Tagless Encodings

Tagless encodings may be specified by encoding tags in template placeholders and in tagless-element sequences. In binary, this allows the opcode to be elided from the encoding of values that fill tagless slots.

Consider the following data:

[
  { sum: 123, sample_count: 12 },
  { sum: 54, sample_count: 8 },
  { sum: 125, sample_count: 15 },
  { sum: 314, sample_count: 30 },
]

With a macro such as (metric {sum: (:? {#int}), sample_count: (:? {#int}), unit: ms}), it can be encoded using a tagless-element list as follows

[{:metric} // {:<macro_name>} specifies a macro-shape
  (123 12),
  (54 8),
  (125 15),
  (314 30),
]

The available tagless encodings are enumerated in the following table.

Ion Type	Encoding Name	Size in bytes	Binary Encoding Tag	Encoding
`int`	`int`	variable	`0x60`	[`FlexInt`][flexint]
	`int8`	1	`0x61`	[`FixedInt`][fixedint]
	`int16`	2	`0x62`	[`FixedInt`][fixedint]
	`int32`	4	`0x64`	[`FixedInt`][fixedint]
	`int64`	8	`0x68`	[`FixedInt`][fixedint]
	`uint`	variable	`0xE0`	[`FlexUInt`][flexuint]
	`uint8`	1	`0xE1`	[`FixedUInt`][fixeduint]
	`uint16`	2	`0xE2`	[`FixedUInt`][fixeduint]
	`uint32`	4	`0xE4`	[`FixedUInt`][fixeduint]
	`uint64`	8	`0xE8`	[`FixedUInt`][fixeduint]
`float`	`float16`	2	`0x6B`	[Little-endian IEEE-754 half-precision float][f16]
	`float32`	4	`0x6C`	[Little-endian IEEE-754 single-precision float][f32]
	`float64`	8	`0x6D`	[Little-endian IEEE-754 double-precision float][f64]
`decimal`	`small_decimal`	variable (2+)	`0x70`	Tuple of (`int`,`int8`) representing the coefficient and exponent respectively.
`timestamp`	`timestamp_day`	2	`0x82`	[Day precision timestamp][t1]
	`timestamp_min`	4	`0x83`	[Minute precision timestamp][t2]
	`timestamp_s`	5	`0x84`	[Second precision timestamp][t3]
	`timestamp_ms`	6	`0x85`	[Millisecond precision timestamp][t4]
	`timestamp_us`	7	`0x86`	[Microsecond precision timestamp][t5]
	`timestamp_ns`	8	`0x87`	[Nanosecond precision timestamp][t6]
`symbol`	`symbol`	variable	`0xEE`	[`FlexSym`][flexsym]

Although both primitive and macro-shape encodings may be used in tagless-element sequences, only the primitive encodings in the above table may be used in tagless template placeholders.

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The Ion 1.1 Draft Specification

Tagless Encodings