build-item.rs

// This is an example use of the extensible builder pattern in Context-Generic Programming (CGP)
// to progressively build a struct with missing fields filled with default values.
//
// This is in response to the question at https://www.reddit.com/r/rust/comments/1m4d8a8/how_to_handle_default_values_for_parameters/.
// More info at https://contextgeneric.dev/blog/extensible-datatypes-part-1/

use cgp::core::field::CanFinalizeWithDefault;
use cgp::prelude::*;

#[derive(Default)]
pub struct Uuid;

#[derive(Default)]
pub struct ItemType;

#[derive(Default)]
pub struct EquipmentType;

#[derive(Default)]
pub struct ItemQuantity(u64);

#[derive(BuildField)]
pub struct Item {
    pub id: Uuid,
    pub name: String,
    pub item_type: ItemType,
    pub equipment_type: EquipmentType,
    pub maximum_quantity: ItemQuantity,
}

#[test]
fn test_default_construction() {
    let _item = Item::builder()
        .build_field(PhantomData::<symbol!("name")>, "my-item".to_owned())
        .build_field(
            PhantomData::<symbol!("maximum_quantity")>,
            ItemQuantity(100),
        )
        .finalize_with_default();
}