permit demo

ERC20Implementation.vy

allowances: HashMap[address, HashMap[address, uint256]]
balanceOf: HashMap[address, uint256]
totalSupply: uint256

bundle: ERC20Bundle

def __init__():
    ... # do things with initializing and totalSupply

@external
@bundle(ERC20Bundle, bind=True)  # bind=True indicates transfer cannot be exported on its own, only as part of ERC20Bundle
def transfer(...):
    ... # implementation

@external
@bundle(ERC20Bundle, bind=True)
def transferFrom(...):
    ... # implementation

@external
@bundle(ERC20Bundle, bind=True)
def approve(...):
    ...  # implementation


... # etc

permit.vy

# permit.vy

import ERC20Implementation as token

_erc20: token
nonces: HashMap[address, uint256]

export: _erc20.ERC20Bundle

def __init__():
    self._erc20.__init__(...)

@external
def permit():
    nonce: uint256 = self.nonces[owner]
    ...  # do digest and verify sig
    self.nonces[_owner] += 1
    self._erc20.allowances[...][...] += ...

token.vy

# the final token contract implemented by a user of the library

import permit

p: permit

export: p.ERC20Bundle, p.permit

def __init__():
    self.p.__init__(...)