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This paper presents a general framework for the design and analysis of
exchange mechanisms between two assets that unifies and enables comparisons
between the two dominant paradigms for exchange, constant function market
markers (CFMMs) and limit order books (LOBs). In our framework, each liquidity
provider (LP) submits to the exchange a downward-sloping demand curve,
specifying the quantity of the risky asset it wishes to hold at each price; the
exchange buys and sells the risky asset so as to satisfy the aggregate
submitted demand. In general, such a mechanism is budget-balanced and enables
price discovery. Different exchange mechanisms correspond to different
restrictions on the set of acceptable demand curves. The primary goal of this
paper is to formalize an approximation-complexity trade-off that pervades the
design of exchange mechanisms. For example, CFMMs give up expressiveness in
favor of simplicity: the aggregate demand curve of the LPs can be described
using constant space, but most demand curves cannot be well approximated by any
function in the corresponding single-dimensional family. LOBs, intuitively,
make the opposite trade-off: any downward-slowing demand curve can be well
approximated by a collection of limit orders, but the space needed to describe
the state of a LOB can be large. This paper introduces a general measure of
{\em exchange complexity}, defined by the minimal set of basis functions that
generate, through their conical hull, all of the demand functions allowed by an
exchange. With this complexity measure in place, we investigate the design of
{\em optimally expressive} exchange mechanisms, meaning the lowest complexity
mechanisms that allow for arbitrary downward-sloping demand curves to be well
approximated. As a case study, we interpret the complexity-approximation
trade-offs in the widely-used Uniswap v3 AMM through the lens of our framework.