Macro zng::var::impl_from_and_into_var

macro_rules! impl_from_and_into_var {
    ($($tt:tt)+) => { ... };
}

Implements T: IntoVar<U>, T: IntoValue<U> and optionally U: From<T> without boilerplate.

The macro syntax is one or more functions with signature fn from(t: T) -> U. The LocalVar<U> type is selected for variables. The syntax also supports generic types and constraints, but not where constraints. You can also destructure the input if it is a tuple using the pattern fn from((a, b): (A, B)) -> U, but no other pattern matching in the input is supported.

The U: From<T> implement is optional, you can use the syntax fn from(t: T) -> U; to only generate the T: IntoVar<U> and T: IntoValue<U> implementations using an already implemented U: From<T>.

Examples

The example declares an enum that represents the values possible in a property foo and then implements conversions from literals the user may want to type in a widget:

#[derive(Debug, Clone, PartialEq)]
pub enum FooValue {
    On,
    Off,
    NotSet
}

impl_from_and_into_var! {
    fn from(b: bool) -> FooValue {
        if b {
            FooValue::On
        } else {
            FooValue::Off
        }
    }

    fn from(s: &str) -> FooValue {
        match s {
            "on" => FooValue::On,
            "off" => FooValue::Off,
            _ => FooValue::NotSet
        }
    }

    fn from(f: Foo) -> FooValue;
}

impl From<Foo> for FooValue {
    fn from(foo: Foo) -> Self {
        Self::On
    }
}///