Stabilize discriminated union reducer

typescriptMEDIUMFILL IN CODE

Stabilize discriminated union reducer

Medium TypeScript arena: tagged unions, exhaustive checks, and reducer safety

XP reward

180

Coins

Target time

24m

Mission locked

Missing prerequisites: typescript-easy-discriminated-union

A billing reducer accepts several action shapes and silently ignores new actions. You are working inside a TypeScript monorepo, and the arena only clears when the result is safe, deterministic, and explainable.

Learning objective

Learn to apply tagged unions, exhaustive checks, and reducer safety in a TypeScript monorepo while explaining the invariant, safety constraints, and hidden edge cases.

Mission order

Actions include invoice.created, invoice.paid, invoice.failed, and invoice.refunded. Handle normal data, malformed data, and deterministic ordering before optimizing. Submit the solution plus enough reasoning to pass hidden edge cases.

Visible checks

sample case

Expected: 2

empty input

Expected: 0

Clear requirements

Demonstrates tagged unions, exhaustive checks, and reducer safety
Handles the visible sample and hidden edge cases
Keeps output deterministic and explainable
Avoids unsafe dynamic execution

Secure validation contract

Judge type

FILL IN CODE

Complexity target

Meets expected complexity under hidden scale cases

Workspace

Code editor

locked

Clear the route first

You can read the mission brief, but the editor, hints, tests, and submit flow stay locked until progression or plan access catches up.

Enter Immersive

Test Results

Run the visible checks when your first pass is ready.

Clear Protocol

1Read the scenario and restate the expected output shape.

2Run visible checks before chasing hidden edge cases.

3Use Genie for one nudge if stuck, then explain the invariant.

4Submit only when the result is deterministic and safe.

Rewards

180

Coins

Mission Route

Previous Next

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Hints

Hints are metered and logged for No Hint Hero runs.

Genie Mentor Core

Hint protocol / contextual guardrails active

Progressive hints

Failed-test aware

Solution guarded

Mission: typescript-medium-discriminated-union0 attempts0 failed tests0 hints used

Progressive hint depth

Genie: Genie online. I use your mission, attempts, failed tests, hints, and path context to coach the next rep without dumping answers first.

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typescriptMEDIUMFILL IN CODE