Checkpoint: Errors You Can Trust

The task

Implement app_run and the two stages (see the challenge). The graded surface is the shape of failure: config vs data vs caller-error are three different answers; the report names the failing stage; a failure in config means data never runs. Swallowing, conflating, or reporting success on failure — the classic sins — all fail the tests.

Passing this proves you can design the error dimension of an API, not just its happy path — the difference between code that works and code that can be operated.

Next module: undefined behavior — the errors the compiler is allowed to make for you.