takeEvents

[martyall]

Add a function takeEvents that can be used as follows

Assume you have some solidity code as in the example project's PaidSimpleStorage

pragma solidity ^0.8.0;

import "openzeppelin-contracts/token/ERC20/IERC20.sol";

contract SimplePaidStorage {
    IERC20 private _token;
    address private _owner;
    uint public count = 0;

    event CountUpdated(uint newCount);

...

    function updateCount(uint _newCount) external {
        require(
            _token.balanceOf(msg.sender) >= 1,
            "Insufficient tokens to update count"
        );
        require(
            _token.allowance(msg.sender, address(this)) >= 1,
            "Token allowance not set for contract"
        );

        _token.transferFrom(msg.sender, address(this), 1);

        count = _newCount;
        emit CountUpdated(_newCount);
    }

...
}
}

If you call this function, two events will get emitted:

1. Transfer coming from the ERC20 when the user pays the contract
2. CountUpdated coming PaidSimpleStorage

If you want to easily catch all the events emitted by a transaction which calls this function, you can now do:

events <- takeEvents someTransaction
      { transfer :: eventFilter (Proxy @ERC20.Transfer) tokenAddress
      , countUpdate :: eventFilter (Proxy @SimplePaidStorage.CountUpdated) paidSimpleStorageAddress
      }
  let (ERC20.Transfer transfer) = unsafePartial $ Array.head events.transfer 
  transfer.value `shouldEqual` events.deposited.value
  transfer.to `shouldEqual` simplePaidStorageAddress

  let (SimplePaidStorage.CountUpdated {newCount} = unsafePartial $ Array.head events.countUpdate
  newCount `shouldEqual` whatever

The events are collected using the transaction hash that caused them. You can actually collect them using any (Applicative f, Monoid (f event), and moreover f can be different for each event if you really wanted. E.g. if the transaction emitted many Transfer events and you only want the first, you could use

  let (SimplePaidStorage.CountUpdated {newCount} = unsafePartial $ Array.head events.countUpdate
  let (ERC20.Transfer transfer) = unsafePartial $ fromJust $ un Last events.transfer 

See the SimplePaidStorageSpec for further usage details