Business rules

Decoding isn't validating

Decode proves the JSON parsed. It says nothing about whether the booking makes sense. Three rules stand between a decoded request and a real booking:

• Known duration — PawWalk sells 30, 45, 60. A request for 37 minutes is nonsense; reject it.
• Not in the past — you can't book a walk for yesterday.
• No double-booking — a walker can't be in two places at once, so their slot must be free.

Each rule that fails is a specific kind of wrong, and the app deserves to be told which. That's what domain errors are for.