feat: proposed fungible token trait

sips/sip-010/sip-010-fungible-token-standard.md

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+## Preamble
+
+- Sip Number: 010
+- Title: Standard Trait Definition for Non-Fungibe Tokens
+- Author: Hank Stoever (hstove@gmail.com)
+- Consideration: Technical
+- Type: Standard
+- Status: Draft
+- Created: 2021-01-25
+- License: CC0-1.0
+- Sign-off:
+
+## Abstract
+
+Fungible tokens are digital assets that can be sent, received, combined, and divided. Most forms of currency and cryptocurrencies are fungible tokens. They have become a building block of almost all blockchains. This SIP aims to provide a flexible and easy to implement standard that can be used by developers on the Stacks blockchain when creating their own tokens.
+
+## License and Copyright
+
+This SIP is made available under the terms of the Creative Commons CC0 1.0 Universal license, available at https://creativecommons.org/publicdomain/zero/1.0/
+This SIP’s copyright is held by the Stacks Open Internet Foundation.
+
+## Introduction
+
+Perhaps the oldest, and most well known, standard for fungible tokens is Ethereum's [ERC20](https://ethereum.org/en/developers/docs/standards/tokens/erc-20/) standard. It has become one of the strongest building blocks for the Ethereum ecosystem. When all fungible tokens follow the same standard, any wallet or application developer can interact with it without having to create custom logic for handling each individual token.
+
+Fungible tokens have become so popular that the Clarity smart contracting language has support for basic fungible token operations built-in. In fact, as you might see in this proposal's reference implementation, very little code is required to implement a fungible token. The important part of this standard is defining a Clarity trait that all fungible tokens can implement. Even though Clarity has fungible token operations built-in, it is important for each contract to define the same methods so that their contracts are easy to integrate.
+
+## Specification
+
+The fungible token trait, `ft-trait`, has a few methods:
+
+### Transfer
+
+`(transfer ((recipient principal) (amount uint)) (response bool uint))`
+
+Transfer the fungible token from the sender of this transaction to the recipient. The `amount` is an unsigned integer. It is recommended that implementing contracts use the built-in `ft-transfer` Clarity method. If the sender does not have enough tokens to complete the transaction, the transaction should abort and return an `(err uint)`.
+
+### Name
+
+`(name () (response (string-ascii 32) uint))`
+
+Return a human-readable name for the contract, such as "CoolPoints", etc.
+
+### Symbol
+
+`(symbol () (response (string-ascii 32) uint))`
+
+Return a symbol that allows for a shorter representation of your token. This is sometimes referred to as a "ticker". Examples: "STX", "COOL", etc. Typically, your token could be referred to as $SYMBOL when referencing it in writing.
+
+### Decimals
+
+`(decimals () (response uint uint))`
+
+The number of decimal places in your token. All fungible token balances must be represented as integers, but providing the number of decimals provides for an abstraction of your token that humans are more familiar dealing with. For example, the US Dollar has 2 decimals, if the base unit is "cents", as is typically done in accounting. Stacks has 6 decimals, Bitcoin has 8 decimals, and so on.
+
+As another example, if your token has 4 decimals, and the `balance-of` a particular user returns `100345000`, wallets and exchanges would likely represent that value as `10034.5`.
+
+### Balance of
+
+`(balance-of (principal) (response uint uint))`
+
+Return the balance of a particular principal (also known as "address" or "account"). Implementations should typically use the built-in Clarity method `ft-get-balance`.
+
+### Total supply
+
+`(total-supply () (response uint uint))`
+
+Return the total supply of this token. Implementations should typically use the built-in Clarity method `ft-get-supply`.
+
+## Trait implementation
+
+An implementation of the proposed trait is provided below.
+
+```clarity
+(define-trait ft-trait
+  (
+    ;; Transfer from the caller to a new principal
+    (transfer (principal uint) (response bool uint))
+
+    ;; the human readable name of the token
+    (name () (response (string-ascii 32) uint))
+
+    ;; the ticker symbol, or empty if none
+    (symbol () (response (string-ascii 32) uint))
+
+    ;; the number of decimals used, e.g. 6 would mean 1_000_000 represents 1 token
+    (decimals () (response uint uint))
+
+    ;; the balance of the passed principal
+    (balance-of (principal) (response uint uint))
+
+    ;; the current total supply (which does not need to be a constant)
+    (total-supply () (response uint uint))
+  )
+)
+```
+
+## Example implementation
+
+I have provided a simple implementation along with a Javascript client and tests. https://github.com/hstove/stacks-fungible-token
+
+## Credit
+
+Credit for the work behind this proposal belongs to [@psq](https://github.com/psq), who wrote almost all of this code originally for use in his projects (such as [Flexr](https://github.com/psq/flexr)). I am simply re-packaging his existing code in a format that can be shared in this proposal.
+
+## Implementing in wallets and other applications
+
+Developers who with to interact with a fungible token contract should first be provided, or keep track of, various different fungible token implementations. When validating a fungible token contract, they should fetch the interface and/or source code for that contract. If the contract implements the trait, then the wallet can use this standard's contract interface for making transfers and getting balances.
+
+### Use of post conditions
+
+In addition to fantastic built-in methods for fungible token contracts, the Stacks blockchain includes an innovation known as Post Conditions. By defining post conditions, users can create transactions that include pre-defined guarantees about what might happen in that contract.
+
+One such post condition could be "I will transfer exactly 100 of X token", where "X token" is referenced as a specific contract's fungible token. When wallets and applications implement the `transfer` method, they should _always_ use post conditions to specify that the user will transfer exactly the amount of tokens that they specify in the `amount` argument of the `transfer` function. Only in very specific circumstances should such a post condition not be included.
+
+## Related work
+
+- [@psq's trait and implementation](https://github.com/psq/flexr/blob/master/contracts/src20-trait.clar)
+- [@friedger's fungible token implementation](https://github.com/friedger/clarity-smart-contracts/blob/master/contracts/tokens/fungible-token.clar)
+- [Ethereum ERC20 standard](https://ethereum.org/en/developers/docs/standards/tokens/erc-20/)