Interface Tracer

A sink for structured trace events emitted by TracedOption chains.

Type-only — no runtime, no opinion on output format, no console-global dependency. Same rationale as Logger: core names the contract, consumers own the implementation. The whole surface is one method, so the common implementations are literals at the call site:

const noop: Tracer = { emit: () => undefined }
const collecting = (target: TraceEvent[]): Tracer => ({ emit: (e) => void target.push(e) })
const viaLogger = (logger: Logger): Tracer => ({ emit: (e) => logger.debug("trace", e) })
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For dependency injection, declare your own Tag — Tag and Layer are public from functype/io, and core deliberately does not reserve an id:

const Tracer = Tag<Tracer>("app/Tracer")
program.provideLayer(Layer.succeed(Tracer, { emit }))
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