A real-life example

Named records, a required array, an optional field, and recursion-free reuse — built once, validated across formats.

An order schema, combining several of the ideas from earlier steps into one working example:

ORDER = '''
record Address  { "street": string, "city": string }
record LineItem { "sku": string, "qty": integer, "price": number }
record Order {
    "id":       string,
    "status":   string,
    "total":    number,
    "address":  Address,
    "items" [1,]: LineItem,        # at least one line item
    "coupon" [0,1]: string,         # optional
}
root Order
'''
s = parse_schema(ORDER)

The records form a graph, linked by references, each with the field's cardinality attached:

graph LR
    Order["Order"] -->|"address [1,1]"| Address["Address"]
    Order -->|"items [1,]"| LineItem["LineItem"]

good = Doc.from_oml('''
id: "A1"
status: "shipped"
total: 29.97
address: { street: "1 Main St"; city: "London" }
items: { sku: "W"; qty: 3; price: 9.99 }
''')
s.validate(good).ok

True

As a tree of labeled edges, this is the same document every format above would read into:

graph LR
    order["(root)"] --> id["id: A1"]
    order --> status["status: shipped"]
    order --> total["total: 29.97"]
    order --> address["address"]
    address --> street["street: 1 Main St"]
    address --> city["city: London"]
    order --> items["items"]
    items --> sku["sku: W"]
    items --> qty["qty: 3"]
    items --> price["price: 9.99"]

bad = Doc.from_oml('''
id: "A2"
status: "shipped"
total: "ten"
address: { street: "x"; city: "y" }
''')
print(s.validate(bad))

invalid:
  at $.total: expected number, got string ('ten')
  at $: field 'items' occurs 0 time(s), expected at least 1

Two independent problems, both reported — a missing type match and a missing required field — since validation collects every failure rather than stopping at the first.