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Inference And Trimming

OptParse is built with trimming in mind, so inference quality is not an optional polish step.

What To Test

Current tests generally use:

  • ordinary behavior tests
  • @test_opt for inference-sensitive paths
  • trimming integration tests under test/trim/

When adding a parser family, add:

  • direct primitive or family tests
  • at least one @test_opt on the constructor itself
  • at least one @test_opt through argparse(...)

Common Inference Traps

  1. Loose parse / complete signatures
  2. Returning Context without preserving the state type parameter
  3. Hidden type changes inside helper functions
  4. Broadly typed higher-order helpers
  5. Accidental materialization into abstract containers

JET-Specific Lesson

A runtime path may be perfectly fine while JET still reports a dispatch problem if wrapped-union branches are not trimmed by method signatures.

When that happens, check:

  • whether the parser family method is too broadly typed
  • whether constructor invariants are being expressed in dispatch
  • whether a helper erased the state parameter from Context{S}

Do not paper over this with assertions first. Tighten the family invariants first.

Why Tight State Dispatch Matters

The public Parser wrapper is a @wrapped union over all parser families. That means inference will consider many possible parser-family arms unless your method signatures rule impossible ones out early.

This is why a family-specific method like:

function parse(p::ArgOption{T, OptionState{T}}, ctx::Context{OptionState{T}})::InnerParseResult{OptionState{T}} where {T}

is much better than:

function parse(p::ArgOption, ctx::Context)

The tight version:

  • documents the actual invariant of the family
  • trims impossible union arms sooner
  • keeps JET focused on reachable execution paths
  • usually improves trimming behavior as well

Current Design Notes

Two internal rules are worth keeping in mind while working on inference-sensitive code:

  1. parser families should only operate on their own state shape
  2. Context should be updated through its helper API so the state parameter stays explicit

If inference starts to widen, look there first before adding local assertions.