Examples

The dynamic API and the generated API write the same Ion. Use the dynamic API when the shape is not known ahead of time. Use a generated struct when a schema already names the fields.

Dynamic document

examples/encode_value.mojo decodes one struct and writes it twice.

from ion import Catalog, EncodeOptions, decode, encode

fn main() raises:
    var cat = Catalog()
    var doc = decode(String("{a:1,b:\"hi\"}\n").as_bytes(), cat)
    var text = encode(doc, EncodeOptions(False, False, 10), cat)
    var binary = encode(doc, EncodeOptions(True, False, 11), cat)
    print("text bytes", len(text))
    print("ion 1.1 binary bytes", len(binary))

Run it from a checkout:

pixi run mojo run -I src examples/encode_value.mojo

Ion 1.0 text starts with $ion_1_0. Ion 1.1 binary starts with E0 01 01 EA and stores the field name a as inline symbol text, so the stream has no symbol table.

Build a value without parsing

from runtime.doc import IonDoc

fn main() raises:
    var doc = IonDoc()
    var st = doc.start_container(13)
    var field = doc.add_symbol_text("n")
    doc.add_child(st, doc.add_i64(150), doc.nodes[field].a)
    doc.add_top(st)

13 is the struct kind. add_i64 accepts any Int64. Wider integers go through add_int and a BigInt. add_decimal keeps the coefficient and the exponent, including a negative zero. add_time keeps precision and offset.

Generated struct

testdata/schema/message.json describes a closed object. After pixi run generate, tests/generated/Message.mojo has the fields below.

Schema field	Mojo field	Required
id	Int64	yes
name	String	yes
ok	Bool	no
weight	Float64	no
tags	List[String]	no
when	String holding Ion timestamp text	no
payload	List[Byte]	no

from runtime.options import EncodeOptions
from Message import Message

fn main() raises:
    var msg = Message()
    msg.id = Int64(7)
    msg.name = String("ada")
    msg.has_when = True
    msg.when = "2023T"
    var buf = List[Byte]()
    msg.encode_to(buf, EncodeOptions(True, False, 10))
    var back = Message.decode_from(buf)

Optional fields use a has_ flag. A missing flag omits the field. A closed schema raises DecodeError.KIND_SCHEMA when a required field is absent or an unknown field is present.

testdata/schema/point.ion is Ion Schema. label is a symbol, so the generated String holds Ion symbol text such as foo, not a quoted string. encode_to with version 11 writes an Ion 1.1 binary struct.