Inspect pipelines with explain()

explain() and explainPure() (from @stopcock/fp/fusion/debug) describe what running a set of steps through pipe does at runtime. Since there’s no runtime fusion engine any more, the answer is always the same one:

import { explain } from '@stopcock/fp/fusion/debug'
import * as A from '@stopcock/fp/array'

explain(
  A.map((value: number) => value * 2),
  A.filter((value: number) => value > 10),
  A.take(3),
)
// 'sequential'

explain() returns the string 'sequential' for any set of steps, always. pipe runs each one in order, one call at a time — there’s no plan to report beyond that. The return type also allows 'compiled site', but you will never observe it: if @stopcock/fp-compiler recognizes and fuses a call at build time, the fused loop replaces the call outright, and this function never runs for it. A truly compiled site never calls explain() at all — there’s nothing left there to ask.

Why this function still exists

Steps built from @stopcock/fp operators and ordinary opaque functions now run identically through pipe: in order, one at a time. explain() is a holdover from the pre-2.0 runtime engine, which used to plan tagged operators into streaming segments, materialization boundaries, and specialized executors, and this function described that plan. That plan doesn’t exist any more, so there’s nothing left to describe beyond the one constant answer above.

If you want to know whether a specific call site in your own source will actually get fused, that’s a build-time question, not a runtime one — ask the compiler directly:

stopcock check --strict src

This reports, per call site, whether @stopcock/fp-compiler recognized and fused it or bailed and left it as an ordinary call, and why. See @stopcock/fp-compiler for the diagnostics options and Fusion for what fusing actually changes.

explainPure() is explain() under a different name, kept for source parity with compilePure’s assumePure option. It also always returns 'sequential'.