Validate output and apply defaults

To validate a value, add constraint keywords to its pairing. To fill a gap, add default:

/profile/handle: { source: /handle, required: true, minLength: 3, pattern: '^[a-z0-9-]+$' }
/profile/plan: { source: /plan, default: free }
/profile/seats: { source: /seats, type: number, minimum: 1 }

A valid input passes through, with the gap filled:

{ "handle": "ada-1", "seats": 2 }

{ "profile": { "handle": "ada-1", "plan": "free", "seats": 2 } }

An invalid input fails the whole mapping, and every failed constraint on the pairing reports. With "handle": "A!" the envelope carries both:

valid: false
errors:
  - { source: /handle, value: 'A!', minLength: 3, message: cannot be less than 3 characters }
  - { source: /handle, value: 'A!', pattern: '^[a-z0-9-]+$', message: must match pattern }

The result is empty. The mapper never returns a partially valid document.

Constraints validate. default finalizes: it runs after validation and only fills undefined. as coerces the type before validation keywords see the value.

Constraints check values that exist. A missing value fails nothing except required, so constrain shape freely and add required: true where absence itself is the error.

See Handle errors for reading the envelope.