Looping

loop repeats a body until it produces a Break signal. The body receives two tokens — recur (restart the loop) and done (exit the loop) — and routes to one of them via branch.

Type-check-and-fix loop

From demos/convert-folder-to-ts/handlers/type-check-fix.ts:

export const typeCheckFix = loop((recur) =>
  typeCheck.then(classifyErrors).branch({
    HasErrors: Iterator.fromArray<TypeError>().map(fix).drop().then(recur),
    Clean: drop,
  }),
);

1. typeCheck runs the TypeScript compiler.
2. classifyErrors returns { kind: "HasErrors", value: TypeError[] } or { kind: "Clean" }.
3. On HasErrors: wrap errors as Iterator, fix each in parallel, then recur to type-check again.
4. On Clean: drop exits the loop.

Loop with retry

From demos/retry-on-error/run.ts:

loop((recur, done) =>
  tryCatch(
    (throwError) =>
      stepA.mapErr(drop).unwrapOr(done).drop()
        .then(withTimeout(constant(2_000), stepB.unwrapOr(throwError))
          .mapErr(constant("stepB: timed out"))
          .unwrapOr(throwError)
          .drop())
        .then(stepC.unwrapOr(throwError).drop()),
    logError.then(recur),
  ),
)

The outer loop provides recur and done. Inside, tryCatch catches errors from the pipeline. On error, logError runs and recur restarts the loop. On catastrophic failure (stepA), done exits immediately.

Adversarial review loop

From demos/identify-and-address-refactors/handlers/refactor.ts:

loop((recur) =>
  judgeRefactor.then(classifyJudgment).branch({
    NeedsWork: applyFeedback.drop()
      .then(params.pick("worktreePath").then(typeCheckFix))
      .drop().then(recur),
    Approved: drop,
  }),
)

A judge evaluates the refactor. If it NeedsWork, feedback is applied, type errors are fixed, and the loop restarts for another review. If Approved, the loop exits.

How it works

Under the hood, loop compiles to a RestartHandle with a Branch. See algebraic effect handlers for the full compilation.