How to best *fake* keyword style function arguments in Rust?

Question

I'm interested to have something functionally similar to keyword arguments in Rust, where they're currently not supported.

For languages that provide keyword argument, something like this is common:

panel.button(label="Some Button")
panel.button(label="Test", align=Center, icon=CIRCLE)

I've seen this handled using the builder-pattern, eg:

ui::Button::new().label("Some Button").build(panel)
ui::Button::new().label("Test").align(Center).icon(CIRCLE).build(panel)

Which is fine but at times a little awkward compared with keyword arguments in Python.

---

However using struct initialization with impl Default and Option<..> members in Rust could be used to get something very close to something which is in practice similar to writing keyword arguments, eg:

ui::button(ButtonArgs { label: "Some Button".to_string(), .. Default::default() } );

ui::button(ButtonArgs {
    label: "Test".to_string(),
    align: Some(Center),
    icon: Some(Circle),
    .. Default::default()
});

This works, but has some down-sides in the context of attempting to use as keyword args:

• Having to prefix the arguments with the name of the struct
  (also needing to explicitly include it in the namespace adds some overhead).
• Putting Some(..) around every optional argument is annoying/verbose.
• .. Default::default() at the end of every use is a little tedious.

Are there ways to reduce some of these issues, (using macros for example) to make this work more easily as a replacement for keyword access?

Answer 1

Disclaimer: I advise against using this solution, because the errors reported are horrid. The cleanest solution, codewise, is most probably the builder pattern.

---

With that out of the way... I whipped together a proof-of-concept demonstrating operator abuse.

Its main advantage over using struct syntax to pass arguments, or using a builder, is that it allows reuse across functions taking different sets of the same parameters.

On the other hand, it does suffer from having to import a whole lot of symbols (each name to be used).

It looks like:

//  Rust doesn't allow overloading `=`, so I picked `<<`.
fn main() {
    let p = Panel;
    p.button(LABEL << "Hello", ALIGNMENT << Alignment::Center);

    p.button(LABEL << "Hello", Alignment::Left);
    p.button(Label::new("Hello"), Alignment::Left);
}

Note that the name is really optional, it merely servers as a builder for the argument itself, but if you already have the argument it can be eschewed. This also means that it's probably not worth creating a name for "obvious" parameters (Alignment here).

The normal definition of button:

#[derive(Debug)]
struct Label(&'static str);

#[derive(Debug)]
enum Alignment { Left, Center, Right }

struct Panel;

impl Panel {
    fn button(&self, label: Label, align: Alignment) {
        println!("{:?} {:?}", label, align)
    }
}

Requires some augmentation:

impl Carrier for Label {
    type Item = &'static str;
    fn new(item: &'static str) -> Self { Label(item) }
}

impl Carrier for Alignment {
    type Item = Alignment;
    fn new(item: Alignment) -> Self { item }
}

const LABEL: &'static Argument<Label> = &Argument { _marker: PhantomData };
const ALIGNMENT: &'static Argument<Alignment> = &Argument { _marker: PhantomData };

And yes, this does mean that you can augment a function/method defined in a 3rd party library.

This is supported by:

trait Carrier {
    type Item;
    fn new(item: Self::Item) -> Self;
}

struct Argument<C: Carrier> {
    _marker: PhantomData<*const C>,
}

impl<C: Carrier> Argument<C> {
    fn create<I>(&self, item: I) -> C
        where I: Into<<C as Carrier>::Item>
    {
        <C as Carrier>::new(item.into())
    }
}

impl<R, C> std::ops::Shl<R> for &'static Argument<C>
    where R: Into<<C as Carrier>::Item>,
          C: Carrier
{
    type Output = C;
    fn shl(self, rhs: R) -> C {
        self.create(rhs)
    }
}

Note that this does NOT address:

• out of order argument passing
• optional arguments

---

If a user is patient enough to enumerate all combinations of optional parameters, a solution like @ljedrz is possible:

struct ButtonArgs {
    label: Label,
    align: Alignment,
    icon: Icon,
}

impl From<Label> for ButtonArgs {
    fn from(t: Label) -> ButtonArgs {
        ButtonArgs { label: t, align: Alignment::Center, icon: Icon::Circle }
    }
}

impl From<(Label, Alignment)> for ButtonArgs {
    fn from(t: (Label, Alignment)) -> ButtonArgs {
        ButtonArgs { label: t.0, align: t.1, icon: Icon::Circle }
    }
}

impl From<(Label, Icon)> for ButtonArgs {
    fn from(t: (Label, Icon)) -> ButtonArgs {
        ButtonArgs { label: t.0, align: Alignment::Center, icon: t.1 }
    }
}

impl From<(Label, Alignment, Icon)> for ButtonArgs {
    fn from(t: (Label, Alignment, Icon)) -> ButtonArgs {
        ButtonArgs { label: t.0, align: t.1, icon: t.2 }
    }
}

impl From<(Label, Icon, Alignment)> for ButtonArgs {
    fn from(t: (Label, Icon, Alignment)) -> ButtonArgs {
        ButtonArgs { label: t.0, align: t.2, icon: t.1 }
    }
}

will then allow all of the following combinations:

fn main() {
    let p = Panel;
    p.button( LABEL << "Hello" );
    p.button((LABEL << "Hello"));
    p.button((LABEL << "Hello", ALIGNMENT << Alignment::Left));
    p.button((LABEL << "Hello", ICON << Icon::Circle));
    p.button((LABEL << "Hello", ALIGNMENT << Alignment::Left, ICON << Icon::Circle));
    p.button((LABEL << "Hello", ICON << Icon::Circle, ALIGNMENT << Alignment::Left));

    p.button(Label::new("Hello"));
    p.button((LABEL << "Hello", Alignment::Left, Icon::Circle));
}

The extra set of parentheses is necessary when there is more than one argument.

However there is big downside: the user experience is degraded when using the wrong set of parameters.

The result of calling p.button("Hello"); is:

error[E0277]: the trait bound `ButtonArgs: std::convert::From<&str>` is not satisfied    --> <anon>:124:7
    | 124 |     p.button("Hello");
    |       ^^^^^^ the trait `std::convert::From<&str>` is not implemented for `ButtonArgs`
    |
    = help: the following implementations were found:
    = help:   <ButtonArgs as std::convert::From<Label>>
    = help:   <ButtonArgs as std::convert::From<(Label, Alignment)>>
    = help:   <ButtonArgs as std::convert::From<(Label, Icon)>>
    = help:   <ButtonArgs as std::convert::From<(Label, Alignment, Icon)>>
    = help: and 1 others
    = note: required because of the requirements on the impl of `std::convert::Into<ButtonArgs>` for `&str`