lib.rs

// This file is part of Substrate.

// Copyright (C) 2017-2020 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! # Balances Module
//!
//! The Balances module provides functionality for handling accounts and balances.
//!
//! - [`balances::Trait`](./trait.Trait.html)
//! - [`Call`](./enum.Call.html)
//! - [`Module`](./struct.Module.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.
//! - 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. When the free plus the reserved balance (i.e. the total balance)
//!   fall below this, then the account is said to be dead; and it loses its functionality as well as any
//!   prior history and all information on it is removed from the chain's state.
//!   No account should ever have a total balance that is strictly between 0 and the existential
//!   deposit (exclusive). If this ever happens, it indicates either a bug in this module or an
//!   erroneous raw mutation of storage.
//!
//! - **Total Issuance:** The total number of units in existence in a system.
//!
//! - **Reaping an account:** The act of removing an account by resetting its nonce. Happens after its
//! total balance has become zero (or, strictly speaking, less than the Existential Deposit).
//!
//! - **Free Balance:** The portion of a balance that is not reserved. The free balance is the only
//!   balance that matters for most operations.
//!
//! - **Reserved Balance:** Reserved balance still belongs to the account holder, but is suspended.
//!   Reserved balance can still be slashed, but only after all the free balance has been slashed.
//!
//! - **Imbalance:** A condition when some funds were credited or debited without equal and opposite accounting
//! (i.e. a difference between total issuance and account balances). Functions that result in an imbalance will
//! return an object of the `Imbalance` trait that can be managed within your runtime logic. (If an imbalance is
//! simply dropped, it should automatically maintain any book-keeping such as total issuance.)
//!
//! - **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".
//!
//! ### 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`](../frame_support/traits/trait.Currency.html): Functions for dealing with a
//! fungible assets system.
//! - [`ReservableCurrency`](../frame_support/traits/trait.ReservableCurrency.html):
//! Functions for dealing with assets that can be reserved from an account.
//! - [`LockableCurrency`](../frame_support/traits/trait.LockableCurrency.html): Functions for
//! dealing with accounts that allow liquidity restrictions.
//! - [`Imbalance`](../frame_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).
//! - [`IsDeadAccount`](../frame_system/trait.IsDeadAccount.html): Determiner to say whether a
//! given account is unused.
//!
//! ## Interface
//!
//! ### Dispatchable Functions
//!
//! - `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.
//!
//! ## Usage
//!
//! The following examples show how to use the Balances module in your custom module.
//!
//! ### Examples from the FRAME
//!
//! The Contract module uses the `Currency` trait to handle gas payment, and its types inherit from `Currency`:
//!
//! ```
//! use frame_support::traits::Currency;
//! # pub trait Trait: frame_system::Trait {
//! # 	type Currency: Currency<Self::AccountId>;
//! # }
//!
//! pub type BalanceOf<T> = <<T as Trait>::Currency as Currency<<T as frame_system::Trait>::AccountId>>::Balance;
//! pub type NegativeImbalanceOf<T> = <<T as Trait>::Currency as Currency<<T as frame_system::Trait>::AccountId>>::NegativeImbalance;
//!
//! # fn main() {}
//! ```
//!
//! The Staking module uses the `LockableCurrency` trait to lock a stash account's funds:
//!
//! ```
//! use frame_support::traits::{WithdrawReasons, LockableCurrency};
//! use sp_runtime::traits::Bounded;
//! pub trait Trait: frame_system::Trait {
//! 	type Currency: LockableCurrency<Self::AccountId, Moment=Self::BlockNumber>;
//! }
//! # struct StakingLedger<T: Trait> {
//! # 	stash: <T as frame_system::Trait>::AccountId,
//! # 	total: <<T as Trait>::Currency as frame_support::traits::Currency<<T as frame_system::Trait>::AccountId>>::Balance,
//! # 	phantom: std::marker::PhantomData<T>,
//! # }
//! # const STAKING_ID: [u8; 8] = *b"staking ";
//!
//! fn update_ledger<T: Trait>(
//! 	controller: &T::AccountId,
//! 	ledger: &StakingLedger<T>
//! ) {
//! 	T::Currency::set_lock(
//! 		STAKING_ID,
//! 		&ledger.stash,
//! 		ledger.total,
//! 		WithdrawReasons::all()
//! 	);
//! 	// <Ledger<T>>::insert(controller, ledger); // Commented out as we don't have access to Staking's storage here.
//! }
//! # fn main() {}
//! ```
//!
//! ## Genesis config
//!
//! The Balances module depends on the [`GenesisConfig`](./struct.GenesisConfig.html).
//!
//! ## Assumptions
//!
//! * Total issued balanced of all accounts should be less than `Trait::Balance::max_value()`.

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

#[macro_use]
mod tests;
mod tests_local;
mod tests_composite;
mod benchmarking;
mod default_weight;

use sp_std::prelude::*;
use sp_std::{cmp, result, mem, fmt::Debug, ops::BitOr, convert::Infallible};
use codec::{Codec, Encode, Decode};
use frame_support::{
	StorageValue, Parameter, decl_event, decl_storage, decl_module, decl_error, ensure,
	weights::Weight,
	traits::{
		Currency, OnKilledAccount, OnUnbalanced, TryDrop, StoredMap,
		WithdrawReason, WithdrawReasons, LockIdentifier, LockableCurrency, ExistenceRequirement,
		Imbalance, SignedImbalance, ReservableCurrency, Get, ExistenceRequirement::KeepAlive,
		ExistenceRequirement::AllowDeath, IsDeadAccount, BalanceStatus as Status,
	}
};
use sp_runtime::{
	RuntimeDebug, DispatchResult, DispatchError,
	traits::{
		Zero, AtLeast32BitUnsigned, StaticLookup, Member, CheckedAdd, CheckedSub,
		MaybeSerializeDeserialize, Saturating, Bounded,
	},
};
use frame_system::{self as system, ensure_signed, ensure_root};
pub use self::imbalances::{PositiveImbalance, NegativeImbalance};

pub trait WeightInfo {
	fn transfer() -> Weight;
	fn transfer_keep_alive() -> Weight;
	fn set_balance_creating() -> Weight;
	fn set_balance_killing() -> Weight;
	fn force_transfer() -> Weight;
}

pub trait Subtrait<I: Instance = DefaultInstance>: frame_system::Trait {
	/// The balance of an account.
	type Balance: Parameter + Member + AtLeast32BitUnsigned + Codec + Default + Copy +
		MaybeSerializeDeserialize + Debug;

	/// The minimum amount required to keep an account open.
	type ExistentialDeposit: Get<Self::Balance>;

	/// The means of storing the balances of an account.
	type AccountStore: StoredMap<Self::AccountId, AccountData<Self::Balance>>;

	/// Weight information for the extrinsics in this pallet.
	type WeightInfo: WeightInfo;
}

pub trait Trait<I: Instance = DefaultInstance>: frame_system::Trait {
	/// The balance of an account.
	type Balance: Parameter + Member + AtLeast32BitUnsigned + Codec + Default + Copy +
		MaybeSerializeDeserialize + Debug;

	/// Handler for the unbalanced reduction when removing a dust account.
	type DustRemoval: OnUnbalanced<NegativeImbalance<Self, I>>;

	/// The overarching event type.
	type Event: From<Event<Self, I>> + Into<<Self as frame_system::Trait>::Event>;

	/// The minimum amount required to keep an account open.
	type ExistentialDeposit: Get<Self::Balance>;

	/// The means of storing the balances of an account.
	type AccountStore: StoredMap<Self::AccountId, AccountData<Self::Balance>>;

	/// Weight information for extrinsics in this pallet.
	type WeightInfo: WeightInfo;
}

impl<T: Trait<I>, I: Instance> Subtrait<I> for T {
	type Balance = T::Balance;
	type ExistentialDeposit = T::ExistentialDeposit;
	type AccountStore = T::AccountStore;
	type WeightInfo = <T as Trait<I>>::WeightInfo;
}

decl_event!(
	pub enum Event<T, I: Instance = DefaultInstance> where
		<T as frame_system::Trait>::AccountId,
		<T as Trait<I>>::Balance
	{
		/// An account was created with some free balance. \[account, free_balance\]
		Endowed(AccountId, Balance),
		/// An account was removed whose balance was non-zero but below ExistentialDeposit,
		/// resulting in an outright loss. \[account, balance\]
		DustLost(AccountId, Balance),
		/// Transfer succeeded. \[from, to, value\]
		Transfer(AccountId, AccountId, Balance),
		/// A balance was set by root. \[who, free, reserved\]
		BalanceSet(AccountId, Balance, Balance),
		/// Some amount was deposited (e.g. for transaction fees). \[who, deposit\]
		Deposit(AccountId, Balance),
		/// Some balance was reserved (moved from free to reserved). \[who, value\]
		Reserved(AccountId, Balance),
		/// Some balance was unreserved (moved from reserved to free). \[who, value\]
		Unreserved(AccountId, Balance),
		/// Some balance was moved from the reserve of the first account to the second account.
		/// Final argument indicates the destination balance type.
		/// \[from, to, balance, destination_status\]
		ReserveRepatriated(AccountId, AccountId, Balance, Status),
	}
);

decl_error! {
	pub enum Error for Module<T: Trait<I>, I: Instance> {
		/// Vesting balance too high to send value
		VestingBalance,
		/// Account liquidity restrictions prevent withdrawal
		LiquidityRestrictions,
		/// Got an overflow after adding
		Overflow,
		/// Balance too low to send value
		InsufficientBalance,
		/// Value too low to create account due to existential deposit
		ExistentialDeposit,
		/// Transfer/payment would kill account
		KeepAlive,
		/// A vesting schedule already exists for this account
		ExistingVestingSchedule,
		/// Beneficiary account must pre-exist
		DeadAccount,
	}
}

/// Simplified reasons for withdrawing balance.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
pub enum Reasons {
	/// Paying system transaction fees.
	Fee = 0,
	/// Any reason other than paying system transaction fees.
	Misc = 1,
	/// Any reason at all.
	All = 2,
}

impl From<WithdrawReasons> for Reasons {
	fn from(r: WithdrawReasons) -> Reasons {
		if r == WithdrawReasons::from(WithdrawReason::TransactionPayment) {
			Reasons::Fee
		} else if r.contains(WithdrawReason::TransactionPayment) {
			Reasons::All
		} else {
			Reasons::Misc
		}
	}
}

impl BitOr for Reasons {
	type Output = Reasons;
	fn bitor(self, other: Reasons) -> Reasons {
		if self == other { return self }
		Reasons::All
	}
}

/// A single lock on a balance. There can be many of these on an account and they "overlap", so the
/// same balance is frozen by multiple locks.
#[derive(Encode, Decode, Clone, PartialEq, Eq, RuntimeDebug)]
pub struct BalanceLock<Balance> {
	/// An identifier for this lock. Only one lock may be in existence for each identifier.
	pub id: LockIdentifier,
	/// The amount which the free balance may not drop below when this lock is in effect.
	pub amount: Balance,
	/// If true, then the lock remains in effect even for payment of transaction fees.
	pub reasons: Reasons,
}

/// All balance information for an account.
#[derive(Encode, Decode, Clone, PartialEq, Eq, Default, RuntimeDebug)]
pub struct AccountData<Balance> {
	/// Non-reserved part of the balance. There may still be restrictions on this, but it is the
	/// total pool what may in principle be transferred, reserved and used for tipping.
	///
	/// 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.
	pub free: Balance,
	/// Balance which is reserved and may not be used at all.
	///
	/// This can still get 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.
	pub reserved: Balance,
	/// The amount that `free` may not drop below when withdrawing for *anything except transaction
	/// fee payment*.
	pub misc_frozen: Balance,
	/// The amount that `free` may not drop below when withdrawing specifically for transaction
	/// fee payment.
	pub fee_frozen: Balance,
}

impl<Balance: Saturating + Copy + Ord> AccountData<Balance> {
	/// How much this account's balance can be reduced for the given `reasons`.
	fn usable(&self, reasons: Reasons) -> Balance {
		self.free.saturating_sub(self.frozen(reasons))
	}
	/// The amount that this account's free balance may not be reduced beyond for the given
	/// `reasons`.
	fn frozen(&self, reasons: Reasons) -> Balance {
		match reasons {
			Reasons::All => self.misc_frozen.max(self.fee_frozen),
			Reasons::Misc => self.misc_frozen,
			Reasons::Fee => self.fee_frozen,
		}
	}
	/// The total balance in this account including any that is reserved and ignoring any frozen.
	fn total(&self) -> Balance {
		self.free.saturating_add(self.reserved)
	}
}

// A value placed in storage that represents the current version of the Balances storage.
// This value is used by the `on_runtime_upgrade` logic to determine whether we run
// storage migration logic. This should match directly with the semantic versions of the Rust crate.
#[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug)]
enum Releases {
	V1_0_0,
	V2_0_0,
}

impl Default for Releases {
	fn default() -> Self {
		Releases::V1_0_0
	}
}

decl_storage! {
	trait Store for Module<T: Trait<I>, I: Instance=DefaultInstance> as Balances {
		/// The total units issued in the system.
		pub TotalIssuance get(fn total_issuance) build(|config: &GenesisConfig<T, I>| {
			config.balances.iter().fold(Zero::zero(), |acc: T::Balance, &(_, n)| acc + n)
		}): T::Balance;

		/// The balance of an account.
		///
		/// NOTE: This is only used in the case that this module is used to store balances.
		pub Account: map hasher(blake2_128_concat) T::AccountId => AccountData<T::Balance>;

		/// Any liquidity locks on some account balances.
		/// NOTE: Should only be accessed when setting, changing and freeing a lock.
		pub Locks get(fn locks): map hasher(blake2_128_concat) T::AccountId => Vec<BalanceLock<T::Balance>>;

		/// Storage version of the pallet.
		///
		/// This is set to v2.0.0 for new networks.
		StorageVersion build(|_: &GenesisConfig<T, I>| Releases::V2_0_0): Releases;
	}
	add_extra_genesis {
		config(balances): Vec<(T::AccountId, T::Balance)>;
		// ^^ begin, length, amount liquid at genesis
		build(|config: &GenesisConfig<T, I>| {
			for (_, balance) in &config.balances {
				assert!(
					*balance >= <T as Trait<I>>::ExistentialDeposit::get(),
					"the balance of any account should always be more than existential deposit.",
				)
			}
			for &(ref who, free) in config.balances.iter() {
				T::AccountStore::insert(who, AccountData { free, .. Default::default() });
			}
		});
	}
}

decl_module! {
	pub struct Module<T: Trait<I>, I: Instance = DefaultInstance> for enum Call where origin: T::Origin {
		type Error = Error<T, I>;

		/// The minimum amount required to keep an account open.
		const ExistentialDeposit: T::Balance = T::ExistentialDeposit::get();

		fn deposit_event() = default;

		/// 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.
		///
		/// # <weight>
		/// - Dependent on arguments but not critical, given proper implementations for
		///   input config types. See related functions below.
		/// - It contains a limited number of reads and writes internally and no complex computation.
		///
		/// Related functions:
		///
		///   - `ensure_can_withdraw` is always called internally but has a bounded complexity.
		///   - Transferring balances to accounts that did not exist before will cause
		///      `T::OnNewAccount::on_new_account` to be called.
		///   - Removing enough funds from an account will trigger `T::DustRemoval::on_unbalanced`.
		///   - `transfer_keep_alive` works the same way as `transfer`, but has an additional
		///     check that the transfer will not kill the origin account.
		/// ---------------------------------
		/// - Base Weight: 73.64 µs, worst case scenario (account created, account removed)
		/// - DB Weight: 1 Read and 1 Write to destination account
		/// - Origin account is already in memory, so no DB operations for them.
		/// # </weight>
		#[weight = T::WeightInfo::transfer()]
		pub fn transfer(
			origin,
			dest: <T::Lookup as StaticLookup>::Source,
			#[compact] value: T::Balance
		) {
			let transactor = ensure_signed(origin)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&transactor, &dest, value, ExistenceRequirement::AllowDeath)?;
		}

		/// Set the balances of a given account.
		///
		/// This will alter `FreeBalance` and `ReservedBalance` in storage. it will
		/// also decrease the total issuance of the system (`TotalIssuance`).
		/// If the new free or reserved balance is below the existential deposit,
		/// it will reset the account nonce (`frame_system::AccountNonce`).
		///
		/// The dispatch origin for this call is `root`.
		///
		/// # <weight>
		/// - Independent of the arguments.
		/// - Contains a limited number of reads and writes.
		/// ---------------------
		/// - Base Weight:
		///     - Creating: 27.56 µs
		///     - Killing: 35.11 µs
		/// - DB Weight: 1 Read, 1 Write to `who`
		/// # </weight>
		#[weight = T::WeightInfo::set_balance_creating() // Creates a new account.
			.max(T::WeightInfo::set_balance_killing()) // Kills an existing account.
		]
		fn set_balance(
			origin,
			who: <T::Lookup as StaticLookup>::Source,
			#[compact] new_free: T::Balance,
			#[compact] new_reserved: T::Balance
		) {
			ensure_root(origin)?;
			let who = T::Lookup::lookup(who)?;
			let existential_deposit = T::ExistentialDeposit::get();

			let wipeout = new_free + new_reserved < existential_deposit;
			let new_free = if wipeout { Zero::zero() } else { new_free };
			let new_reserved = if wipeout { Zero::zero() } else { new_reserved };

			let (free, reserved) = Self::mutate_account(&who, |account| {
				if new_free > account.free {
					mem::drop(PositiveImbalance::<T, I>::new(new_free - account.free));
				} else if new_free < account.free {
					mem::drop(NegativeImbalance::<T, I>::new(account.free - new_free));
				}

				if new_reserved > account.reserved {
					mem::drop(PositiveImbalance::<T, I>::new(new_reserved - account.reserved));
				} else if new_reserved < account.reserved {
					mem::drop(NegativeImbalance::<T, I>::new(account.reserved - new_reserved));
				}

				account.free = new_free;
				account.reserved = new_reserved;

				(account.free, account.reserved)
			});
			Self::deposit_event(RawEvent::BalanceSet(who, free, reserved));
		}

		/// Exactly as `transfer`, except the origin must be root and the source account may be
		/// specified.
		/// # <weight>
		/// - Same as transfer, but additional read and write because the source account is
		///   not assumed to be in the overlay.
		/// # </weight>
		#[weight = T::WeightInfo::force_transfer()]
		pub fn force_transfer(
			origin,
			source: <T::Lookup as StaticLookup>::Source,
			dest: <T::Lookup as StaticLookup>::Source,
			#[compact] value: T::Balance
		) {
			ensure_root(origin)?;
			let source = T::Lookup::lookup(source)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&source, &dest, value, ExistenceRequirement::AllowDeath)?;
		}

		/// Same as the [`transfer`] call, but with a check that the transfer will not kill the
		/// origin account.
		///
		/// 99% of the time you want [`transfer`] instead.
		///
		/// [`transfer`]: struct.Module.html#method.transfer
		/// # <weight>
		/// - Cheaper than transfer because account cannot be killed.
		/// - Base Weight: 51.4 µs
		/// - DB Weight: 1 Read and 1 Write to dest (sender is in overlay already)
		/// #</weight>
		#[weight = T::WeightInfo::transfer_keep_alive()]
		pub fn transfer_keep_alive(
			origin,
			dest: <T::Lookup as StaticLookup>::Source,
			#[compact] value: T::Balance
		) {
			let transactor = ensure_signed(origin)?;
			let dest = T::Lookup::lookup(dest)?;
			<Self as Currency<_>>::transfer(&transactor, &dest, value, KeepAlive)?;
		}
	}
}

impl<T: Trait<I>, I: Instance> Module<T, I> {
	// PRIVATE MUTABLES

	/// Get the free balance of an account.
	pub fn free_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).free
	}

	/// Get the balance of an account that can be used for transfers, reservations, or any other
	/// non-locking, non-transaction-fee activity. Will be at most `free_balance`.
	pub fn usable_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).usable(Reasons::Misc)
	}

	/// Get the balance of an account that can be used for paying transaction fees (not tipping,
	/// or any other kind of fees, though). Will be at most `free_balance`.
	pub fn usable_balance_for_fees(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).usable(Reasons::Fee)
	}

	/// Get the reserved balance of an account.
	pub fn reserved_balance(who: impl sp_std::borrow::Borrow<T::AccountId>) -> T::Balance {
		Self::account(who.borrow()).reserved
	}

	/// Get both the free and reserved balances of an account.
	fn account(who: &T::AccountId) -> AccountData<T::Balance> {
		T::AccountStore::get(&who)
	}

	/// Places the `free` and `reserved` parts of `new` into `account`. Also does any steps needed
	/// after mutating an account. This includes DustRemoval unbalancing, in the case than the `new`
	/// account's total balance is non-zero but below ED.
	///
	/// Returns the final free balance, iff the account was previously of total balance zero, known
	/// as its "endowment".
	fn post_mutation(
		who: &T::AccountId,
		new: AccountData<T::Balance>,
	) -> Option<AccountData<T::Balance>> {
		let total = new.total();
		if total < T::ExistentialDeposit::get() {
			if !total.is_zero() {
				T::DustRemoval::on_unbalanced(NegativeImbalance::new(total));
				Self::deposit_event(RawEvent::DustLost(who.clone(), total));
			}
			None
		} else {
			Some(new)
		}
	}

	/// Mutate an account to some new value, or delete it entirely with `None`. Will enforce
	/// `ExistentialDeposit` law, annulling the account as needed.
	///
	/// NOTE: 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.
	pub fn mutate_account<R>(
		who: &T::AccountId,
		f: impl FnOnce(&mut AccountData<T::Balance>) -> R
	) -> R {
		Self::try_mutate_account(who, |a, _| -> Result<R, Infallible> { Ok(f(a)) })
			.expect("Error is infallible; qed")
	}

	/// Mutate an account to some new value, or delete it entirely with `None`. Will enforce
	/// `ExistentialDeposit` law, annulling the account as needed. This will do nothing if the
	/// result of `f` is an `Err`.
	///
	/// NOTE: 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.
	fn try_mutate_account<R, E>(
		who: &T::AccountId,
		f: impl FnOnce(&mut AccountData<T::Balance>, bool) -> Result<R, E>
	) -> Result<R, E> {
		T::AccountStore::try_mutate_exists(who, |maybe_account| {
			let is_new = maybe_account.is_none();
			let mut account = maybe_account.take().unwrap_or_default();
			f(&mut account, is_new).map(move |result| {
				let maybe_endowed = if is_new { Some(account.free) } else { None };
				*maybe_account = Self::post_mutation(who, account);
				(maybe_endowed, result)
			})
		}).map(|(maybe_endowed, result)| {
			if let Some(endowed) = maybe_endowed {
				Self::deposit_event(RawEvent::Endowed(who.clone(), endowed));
			}
			result
		})
	}

	/// Update the account entry for `who`, given the locks.
	fn update_locks(who: &T::AccountId, locks: &[BalanceLock<T::Balance>]) {
		Self::mutate_account(who, |b| {
			b.misc_frozen = Zero::zero();
			b.fee_frozen = Zero::zero();
			for l in locks.iter() {
				if l.reasons == Reasons::All || l.reasons == Reasons::Misc {
					b.misc_frozen = b.misc_frozen.max(l.amount);
				}
				if l.reasons == Reasons::All || l.reasons == Reasons::Fee {
					b.fee_frozen = b.fee_frozen.max(l.amount);
				}
			}
		});

		let existed = Locks::<T, I>::contains_key(who);
		if locks.is_empty() {
			Locks::<T, I>::remove(who);
			if existed {
				// TODO: use Locks::<T, I>::hashed_key
				// https://github.com/paritytech/substrate/issues/4969
				system::Module::<T>::dec_ref(who);
			}
		} else {
			Locks::<T, I>::insert(who, locks);
			if !existed {
				system::Module::<T>::inc_ref(who);
			}
		}
	}
}

// wrapping these imbalances in a private module is necessary to ensure absolute privacy
// of the inner member.
mod imbalances {
	use super::{
		result, Subtrait, DefaultInstance, Imbalance, Trait, Zero, Instance, Saturating,
		StorageValue, TryDrop,
	};
	use sp_std::mem;

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been created without any equal and opposite accounting.
	#[must_use]
	pub struct PositiveImbalance<T: Subtrait<I>, I: Instance=DefaultInstance>(T::Balance);

	impl<T: Subtrait<I>, I: Instance> PositiveImbalance<T, I> {
		/// Create a new positive imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			PositiveImbalance(amount)
		}
	}

	/// Opaque, move-only struct with private fields that serves as a token denoting that
	/// funds have been destroyed without any equal and opposite accounting.
	#[must_use]
	pub struct NegativeImbalance<T: Subtrait<I>, I: Instance=DefaultInstance>(T::Balance);

	impl<T: Subtrait<I>, I: Instance> NegativeImbalance<T, I> {
		/// Create a new negative imbalance from a balance.
		pub fn new(amount: T::Balance) -> Self {
			NegativeImbalance(amount)
		}
	}

	impl<T: Trait<I>, I: Instance> TryDrop for PositiveImbalance<T, I> {
		fn try_drop(self) -> result::Result<(), Self> {
			self.drop_zero()
		}
	}

	impl<T: Trait<I>, I: Instance> Imbalance<T::Balance> for PositiveImbalance<T, I> {
		type Opposite = NegativeImbalance<T, I>;

		fn zero() -> Self {
			Self(Zero::zero())
		}
		fn drop_zero(self) -> result::Result<(), Self> {
			if self.0.is_zero() {
				Ok(())
			} else {
				Err(self)
			}
		}
		fn split(self, amount: T::Balance) -> (Self, Self) {
			let first = self.0.min(amount);
			let second = self.0 - first;

			mem::forget(self);
			(Self(first), Self(second))
		}
		fn merge(mut self, other: Self) -> Self {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);

			self
		}
		fn subsume(&mut self, other: Self) {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);
		}
		fn offset(self, other: Self::Opposite) -> result::Result<Self, Self::Opposite> {
			let (a, b) = (self.0, other.0);
			mem::forget((self, other));

			if a >= b {
				Ok(Self(a - b))
			} else {
				Err(NegativeImbalance::new(b - a))
			}
		}
		fn peek(&self) -> T::Balance {
			self.0.clone()
		}
	}

	impl<T: Trait<I>, I: Instance> TryDrop for NegativeImbalance<T, I> {
		fn try_drop(self) -> result::Result<(), Self> {
			self.drop_zero()
		}
	}

	impl<T: Trait<I>, I: Instance> Imbalance<T::Balance> for NegativeImbalance<T, I> {
		type Opposite = PositiveImbalance<T, I>;

		fn zero() -> Self {
			Self(Zero::zero())
		}
		fn drop_zero(self) -> result::Result<(), Self> {
			if self.0.is_zero() {
				Ok(())
			} else {
				Err(self)
			}
		}
		fn split(self, amount: T::Balance) -> (Self, Self) {
			let first = self.0.min(amount);
			let second = self.0 - first;

			mem::forget(self);
			(Self(first), Self(second))
		}
		fn merge(mut self, other: Self) -> Self {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);

			self
		}
		fn subsume(&mut self, other: Self) {
			self.0 = self.0.saturating_add(other.0);
			mem::forget(other);
		}
		fn offset(self, other: Self::Opposite) -> result::Result<Self, Self::Opposite> {
			let (a, b) = (self.0, other.0);
			mem::forget((self, other));

			if a >= b {
				Ok(Self(a - b))
			} else {
				Err(PositiveImbalance::new(b - a))
			}
		}
		fn peek(&self) -> T::Balance {
			self.0.clone()
		}
	}

	impl<T: Subtrait<I>, I: Instance> Drop for PositiveImbalance<T, I> {
		/// Basic drop handler will just square up the total issuance.
		fn drop(&mut self) {
			<super::TotalIssuance<super::ElevatedTrait<T, I>, I>>::mutate(
				|v| *v = v.saturating_add(self.0)
			);
		}
	}

	impl<T: Subtrait<I>, I: Instance> Drop for NegativeImbalance<T, I> {
		/// Basic drop handler will just square up the total issuance.
		fn drop(&mut self) {
			<super::TotalIssuance<super::ElevatedTrait<T, I>, I>>::mutate(
				|v| *v = v.saturating_sub(self.0)
			);
		}
	}
}

// TODO: #2052
// Somewhat ugly hack in order to gain access to module's `increase_total_issuance_by`
// using only the Subtrait (which defines only the types that are not dependent
// on Positive/NegativeImbalance). Subtrait must be used otherwise we end up with a
// circular dependency with Trait having some types be dependent on PositiveImbalance<Trait>
// and PositiveImbalance itself depending back on Trait for its Drop impl (and thus
// its type declaration).
// This works as long as `increase_total_issuance_by` doesn't use the Imbalance
// types (basically for charging fees).
// This should eventually be refactored so that the type item that
// depends on the Imbalance type (DustRemoval) is placed in its own pallet.
struct ElevatedTrait<T: Subtrait<I>, I: Instance>(T, I);
impl<T: Subtrait<I>, I: Instance> Clone for ElevatedTrait<T, I> {
	fn clone(&self) -> Self { unimplemented!() }
}
impl<T: Subtrait<I>, I: Instance> PartialEq for ElevatedTrait<T, I> {
	fn eq(&self, _: &Self) -> bool { unimplemented!() }
}
impl<T: Subtrait<I>, I: Instance> Eq for ElevatedTrait<T, I> {}
impl<T: Subtrait<I>, I: Instance> frame_system::Trait for ElevatedTrait<T, I> {
	type BaseCallFilter = T::BaseCallFilter;
	type Origin = T::Origin;
	type Call = T::Call;
	type Index = T::Index;
	type BlockNumber = T::BlockNumber;
	type Hash = T::Hash;
	type Hashing = T::Hashing;
	type AccountId = T::AccountId;
	type Lookup = T::Lookup;
	type Header = T::Header;
	type Event = ();
	type BlockHashCount = T::BlockHashCount;
	type MaximumBlockWeight = T::MaximumBlockWeight;
	type DbWeight = T::DbWeight;
	type BlockExecutionWeight = T::BlockExecutionWeight;
	type ExtrinsicBaseWeight = T::ExtrinsicBaseWeight;
	type MaximumExtrinsicWeight = T::MaximumBlockWeight;
	type MaximumBlockLength = T::MaximumBlockLength;
	type AvailableBlockRatio = T::AvailableBlockRatio;
	type Version = T::Version;
	type ModuleToIndex = T::ModuleToIndex;
	type OnNewAccount = T::OnNewAccount;
	type OnKilledAccount = T::OnKilledAccount;
	type AccountData = T::AccountData;
	type SystemWeightInfo = T::SystemWeightInfo;
}
impl<T: Subtrait<I>, I: Instance> Trait<I> for ElevatedTrait<T, I> {
	type Balance = T::Balance;
	type Event = ();
	type DustRemoval = ();
	type ExistentialDeposit = T::ExistentialDeposit;
	type AccountStore = T::AccountStore;
	type WeightInfo = <T as Subtrait<I>>::WeightInfo;
}

impl<T: Trait<I>, I: Instance> Currency<T::AccountId> for Module<T, I> where
	T::Balance: MaybeSerializeDeserialize + Debug
{
	type Balance = T::Balance;
	type PositiveImbalance = PositiveImbalance<T, I>;
	type NegativeImbalance = NegativeImbalance<T, I>;

	fn total_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).total()
	}

	// Check if `value` amount of free balance can be slashed from `who`.
	fn can_slash(who: &T::AccountId, value: Self::Balance) -> bool {
		if value.is_zero() { return true }
		Self::free_balance(who) >= value
	}

	fn total_issuance() -> Self::Balance {
		<TotalIssuance<T, I>>::get()
	}

	fn minimum_balance() -> Self::Balance {
		T::ExistentialDeposit::get()
	}

	// Burn funds from the total issuance, returning a positive imbalance for the amount burned.
	// Is a no-op if amount to be burned is zero.
	fn burn(mut amount: Self::Balance) -> Self::PositiveImbalance {
		if amount.is_zero() { return PositiveImbalance::zero() }
		<TotalIssuance<T, I>>::mutate(|issued| {
			*issued = issued.checked_sub(&amount).unwrap_or_else(|| {
				amount = *issued;
				Zero::zero()
			});
		});
		PositiveImbalance::new(amount)
	}

	// Create new funds into the total issuance, returning a negative imbalance
	// for the amount issued.
	// Is a no-op if amount to be issued it zero.
	fn issue(mut amount: Self::Balance) -> Self::NegativeImbalance {
		if amount.is_zero() { return NegativeImbalance::zero() }
		<TotalIssuance<T, I>>::mutate(|issued|
			*issued = issued.checked_add(&amount).unwrap_or_else(|| {
				amount = Self::Balance::max_value() - *issued;
				Self::Balance::max_value()
			})
		);
		NegativeImbalance::new(amount)
	}

	fn free_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).free
	}

	// Ensure that an account can withdraw from their free balance given any existing withdrawal
	// restrictions like locks and vesting balance.
	// Is a no-op if amount to be withdrawn is zero.
	//
	// # <weight>
	// Despite iterating over a list of locks, they are limited by the number of
	// lock IDs, which means the number of runtime modules that intend to use and create locks.
	// # </weight>
	fn ensure_can_withdraw(
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
		new_balance: T::Balance,
	) -> DispatchResult {
		if amount.is_zero() { return Ok(()) }
		let min_balance = Self::account(who).frozen(reasons.into());
		ensure!(new_balance >= min_balance, Error::<T, I>::LiquidityRestrictions);
		Ok(())
	}

	// Transfer some free balance from `transactor` to `dest`, respecting existence requirements.
	// Is a no-op if value to be transferred is zero or the `transactor` is the same as `dest`.
	fn transfer(
		transactor: &T::AccountId,
		dest: &T::AccountId,
		value: Self::Balance,
		existence_requirement: ExistenceRequirement,
	) -> DispatchResult {
		if value.is_zero() || transactor == dest { return Ok(()) }

		Self::try_mutate_account(dest, |to_account, _| -> DispatchResult {
			Self::try_mutate_account(transactor, |from_account, _| -> DispatchResult {
				from_account.free = from_account.free.checked_sub(&value)
					.ok_or(Error::<T, I>::InsufficientBalance)?;

				// NOTE: total stake being stored in the same type means that this could never overflow
				// but better to be safe than sorry.
				to_account.free = to_account.free.checked_add(&value).ok_or(Error::<T, I>::Overflow)?;

				let ed = T::ExistentialDeposit::get();