Slow

[thumphries]

The typechecker is fast, and the parser is fast enough, but the last mile introduces a fair amount of slow.

When this really starts to grate and we decide to do something about it, there's a fair bit of low hanging fruit:

• Serialise intermediate state to disk so we can actually run incrementally
  • maybe we don't want to do this, would be finicky, but is the quickest solution
• Builds should be streaming, currently there's a pointless outer Either restricting throughput
• Fuse various traversals together. We probably walk every expression about 30 times before it hits the disk, for typechecking, type substitution, rewriting, let substitution, backend predicates, warnings, and evaluation/interpretation
• Dependency order could be a rose tree instead of a list, exposing plenty of parallelism
• eval and subst are extremely naive and slow, quadratic even, with some effort we could knock an order of magnitude off

[thumphries]

I benchmarked loom on Friday and subst was a major bottleneck.

I also experimented a bit with parallel codegen and parsing. Codegen saw some payoff from parMap / chunked list strategy, but overall there are too many eithers forcing values prematurely to get much value from this. It knocked about 150ms off overall.