Macro from

macro_rules! from {
    (
      $(,)?
    ) => { ... };
    (
      $Arg1 : expr $(,)?
    ) => { ... };
    (
      $Arg1 : expr, $Arg2 : expr $(,)?
    ) => { ... };
    (
      $Arg1 : expr, $Arg2 : expr, $Arg3 : expr $(,)?
    ) => { ... };
    (
      $( $Rest : tt )+
    ) => { ... };
}

Reusing main crate.

Variadic constructor.

Implement traits From1 from tuple with fields and std::convert::From from tuple with fields to provide the interface to construct your structure with a different set of arguments. In this example structure, Struct1 could be constructed either without arguments, with a single argument, or with two arguments.

• Constructor without arguments fills fields with zero.
• Constructor with a single argument sets both fields to the value of the argument.
• Constructor with 2 arguments set individual values of each field.

  use variadic_from::prelude::*;

  #[ derive( Debug, PartialEq ) ]
  struct Struct1
  {
    a : i32,
    b : i32,
  }

  impl Default for Struct1
  {
    fn default() -> Self
    {
      Self { a : 0, b : 0 }
    }
  }

  impl From1< i32 > for Struct1
  {
    fn from1( val : i32 ) -> Self
    {
      Self { a : val, b : val }
    }
  }

  impl From2< i32, i32 > for Struct1
  {
    fn from2( val1 : i32, val2 : i32 ) -> Self
    {
      Self { a : val1, b : val2 }
    }
  }

  let got : Struct1 = from!();
  let exp = Struct1{ a : 0, b : 0 };
  assert_eq!( got, exp );

  let got : Struct1 = from!( 13 );
  let exp = Struct1{ a : 13, b : 13 };
  assert_eq!( got, exp );

  let got : Struct1 = from!( 1, 3 );
  let exp = Struct1{ a : 1, b : 3 };
  assert_eq!( got, exp );

To add to your project

cargo add type_constructor

Try out from the repository

git clone https://github.com/Wandalen/wTools
cd wTools
cd examples/type_constructor_trivial
cargo run