Errors And Results

build and use return Result values. Expected construction failures do not throw through your program.

const built = await build(graph)

if (Result.isErr(built)) {
  console.error(built.error._tag)
}

Result

Result<A, E> is deliberately small.

type Result<A, E> =
  | { readonly _tag: 'ok'; readonly value: A }
  | { readonly _tag: 'err'; readonly error: E }

Use Result.isOk, Result.isErr, Result.map, Result.mapError, and Result.match for boring control flow. Boring is underrated. Most production incidents are exciting enough already.

BuildError

Build failures are normalized into four variants.

Variant	Meaning
`missing_service`	A dependency required a tag absent from the build environment.
`duplicate_service`	Two built contexts tried to expose the same output tag.
`circular_dependency`	The evaluator detected a cycle through memoized construction.
`construction_failed`	A constructor returned `Result.err` or threw/rejected.

construction_failed distinguishes typed errors from defects with a cause field:

if (error._tag === 'construction_failed') {
  if (error.cause === 'error') {
    console.error(error.error)
  } else {
    console.error(error.defect)
  }
}

Paths

Every BuildError includes a path showing where construction was when the error happened.

Use it for diagnostics, logs, and test assertions. Do not parse it as a public protocol unless you enjoy making your future self mutter at a laptop.