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Documentation for balances module (#1943)
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* comment updates

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* Update srml/balances/src/lib.rs

Co-Authored-By: joepetrowski <25483142+joepetrowski@users.noreply.github.com>

* Update srml/balances/src/lib.rs

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* Update srml/balances/src/lib.rs

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joepetrowski authored and gavofyork committed Mar 26, 2019
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214 changes: 184 additions & 30 deletions srml/balances/src/lib.rs
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Expand Up @@ -14,13 +14,158 @@
// You should have received a copy of the GNU General Public License
// along with Substrate. If not, see <http://www.gnu.org/licenses/>.

//! Balances: Handles setting and retrieval of free balance,
//! retrieving total balance, reserve and unreserve balance,
//! repatriating a reserved balance to a beneficiary account that exists,
//! transfering a balance between accounts (when not reserved),
//! slashing an account balance, account removal, rewards,
//! lookup of an index to reclaim an account (when not balance not reserved),
//! increasing total stake.
//! # Balances Module
//!
//! The balances module provides functionality for handling accounts and balances. To use the balances module, you need
//! to implement the [balances Trait](https://crates.parity.io/srml_balances/trait.Trait.html). Supported dispatchables
//! are documented in the [`Call` enum](https://crates.parity.io/srml_balances/enum.Call.html).
//!
//! ## Overview
//!
//! The balances module provides functions for:
//!
//! - Getting and setting free balances
//! - Retrieving total, reserved and unreserved balances
//! - Repatriating a reserved balance to a beneficiary account that exists
//! - Transferring a balance between accounts (when not reserved)
//! - Slashing an account balance
//! - Account creation and removal
//! - Lookup of an index to reclaim an account
//! - Managing total issuance
//! - Setting and managing locks
//!
//! ### Terminology
//!
//! - **Existential Deposit:** The minimum balance required to create or keep an account open. This prevents
//! "dust accounts" from filling storage.
//! - **Total Issuance:** The total amount of units in existence in a system.
//! - **Reaping an account:** The act of removing an account by resetting its nonce. Happens after its balance is set
//! to zero.
//! - **Free Balance:** The portion of a balance that is not reserved. The free balance is the only balance that matters
//! for most operations. When this balance falls below the existential deposit, most functionality of the account is
//! removed. When both it and the reserved balance are deleted, then the account is said to be dead.
//! - **Reserved Balance:** Reserved balance still belongs to the account holder, but is suspended. Reserved balance
//! can still be slashed, but only after all of free balance has been slashed. If the reserved balance falls below the
//! existential deposit then it and any related functionality will be deleted. When both it and the free balance are
//! deleted, then the account is said to be dead.
//! - **Imbalance:** A condition when some funds were created or deducted without equal and opposite accounting.
//! Functions that result in an imbalance will return an object of the `Imbalance` trait that must be handled.
//! - **Lock:** A freeze on a specified amount of an account's free balance until a specified block number. Multiple
//! locks always operate over the same funds, so they "overlay" rather than "stack".
//! - **Vesting:** Similar to a lock, this is another, but independent, liquidity restriction that reduces linearly
//! over time.
//!
//! ### Implementations
//!
//! The balances module provides implementations for the following traits. If these traits provide the functionality
//! that you need, then you can avoid coupling with the balances module.
//!
//! - [`Currency`](https://crates.parity.io/srml_support/traits/trait.Currency.html): Functions for dealing with a
//! fungible assets system.
//! - [`LockableCurrency`](https://crates.parity.io/srml_support/traits/trait.LockableCurrency.html): Functions for
//! dealing with accounts that allow liquidity restrictions.
//! - [`Imbalance`](https://crates.parity.io/srml_support/traits/trait.Imbalance.html): Functions for handling
//! imbalances between total issuance in the system and account balances. Must be used when a function
//! creates new funds (e.g. a reward) or destroys some funds (e.g. a system fee).
//! - [`MakePayent`](https://crates.parity.io/srml_support/traits/trait.MakePayment.html): Simple trait designed
//! for hooking into a transaction payment.
//! - [`IsDeadAccount`](https://crates.parity.io/srml_system/trait.IsDeadAccount.html): Determiner to say whether a
//! given account is unused.
//!
//! Example of using the `Currency` trait from the treasury module:
//!
//! ```rust,ignore
//! pub trait Trait: system::Trait {
//! /// The staking balance.
//! type Currency: Currency<Self::AccountId>;
//! }
//! ```
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! The `Call` enum is documented [here](https://crates.parity.io/srml_balances/enum.Call.html).
//!
//! - `transfer` - Transfer some liquid free balance to another account.
//! - `set_balance` - Set the balances of a given account. The origin of this call must be root.
//!
//! ### Public Functions
//!
//! See the [module](https://crates.parity.io/srml_balances/struct.Module.html) for details on publicly available
//! functions.
//!
//! ## Usage
//!
//! The following examples show how to use the balances module in your custom module.
//!
//! ### Import and Balance Transfer
//!
//! Import the `balances` module and derive your module configuration trait with the balances trait. You can now call
//! functions from the module.
//!
//! ```rust,ignore
//! use support::{decl_module, dispatch::Result};
//! use system::ensure_signed;
//!
//! pub trait Trait: balances::Trait {}
//!
//! decl_module! {
//! pub struct Module<T: Trait> for enum Call where origin: T::Origin {
//! fn transfer_proxy(origin, to: T::AccountId, value: T::Balance) -> Result {
//! let sender = ensure_signed(origin)?;
//! <balances::Module<T>>::make_transfer(&sender, &to, value)?;
//!
//! Ok(())
//! }
//! }
//! }
//! ```
//!
//! ### Real Use Example
//!
//! Use in the `contract` module (gas.rs):
//!
//! ```rust,ignore
//! pub fn refund_unused_gas<T: Trait>(
//! transactor: &T::AccountId,
//! gas_meter: GasMeter<T>,
//! imbalance: balances::NegativeImbalance<T>,
//! ) {
//! let gas_spent = gas_meter.spent();
//! let gas_left = gas_meter.gas_left();
//!
//! // Increase total spent gas.
//! <GasSpent<T>>::mutate(|block_gas_spent| *block_gas_spent += gas_spent);
//!
//! let refund = <T::Gas as As<T::Balance>>::as_(gas_left) * gas_meter.gas_price;
//! // Refund gas using balances module
//! let refund_imbalance = <balances::Module<T>>::deposit_creating(transactor, refund);
//! // Handle imbalance
//! if let Ok(imbalance) = imbalance.offset(refund_imbalance) {
//! T::GasPayment::on_unbalanced(imbalance);
//! }
//! }
//! ```
//!
//! ## Genesis config
//!
//! The following storage items depend on the genesis config:
//!
//! - `TotalIssuance`
//! - `ExistentialDeposit`
//! - `TransferFee`
//! - `CreationFee`
//! - `Vesting`
//! - `FreeBalance`
//! - `TransactionBaseFee`
//! - `TransactionByteFee`
//!
//! ## Related Modules
//!
//! The balances module depends on the [`system`](https://crates.parity.io/srml_system/index.html) and
//! [`srml_support`](https://crates.parity.io/srml_support/index.html) modules as well as Substrate Core
//! libraries and the Rust standard library.

#![cfg_attr(not(feature = "std"), no_std)]

Expand All @@ -47,7 +192,7 @@ pub trait Subtrait<I: Instance = DefaultInstance>: system::Trait {
/// The balance of an account.
type Balance: Parameter + Member + SimpleArithmetic + Codec + Default + Copy + As<usize> + As<u64> + MaybeSerializeDebug;

/// A function which is invoked when the free-balance has fallen below the existential deposit and
/// A function that is invoked when the free-balance has fallen below the existential deposit and
/// has been reduced to zero.
///
/// Gives a chance to clean up resources associated with the given account.
Expand All @@ -61,7 +206,7 @@ pub trait Trait<I: Instance = DefaultInstance>: system::Trait {
/// The balance of an account.
type Balance: Parameter + Member + SimpleArithmetic + Codec + Default + Copy + As<usize> + As<u64> + MaybeSerializeDebug;

/// A function which is invoked when the free-balance has fallen below the existential deposit and
/// A function that is invoked when the free-balance has fallen below the existential deposit and
/// has been reduced to zero.
///
/// Gives a chance to clean up resources associated with the given account.
Expand Down Expand Up @@ -136,15 +281,15 @@ pub struct BalanceLock<Balance, BlockNumber> {

decl_storage! {
trait Store for Module<T: Trait<I>, I: Instance=DefaultInstance> as Balances {
/// The total amount of stake on the system.
/// The total units issued in the system.
pub TotalIssuance get(total_issuance) build(|config: &GenesisConfig<T, I>| {
config.balances.iter().fold(Zero::zero(), |acc: T::Balance, &(_, n)| acc + n)
}): T::Balance;
/// The minimum amount allowed to keep an account open.
/// The minimum amount required to keep an account open.
pub ExistentialDeposit get(existential_deposit) config(): T::Balance;
/// The fee required to make a transfer.
pub TransferFee get(transfer_fee) config(): T::Balance;
/// The fee required to create an account. At least as big as ReclaimRebate.
/// The fee required to create an account.
pub CreationFee get(creation_fee) config(): T::Balance;
/// The fee to be paid for making a transaction; the base.
pub TransactionBaseFee get(transaction_base_fee) config(): T::Balance;
Expand Down Expand Up @@ -175,11 +320,11 @@ decl_storage! {

/// The 'free' balance of a given account.
///
/// This is the only balance that matters in terms of most operations on tokens. It is
/// alone used to determine the balance when in the contract execution environment. When this
/// This is the only balance that matters in terms of most operations on tokens. It
/// alone is used to determine the balance when in the contract execution environment. When this
/// balance falls below the value of `ExistentialDeposit`, then the 'current account' is
/// deleted: specifically `FreeBalance`. Furthermore, `OnFreeBalanceZero` callback
/// is invoked, giving a chance to external modules to cleanup data associated with
/// deleted: specifically `FreeBalance`. Further, the `OnFreeBalanceZero` callback
/// is invoked, giving a chance to external modules to clean up data associated with
/// the deleted account.
///
/// `system::AccountNonce` is also deleted if `ReservedBalance` is also zero (it also gets
Expand All @@ -190,14 +335,13 @@ decl_storage! {
/// slashed, but gets slashed last of all.
///
/// This balance is a 'reserve' balance that other subsystems use in order to set aside tokens
/// that are still 'owned' by the account holder, but which are suspendable. (This is different
/// and wholly unrelated to the `Bondage` system used in the staking module.)
/// that are still 'owned' by the account holder, but which are suspendable.
///
/// When this balance falls below the value of `ExistentialDeposit`, then this 'reserve account'
/// is deleted: specifically, `ReservedBalance`.
///
/// `system::AccountNonce` is also deleted if `FreeBalance` is also zero (it also gets
/// collapsed to zero if it ever becomes less than `ExistentialDeposit`.
/// collapsed to zero if it ever becomes less than `ExistentialDeposit`.)
pub ReservedBalance get(reserved_balance): map T::AccountId => T::Balance;

/// Any liquidity locks on some account balances.
Expand All @@ -214,7 +358,14 @@ decl_module! {
pub struct Module<T: Trait<I>, I: Instance = DefaultInstance> for enum Call where origin: T::Origin {
fn deposit_event<T, I>() = default;

/// Transfer some liquid free balance to another staker.
/// Transfer some liquid free balance to another account.
///
/// `transfer` will set the `FreeBalance` of the sender and receiver.
/// It will decrease the total issuance of the system by the `TransferFee`.
/// If the sender's account is below the existential deposit as a result
/// of the transfer, the account will be reaped.
///
/// The dispatch origin for this call must be `Signed` by the transactor.
pub fn transfer(
origin,
dest: <T::Lookup as StaticLookup>::Source,
Expand All @@ -226,6 +377,13 @@ decl_module! {
}

/// Set the balances of a given account.
///
/// This will alter `FreeBalance` and `ReservedBalance` in storage.
/// If the new free or reserved balance is below the existential deposit,
/// it will also decrease the total issuance of the system (`TotalIssuance`)
/// and reset the account nonce (`system::AccountNonce`).
///
/// The dispatch origin for this call is `root`.
fn set_balance(
who: <T::Lookup as StaticLookup>::Source,
#[compact] free: T::Balance,
Expand All @@ -238,7 +396,6 @@ decl_module! {
}
}

// For funding methods, see Currency trait
impl<T: Trait<I>, I: Instance> Module<T, I> {

// PUBLIC IMMUTABLES
Expand All @@ -254,10 +411,11 @@ impl<T: Trait<I>, I: Instance> Module<T, I> {

// PRIVATE MUTABLES

/// Set the free balance of an account to some new value.
/// Set the reserved balance of an account to some new value. Will enforce `ExistentialDeposit`
/// law, annulling the account as needed.
///
/// Will enforce ExistentialDeposit law, annulling the account as needed.
/// In that case it will return `AccountKilled`.
/// Doesn't do any preparatory work for creating a new account, so should only be used when it
/// is known that the account already exists.
///
/// NOTE: LOW-LEVEL: This will not attempt to maintain total issuance. It is expected that
/// the caller will do this.
Expand All @@ -272,14 +430,12 @@ impl<T: Trait<I>, I: Instance> Module<T, I> {
}
}

/// Set the free balance of an account to some new value. Will enforce ExistentialDeposit
/// law annulling the account as needed.
/// Set the free balance of an account to some new value. Will enforce `ExistentialDeposit`
/// law, annulling the account as needed.
///
/// Doesn't do any preparatory work for creating a new account, so should only be used when it
/// is known that the account already exists.
///
/// Returns if the account was successfully updated or update has led to killing of the account.
///
/// NOTE: LOW-LEVEL: This will not attempt to maintain total issuance. It is expected that
/// the caller will do this.
fn set_free_balance(who: &T::AccountId, balance: T::Balance) -> UpdateBalanceOutcome {
Expand Down Expand Up @@ -684,8 +840,6 @@ where
// Free balance can never be less than ED. If that happens, it gets reduced to zero
// and the account information relevant to this subsystem is deleted (i.e. the
// account is reaped).
// NOTE: This is orthogonal to the `Bondage` value that an account has, a high
// value of which makes even the `free_balance` unspendable.
let outcome = if balance < <Module<T, I>>::existential_deposit() {
Self::set_free_balance(who, balance);
UpdateBalanceOutcome::AccountKilled
Expand Down
2 changes: 1 addition & 1 deletion srml/contract/src/lib.rs
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Expand Up @@ -366,7 +366,7 @@ decl_event! {

decl_storage! {
trait Store for Module<T: Trait> as Contract {
/// The fee required to create a contract. At least as big as staking's ReclaimRebate.
/// The fee required to create a contract.
ContractFee get(contract_fee) config(): T::Balance = T::Balance::sa(21);
/// The fee charged for a call into a contract.
CallBaseFee get(call_base_fee) config(): T::Gas = T::Gas::sa(135);
Expand Down
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