VIP: Add private Visibility

[pcaversaccio]

Simple Summary

Support private visibility for functions, state variables as well as constants and immutables to restrict access exclusively to the declaring contract.

To frontrun your arguments against it, I know it's not pythonic :)

Motivation

Please note that the perspective shared here reflects (mostly) my viewpoint as a library author.

Adding private visibility for functions and state variables reinforces the principle of encapsulation within a module contract. Ok cool, but what does this exactly mean? Well, it prevents importing contracts (e.g. a contract using and implementing snekmate's erc20 contract) from directly altering critical variables (e.g. balances or allowances), safeguarding against unintended or harmful changes by the main contract implementoooor. By exposing controlled access through e.g. getters or internal setters when modification is appropriate, the module can retain both security and flexibility in this way. All these features are important to library writers.

Something like that is currently possible:

# pragma version ~=0.4.0
from ethereum.ercs import IERC20
from ethereum.ercs import IERC20Detailed
from snekmate.auth import ownable
from snekmate.tokens import erc20


initializes: ownable
initializes: erc20[ownable := ownable]
exports: (
    erc20.owner,
    erc20.IERC20,
    erc20.IERC20Detailed,
    erc20.mint,
    erc20.set_minter,
)


@deploy
def __init__():
    ownable.__init__()
    erc20.__init__("Vyper", "VY", 18, "Vyper", "1")
    # Modifies the `totalSupply` without updating the balances
    # (invariant "sum of total balances = total supply" breaks).
    # With `private` visibility, I could rewrite snekmate `erc20` in
    # a way, that users can't directly modify this variable without
    # breaking the invariant.
    erc20.totalSupply += 1000

Specification

Add the private visibility to state variable, immutable, constant definitions as well as to function declarations:

_CONSTANT: private(constant(bytes1)) = 0xFF
_IMMUTABLE: private(immutable(bytes1))
_balanceOf: private(HashMap[address, uint256])

@private
def _count():
    count += 1

Backwards Compatibility

This VIP introduces a new visibility decorator that is unavailable in older versions, making it inherently backward-incompatible.

Dependencies

N/A.

References

N/A.

Copyright

Copyright and related rights waived via CC0.