The Ion 1.1 Draft Specification

Encoding Expressions

E-expression with the address in the opcode

Opcodes 0x00-0x47 are single byte macro addresses.

If the value of the opcode is less than 72 (0x48), it represents an E-expression invoking the macro at the corresponding address—-an offset within the local macro table.

Invocation of macro address 7

┌──── Opcode in 00-47 range indicates an e-expression
│     where the opcode value is the macro address
│
07
└── FixedUInt 7

Invocation of macro address 31

┌──── Opcode in 00-47 range indicates an e-expression
│     where the opcode value is the macro address
│
1F
└── FixedUInt 31

Note that the opcode alone tells us which macro is being invoked, but it does not supply enough information for the reader to parse any arguments that may follow. The parsing of arguments is described in detail in the section E-expression argument encoding.

E-expressions with extended addresses

Opcodes 0x48-0x4F are extensible macro addresses, with an offset of 72. The opcodes 0x48 through 0x4F share the same 5 most-significant bits. The 3 least-significant bits are used as the 3 least-significant bits of the macro address. The opcode is followed by a FlexUInt, which, once decoded, represents the most-significant bits of the macro address. Finally, the offset of 72 is added.

To get the macro address from the opcode and FlexUInt is straightforward, and can be implemented using bitwise operations or simple arithmetic operations.

// Given an `opcode` and `flexUInt`...
let lsb = opcode & 0b111           // or opcode - 0x48
let msb = flexUInt << 3            // or flexUInt * 8
let macroId = (msb | lsb) + 72     // or msb + lsb + 72

The reverse transformation is also simple:

// Given `macroId`...
let opcode = 0x48 | (macroId & 0b111)   // or 0x48 + (macroId % 8)
let flexUInt = macroId >>> 3            // or macroId / 8 

This table stops at 16777287, but the encoding does not impose any limit on the number of macro addresses. Practically, Ion implementations will have limits based on the programming language and the runtime environment used.

Invocation of macro address 287

To encode macro address 287:

• Subtract 72 to get 215 (0b11010111)
• Take the 3 least-significant bits (111) and add them to 0x48 (0b01001000) to get 0x4F (0b01001111). The opcode will be 0x4F.
• Shift 215 left by 3 (discarding the 3 least-significant bits), and then encode the result (26) as a FlexUInt. The FlexUInt encoding of 26 is 0x35.

┌──── Opcode in range 48-4F indicates a macro address with extended address.
│     Least-significant 3 bits are `111`
│  ┌──── FlexUInt 26
4F 35

Length-prefixed E-Expressions

The opcode F4 represents an E-expression with a FlexUInt macro address and a FlexUInt length prefix. The length prefix indicates the number of argument bytes for the e-expression. The encoding of the arguments themselves are covered in E-expression argument encoding.

┌──── Opcode F4 indicates FlexUInt address and FlexUInt length prefix     
│  ┌──── FlexUInt 26
│  │   ┌──── FlexUInt 6
F4 35 0D __ __ __ __ __ __
         └───────┬───────┘
          6 argument bytes

E-expression argument encoding

The example invocations in prior sections have demonstrated how to encode an invocation of the simplest form of macro--one with no parameters. This section explains how to encode macro invocations when they take parameters.

The encoding of E-Expression arguments follows the macro address (and length-prefix if present). For every placeholder in the macro template, there must be exactly one argument expression provided.

Tagged Arguments

When a macro parameter does not specify an encoding (the parameter name is not annotated), arguments passed to that parameter use the 'tagged' encoding. The argument begins with a leading opcode that dictates how to interpret the bytes that follow.

This is the same encoding used for values in other Ion 1.1 contexts like lists, s-expressions, or at the top level.

When invoking a template macro, the E-expression must have one argument for each parameter in the macro signature. Every argument must be exactly one value or explicitly an absent argument.

The absent argument is a special construct used in macro invocations to explicitly indicate that no value is provided for a particular parameter. The absent argument is distinct from NOP and serves a different purpose. Opcode 0xEB indicates an absent argument; no additional bytes follow.

Example – two tagged arguments

Given a macro definition:

(point { x: (:?), y: (:? 0) })

This macro has two tagged parameters, the second of which has a default value.

Encoding of (:point 2 3)

┌──── E-expression with macro address 0 (assuming point is macro 0)
│  ┌─── Argument 1: integer 2
│  │     ┌─── Argument 2: integer 3
00 61 02 61 03

This would expand to { x: 2, y: 3 }.

Encoding of (:point 5 (:))

┌──── E-expression with macro address 0 (assuming point is macro 0)
│  ┌─── Argument 1: integer 5
│  │     ┌─── Argument 2: absent argument
00 61 05 EB

This would expand to { x: 5, y: 0 } since the second argument is absent and y has a default value of 0.

Encoding of (:point (:) 10)

┌──── E-expression with macro address 0
│  ┌─── Argument 1: absent argument
│  │  ┌─── Argument 2: integer 10
00 EB 61 0A

This would expand to { y: 10 } since the first argument is absent and x has no default value.

An absent argument is encoded the same way regardless of whether the placeholder has a default value.

Tagless Arguments

In contrast to the tagged encoding, tagless encodings do not begin with an opcode. This means that they are potentially more compact than a tagged type, but are also less flexible. Because tagless encodings do not have an opcode, tagless arguments cannot have annotation sequences nor can the argument itself be absent.

Primitive encodings are self-delineating, either by having a statically known size in bytes or by including length information in their serialized form.

Given the following macro definition

(foo { foo: (:?\int8\), bar: (:?\int16\), baz: (?:\string\) })

The text E-expression (:foo 1 2 "three") would be encoded like this:

┌──── Opcode 0x00 is less than 0x48; this is an e-expression
│     invoking the macro at address 0.
│  ┌─── First argument: 1-byte FixedInt 1
│  │    ┌─── Second argument: 2-byte FixedInt 2
│  │    │           ┌─── Third argument: length-prefixed string "three"
│  │  ┌─┴─┐ ┌───────┴───────┐    
00 03 02 00 0B 74 68 72 65 65
            │  └──────┬─────┘
            │         └── 5 UTF-8 bytes
            └──────────── FlexUInt (Length) 5