long years of working with scripting languages I have finally
found a strongly-typed language that is worth to learn (for me). Cheers.

The Guide is awesome. There is only one thing I missed and that’s
between chapters 22 and 23, Generics and Traits. The Guide says that we
have to learn about Traits to fix this message:

error: binary operation `==` cannot be applied to type `T`

I think the example how to fix it at the end is missing. I am not sure
whether this is the right place to provide such a feedback, however, I
could not find a link in the Guide.

Thank you.

        Péter

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Re: [rust-dev] Rust Guide 22 and 23

[Nathan Sizemore]

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I believe that was just showing an example of the error the compiler will
generate when not using trait constraints.  If you read a little further, a
similar error is produced from

fn print_area<T>(shape: T) {
    println!("This shape has an area of {}", shape.area());
}

Because T can be any type, we can't be sure that it implements the area method.
But we can add a *trait constraint* to our generic T, ensuring that it does:

fn print_area<T: HasArea>(shape: T) {
    println!("This shape has an area of {}", shape.area());
}


If you're wanting to do comparisons, you will probably want to place a
trait constraint on your functions from the following module:
http://doc.rust-lang.org/std/cmp/index.html#traits


Nathan Sizemore
@nathansizemore | 937.823.7229


On Fri, Dec 5, 2014 at 6:33 AM, Péter Mózes Merl <mage@mage.li> wrote:

>          Hi Everyone,
>
> after long years of working with scripting languages I have finally found
> a strongly-typed language that is worth to learn (for me). Cheers.
>
> The Guide is awesome. There is only one thing I missed and that’s between
> chapters 22 and 23, Generics and Traits. The Guide says that we have to
> learn about Traits to fix this message:
>
> error: binary operation `==` cannot be applied to type `T`
>
> I think the example how to fix it at the end is missing. I am not sure
> whether this is the right place to provide such a feedback, however, I
> could not find a link in the Guide.
>
> Thank you.
>
>         Péter
>
> _______________________________________________
> Rust-dev mailing list
> Rust-dev@mozilla.org
> https://mail.mozilla.org/listinfo/rust-dev
>
>

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Re: [rust-dev] Rust Guide 22 and 23

[Péter Mózes Merl]

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Am 05/12/14 um 14:15 schrieb Nathan Sizemore:
>
> If you're wanting to do comparisons, you will probably want to place a
> trait constraint on your functions from the following
> module: http://doc.rust-lang.org/std/cmp/index.html#traits
>
I tried to write the function (my second Rust code ever). I thought that
returning T makes no sense since the inverse of an integer should be a
float.

Is this the right way?

use std::num;

fn main() {
    fn inverse<T: num::NumCast>(x: T) -> Result<f64, String> {
        let local: f64 = num::cast(x).unwrap();
        if 0f64 == local { return Err("x cannot be zero!".to_string()); }
        Ok(1f64 / local)
    }

    match inverse(5.2f32) {
        Ok(n) => println!("{}", n),
        Err(s) => println!("{}", s)
    }

    match inverse(4i) {
        Ok(n) => println!("{}", n),
        Err(s) => println!("{}", s)
    }
}




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Re: [rust-dev] Rust Guide 22 and 23

[Kai Noda]

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Hi Peter,

I would do in this way:

use std::fmt::Show;
use std::num::{mod, NumCast};

fn inverse<T: NumCast>(x: T) -> Result<f64, String> {
    let local = num::cast(x).expect("fail to cast into f64");
    if 0. == local { Err("x cannot be zero!".to_string()); }
    Ok(1. / local)
}

fn dotest<T: NumCast + Show + Clone> (x: T) {
    println!("1/{} => {}", x.clone(), inverse(x));
}

fn main() {
    dotest(5.2f32);
    dotest(4i32);
    dotest(0i);
}

(Disclaimer: it's only three months since I started to learn Rust!)

There is an online compiler, though it's one month old and details of
the language and the stdlib somewhat differ from those of the nightly
version:
http://is.gd/13zBYc (Hit the [evaluate] button)
I think Reddit, StackOverflow and IRC (there's a web interface) are more
active than this list.

Hope this helps.

Kai

野田  開 <nodakai@gmail.com>

2014-12-05 22:52 GMT+08:00 Péter Mózes Merl <mage@mage.li>:

>  Am 05/12/14 um 14:15 schrieb Nathan Sizemore:
>
>
> If you're wanting to do comparisons, you will probably want to place a
> trait constraint on your functions from the following module:
> http://doc.rust-lang.org/std/cmp/index.html#traits
>
>  I tried to write the function (my second Rust code ever). I thought that
> returning T makes no sense since the inverse of an integer should be a
> float.
>
> Is this the right way?
>
> use std::num;fn main() {
>     fn inverse<T: num::NumCast>(x: T) -> Result<f64, String> {
>         let local: f64 = num::cast(x).unwrap();        if 0f64 == local { return Err("x cannot be zero!".to_string()); }
>         Ok(1f64 / local)
>     }
>
>     match inverse(5.2f32) {
>         Ok(n) => println!("{}", n),        Err(s) => println!("{}", s)
>     }
>
>     match inverse(4i) {
>         Ok(n) => println!("{}", n),        Err(s) => println!("{}", s)
>     }
> }
>
>
>
>
> _______________________________________________
> Rust-dev mailing list
> Rust-dev@mozilla.org
> https://mail.mozilla.org/listinfo/rust-dev
>
>

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