Can we consolidate trade and swap into one helper?

[erights]

I wonder if we really need trade and swap as distinct abstractions. Swap begins by checking if keepSeat has exited. However, it should also error if trySeat has exited. trade should also error if either has exited, so both checks could be moved into trade.

Looking at the callers of trade, a major reason they can't use swap is that the keepSeat remains open. They all do exit the trySeat.

Looking at the callers of swap, since swap throws on failure and only closes the keepSeat on success, it would not be an undue burden for swap's callers to exit the keepSeat after the call to swap. Since we did presciently call it the keepSeat, it would also not be surprising for swap itself not to close it.

_Originally posted by @erights in #1414 (comment)

[katelynsills]

Let's look into checking whether the seats have exited and otherwise making the two functions more uniform but let's make sure to keep swap for the specialized use cases. Open issue in documentation to clarify use cases for each.

[katelynsills]

Just as a study of how we use swap, we currently use it in 3 contracts:

agoric-sdk/packages/zoe/src/contracts/atomicSwap.js

Lines 31 to 34 in da2b92d

	const matchingSeatOfferHandler = matchingSeat => {
	const swapResult = swap(zcf, firstSeat, matchingSeat);
	zcf.shutdown();
	return swapResult;

agoric-sdk/packages/zoe/src/contracts/coveredCall.js

Lines 49 to 54 in da2b92d

	const rejectMsg = `The covered call option is expired.`;
	const exerciseOption = exerciserSeat => {
	const swapResult = swap(zcf, sellerSeat, exerciserSeat, rejectMsg);
	zcf.shutdown();
	return swapResult;

agoric-sdk/packages/zoe/src/contracts/simpleExchange.js

Lines 68 to 79 in da2b92d

	function swapIfCanTrade(offers, seat) {
	for (const offer of offers) {
	const satisfiedBy = (xSeat, ySeat) =>
	satisfies(zcf, xSeat, ySeat.getCurrentAllocation());
	if (satisfiedBy(offer, seat) && satisfiedBy(seat, offer)) {
	swap(zcf, seat, offer);
	// return handle to remove
	return offer;
	}
	}
	return undefined;
	}