Suppose I want to trade ARDOR for IGNIS. As I understand it, any transaction that changes an account's ARDOR balance has to be stored on the Ardor chain, and cannot be pruned, for security reasons. Am I right in thinking this means even though I am using IGNIS to buy the tokens, I also need some ARDOR to pay for transaction fees?As far as I know the only way to sell ARDOR token is to send it manually to the buyer since there is no AE on Ardor chain. In your case as a buyer you need to pay IGNIS fee and as a seller you need to pay ARDOR fee.
There will be an Asset Exchange on the Ignis child-chain. I would hope transactions between child-chains would be possible. They raise the same issues as transactions between child-chains and the Ardor chain. Maybe there is only one AE across all the child-chains, and offers to sell tokens can only be issued on that token's chain, so the fees would always be in the same token.
I do hope it is possible to trade Ardor in a simple and trust-free way. I was thinking the buy and sell orders would be issued and matched on the Ignis child-chain, and then for the trade to generate an extra transaction on the Ardor chain to represent the change in Ardor balance. The transactions would be linked so they go through atomically (ie, if one fails they both fail).
I'm speculating. Has this part of the system been designed yet?
If I forge an Ardor block, and include some Ignis transactions that I bundled myself, and therefore gained some IGNIS fees, can I sell those fees for Ardor in the same block?You may be able to send IGNIS in the same block (I doubt it - try sending NXT you received in the same block) but you will likely not get ARDOR in the same block since it requires great coordination with the seller.
The Nxt situation isn't very comparable. In order to forge you need at least 1000 NXT. In order to try what you suggest, I'd need a much larger balance in order to forge in reasonable time, and then try to transfer that amount + the fees received in that block. I'm not going to bother. Selling/sending IGNIS would not affect the balance needed for forging Ardor, so it's a very different situation.
I guess it doesn't much matter, though. You could keep a float of the child-chain tokens, and sell/send some of those old tokens while replenishing them with an equal number of new ones from fees. The value of the float could be small. (I'm exploring how much faith a forger needs to have in a child-chain in order to bundle it themselves rather than accept Ardor fees from a third-party bundler.)