Optimizing interceptors with function composition

[grmble]

Effectless interceptors and handlers could be composed, reducing the number of steps for the executor and helping with thread pool overhead for async code.

I made a proof of concept here: https://github.com/grmble/interceptor-optimizer

For detailed explanations see the linked README, but the gist is

• interceptors and handlers can opt into optimization by declaring themselves :composable via metadata - no async results, no changing the flow of control (queue manipulations or catching exceptions)
• the optimizer finds all runs of consecutive composables and composes them

A benchmark for a simple sync example (1 :error, 1 :enter, 1 :leave, 1 handler) gives a 14% improvement. That being said, I care more about composing :leave blocks that come after async code, but that is harder to measure.

It is not througly tested - I only ever ran the one same example, and I only ever used the sieppari executor, but I do think that the approach shows promise.

The benchmark numbers are from https://github.com/grmble/interceptor-optimizer/blob/master/src/grmble/interceptor_optimizer/dev.clj#L39-L58

Edit: updated line numbers

[opqdonut]

Related: metosin/sieppari#51