OrderBook: Use ratio instead of rate

[lemunozm]

Description

Ideally, order-book should use an 18-decimal fixed point number too.

Changes and Descriptions

• Change type in dev runtime.
• Remove dependency from Rate type in tests/benchs.

[mustermeiszer]

Why ideally? Changes look fine though

[lemunozm]

As I see it, the SellRate is used to denominate prices in units of other prices. So, they are specifying quantities or ratios, not 27-decimal rates (whose purpose is to be used for accruing interest per second). Actually, if some currency is 10^9 more valued than others, with 27-decimal, we can no longer use order-book for those.

[wischli]

Thanks for picking this up! LGTM

[wischli]

Actually, if some currency is 10^9 more valued than others, with 27-decimal, we can no longer use order-book for those.

Thanks so much for making this explicit! Given that DAI has 18 decimals and USDT having only 6, this fix is a must-have.

[lemunozm]

DAI has 18 decimals and USDT having only 6

To expand this, here we have two kinds of relations:

• The amount of DAIs in 1 USDT (how many DAIs compound an USDT, that now we suppose 1:1).
• The precission difference between both.

Right now, I do not know if SellRate contains the information of one or both of the above relations. If it also tracks the second relation about precision, then it's a must-have, as you've said.