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Module 0x2::borrow

A simple library that enables hot-potato-locked borrow mechanics.

With Programmable transactions, it is possible to borrow a value within a transaction, use it and put back in the end. Hot-potato Borrow makes sure the object is returned and was not swapped for another one.

use 0x1::option;
use 0x2::object;
use 0x2::tx_context;

Struct Referent​

An object wrapping a T and providing the borrow API.

struct Referent<T: store, key> has store
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Fields
id: address
value: option::Option<T>

Struct Borrow​

A hot potato making sure the object is put back once borrowed.

struct Borrow
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Fields
ref: address
obj: object::ID

Constants​

The Borrow does not match the Referent.

const EWrongBorrow: u64 = 0;

An attempt to swap the Referent.value with another object of the same type.

const EWrongValue: u64 = 1;

Function new​

Create a new Referent struct

public fun new<T: store, key>(value: T, ctx: &mut tx_context::TxContext): borrow::Referent<T>
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Implementation
public fun new<T: key + store>(value: T, ctx: &mut TxContext): Referent<T> {
    Referent {
        id: tx_context::fresh_object_address(ctx),
        value: option::some(value),
    }
}

Function borrow​

Borrow the T from the Referent receiving the T and a Borrow hot potato.

public fun borrow<T: store, key>(self: &mut borrow::Referent<T>): (T, borrow::Borrow)
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Implementation
public fun borrow<T: key + store>(self: &mut Referent<T>): (T, Borrow) {
    let value = self.value.extract();
    let id = object::id(&value);

    (
        value,
        Borrow {
            ref: self.id,
            obj: id,
        },
    )
}

Function put_back​

Put an object and the Borrow hot potato back.

public fun put_back<T: store, key>(self: &mut borrow::Referent<T>, value: T, borrow: borrow::Borrow)
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Implementation
public fun put_back<T: key + store>(self: &mut Referent<T>, value: T, borrow: Borrow) {
    let Borrow { ref, obj } = borrow;

    assert!(object::id(&value) == obj, EWrongValue);
    assert!(self.id == ref, EWrongBorrow);
    self.value.fill(value);
}

Function destroy​

Unpack the Referent struct and return the value.

public fun destroy<T: store, key>(self: borrow::Referent<T>): T
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Implementation
public fun destroy<T: key + store>(self: Referent<T>): T {
    let Referent { id: _, value } = self;
    value.destroy_some()
}