The Ion 1.1 Draft Specification

Symbols

Symbols With Inline Text

If the high nibble of the opcode is 0xA_, it represents a symbol whose text follows the opcode. The low nibble of the opcode indicates how many UTF-8 bytes follow. Opcode 0xA0 represents a symbol with empty text ('').

0xEB x06 represents null.symbol.

Encoding of a symbol with empty text ('')

┌──── Opcode in range A0-AF indicates a symbol with inline text
│┌─── Low nibble 0 indicates that no UTF-8 bytes follow
A0

Encoding of a symbol with 14 bytes of inline text

┌──── Opcode in range A0-AF indicates a symbol with inline text
│┌─── Low nibble E indicates that 14 UTF-8 bytes follow
││  f  o  u  r  t  e  e  n     b  y  t  e  s
AE 66 6F 75 72 74 65 65 6E 20 62 79 74 65 73
   └──────────────────┬────────────────────┘
                 UTF-8 bytes

Encoding of a symbol with 24 bytes of inline text

┌──── Opcode FA indicates a variable-length symbol with inline text
│  ┌─── Length: FlexUInt 24
│  │   v  a  r  i  a  b  l  e     l  e  n  g  t  h     e  n  c  o  d  i  n  g
FA 31 76 61 72 69 61 62 6C 65 20 6C 65 6E 67 74 68 20 65 6E 63 6f 64 69 6E 67
      └────────────────────────────────┬────────────────────────────────────┘
                                  UTF-8 bytes

Encoding of null.symbol

┌──── Opcode 0xEB indicates a typed null; a byte follows specifying the type
│  ┌─── Null type: symbol
│  │
EB 06

Symbols With a Symbol Address

Symbol values whose text can be found in the local symbol table are encoded using opcodes 0xE1 through 0xE3:

• 0xE1 represents a symbol whose address in the symbol table (aka its symbol ID) is a 1-byte FixedUInt that follows the opcode.
• 0xE2 represents a symbol whose address in the symbol table is a 2-byte FixedUInt that follows the opcode.
• 0xE3 represents a symbol whose address in the symbol table is a FlexUInt that follows the opcode.

Writers MUST encode a symbol address in the smallest number of bytes possible. For each opcode above, the symbol address that is decoded is biased by the number of addresses that can be encoded in fewer bytes.

System Symbols

System symbols (that is, symbols defined in the system module) can be encoded using the 0xEE opcode followed by a 1-byte FixedUInt representing an index in the system symbol table.

Unlike Ion 1.0, symbols are not required to use the lowest available SID for a given text, and system symbols MAY be encoded using other SIDs.

Encoding of the system symbol $ion

┌──── Opcode 0xEF indicates a system symbol or macro invocation
│  ┌─── FixedUInt 1 indicates system symbol 1
│  │
EE 01