erc20-pool

A token pool implementation that allows deposits in the form of liquidity donations, and withdrawals in the form of token swaps.

It satisfies the CIC TokenSwap interface.

Synopsis

agent	action	agent change	pool change
alice	deposit(FOO, 1000000)	-1000000 FOO	+1000000 FOO
bob	withdraw(FOO, BAR, 1000)	-1000 BAR, +1000 FOO	+1000 BAR, -1000 FOO
alice	withdraw(BAR, FOO, 1000)	+1000 BAR, -1000 FOO	-1000 BAR, +1000 FOO

Publishing the contract

There are six constructor arguments.

The first three, name, symbol and decimals have matching getter methods, are analogous to the ERC20 methods of the same name.

The declaration parameter is optional and can be any arbitrary content. Typically it defines a content hash of data describing the pool resource. A value of bytes32(0x0) means "no declaration defined."

The tokenRegistry parameter takes an address to a smart contract controlling which tokens are allowed in the pool. See "Token approval" below. A value of address(0x0) deactivates this control, and allows the pool to hold all tokens by default (although they may still be subject to value limits).

The tokenLimiter parameter takes an address to a smart contract controlling value limits of tokens in the pool. See "Token limits" below. A value of address(0x0) deactivates this control, and allows any value of (approved) tokens to be held by the pool.

Token approval

By specifying a non-zero contract address for the tokenRegistry property that implements the CIC ACL interface, that contract can be used to allow and disallow which tokens can be used as input tokens to deposit() and withdraw().

Tokens that are disallowed while the pool still holds a balance can still be withdrawn in full.

Token limits

By specifying a non-zero contract address for the tokenRegistry property that implements the CIC TokenLimit interface, that contract can be used to control the value limit allowed for each token in the pool.

Tokens that are limited below the current balance held by the pool can still be withdrawn. Once the balance goes below the limit, additional tokens values may again be swapped, up to the limit.

Using limiter as registry

The erc20-limiter repository contains the smart contract implementation LimiterIndex.sol. This uses the token limit state to satisfy the CIC ACL interface. Specifically, any token limit higher than 0 will be defined as allowed.

This enables to publisher to use the same smart contract for both constructor arguments tokenRegistry and tokenLimiter.

Handling values

The pool contract does no checking whatsoever regarding the sanity of allowing tokens in the pool.

It is therefore the responsibility of the maintainer of the list of allowed tokens to ensure that tokens in the pool are exchangeable in a sensible way.

Some obvious concerns are:

• Tokens are swapped denominated in their smallest unit (regardless of "decimals").
• The unit of account of the tokens may differ.
• The value of the tokens in relation to unit of account may differ.
• The tokens may be subject to different rates of change.

Providing quotes

Using the setQuoter() method, a smart contract address can be defined that translates value between tokens when exchanging tokens in the pool.

The value returned from the "quoter" is the value of output tokens that will be received in return for the value of input tokens specified.

The "quoter" smart contract must satisfy the CIC TokenQuote interface.

An example quoter contract DecimalQuote.sol can be found in this repository. The contract translates values according to the decimal count reported by the respective ERC20 tokens.

Fees

Using the setFee method, a fee may be specified, in parts-per-million, to be deducted from each token swap.

The fee will be deducted from the input token before the value is sent to the "quoter" (if defined).

Fee is defined in parts-per-million, i.e. 1000000 equals 100%. Any value less than 1000000 is valid.

Fee recipient

By default, all deducted fees are credited to the pool contract.

Using the setFeeAddress method, an external beneficiary for the fees may be defined. That beneficiary will be eligible to receive all fees pending external payment from that moment on. Note that this does also include any fees that were not already claimed by a previous beneficiary.

Withdrawing fees

Fees to be paid externally are accounted for internally in the contract, and may be withdrawn at any time using either the withdraw(outToken) or withdraw(outToken, value) method. (Note the difference in method signature from the exchange method: withdraw(outToken, inToken, value).

Sealing the contract

The contract implements the CIC Seal interface for the following properties:

• Fee value
• Fee address
• Quoter contract address