Docs/ Specification

The Amazon Ion specification has three parts:

All three views are semantically isomorphic, meaning they can represent exactly the same data structures, and an Ion processor can transcode between the formats without loss of data. This allows applications to optimize different areas for different uses – say, using text for human readability and binary for compact persistence – by transcribing between the formats with almost complete fidelity. (“Almost” because converting from text to binary does not preserve whitespace and comments.)

The Ion text encoding is intended to be easy to read and write. It may be more suitable for streaming applications since sequences don’t need to be length-prefixed. Whitespace is insignificant and is only required where necessary to separate tokens. C-style comments (either block or in-line) are treated as whitespace, and are not part of the binary encoding.

The binary encoding is much more compact and efficient. An important feature is that parts of the whole can be accessed without “preparation”, meaning you don’t have to load it into another form before accessing the values.

The Ion Data Model

The semantic basis of Ion is an abstract data model, composed of a set of primitive types and a set of recursively-defined container types. All types support null values and user-defined type annotations.

It’s important to note that the data model is value-based and does not include references. As a result, the data model can express data hierarchies (we can nest things to arbitrary depth), but not general directed graphs.

Here’s an overview of the core data types:

Primitive Types

Null Values

Ion supports distinct null values for every core type, as well as a separate null type that’s distinct from all other types.

The null type has a single value, denoted in the text format by the keyword null. Null values for all core types are denoted by suffixing the keyword with a period and the desired type. Thus we can enumerate all possible null values as follows:

null.null       // Identical to unadorned null

The text format treats all of these as reserved tokens; to use those same characters as a symbol, they must be enclosed in single-quotes:

null        // The type is null
'null'      // The type is symbol
null.list   // The type is list
''  // The type is symbol

(As a historical aside, the null type exists primarily for compatibility with JSON, which has only the untyped null value.)


The bool type is self-explanatory, but note that (as with all Ion types) there’s a null value. Thus the set of all Boolean values consists of the following three reserved tokens:


(As with the null values, one can single-quote those tokens to force them to be parsed as symbols.)


The int type consists of signed integers of arbitrary size. The binary format uses a very compact encoding that uses “just enough” bits to hold the value.

The text format allows hexadecimal and binary (but not octal) notation, but such notation will not be maintained during binary-to-text conversions. It also allows for the use of underscores to separate digits.   // A null int value
0          // Zero.  Surprise!
-0         //   ...the same value with a minus sign
123        // A normal int
-123       // Another negative int
0xBeef     // An int denoted in hexadecimal
0b0101     // An int denoted in binary
1_2_3      // An int with underscores
0xFA_CE    // An int denoted in hexadecimal with underscores
0b10_10_10 // An int denoted in binary with underscores

+1         // ERROR: leading plus not allowed
0123       // ERROR: leading zeros not allowed (no support for octal notation)
1_         // ERROR: trailing underscore not allowed
1__2       // ERROR: consecutive underscores not allowed
0x_12      // ERROR: underscore can only appear between digits (the radix prefix is not a digit)
_1         // A symbol (ints cannot start with underscores)

In the text notation, integer values must be followed by one of the fifteen numeric stop-characters: {}[](),\"\'\ \t\n\r\v\f.

Real Numbers

Ion supports both binary and lossless decimal encodings of real numbers as, respectively, types float and decimal. In the text format, float values are denoted much like the decimal formats in C or Java; decimal values use d instead of e to start the exponent. Reals without an exponent are treated as decimal. As with JSON, extra leading zeros are not allowed. Digits may be separated with an underscore.

null.decimal      // A null decimal value
null.float        // A null float value

0.123             // Type is decimal
-0.12e4           // Type is float
-0.12d4           // Type is decimal

0E0               // Zero as float
0D0               // Zero as decimal
0.                //   ...the same decimal value with different notation
-0e0              // Negative zero float   (distinct from positive zero)
-0d0              // Negative zero decimal (distinct from positive zero)
-0.               //   ...the same decimal value with different notation
-0d-1             // Decimal maintains precision: -0. != -0.0

123_456.789_012   // Decimal with underscores

123_._456         // ERROR: underscores may not appear next to the decimal point
12__34.56         // ERROR: consecutive underscores not allowed
123.456_          // ERROR: trailing underscore not allowed
-_123.456         // ERROR: underscore after negative sign not allowed
_123.456          // ERROR: the symbol '_123' followed by an unexpected dot

The float type denotes either 32-bit or 64-bit IEEE-754 floating-point values; other sizes may be supported in future versions of this specification.

In the text notation, real values must be followed by one of the fifteen numeric stop-characters: {}[](),\"\'\ \t\n\r\v\f.

The precision of decimal values, including trailing zeros, is significant and is preserved through round-trips. Because most decimal values cannot be represented exactly in binary floating-point, float values may change “appearance” and precision when reading or writing Ion text.

See also Ion Float and Ion Decimals for more notes.


Timestamps represent a specific moment in time, always include a local offset, and are capable of arbitrary precision.

In the text format, timestamps follow the W3C note on date and time formats, but they must end with the literal “T” if not at least whole-day precision. Fractional seconds are allowed, with at least one digit of precision and an unlimited maximum. Local-time offsets may be represented as either hour:minute offsets from UTC, or as the literal “Z” to denote a local time of UTC. They are required on timestamps with time and are not allowed on date values.

Ion follows the “Unknown Local Offset Convention” of RFC3339:

If the time in UTC is known, but the offset to local time is unknown, this can be represented with an offset of “-00:00”. This differs semantically from an offset of “Z” or “+00:00”, which imply that UTC is the preferred reference point for the specified time. RFC2822 describes a similar convention for email.

Values that are precise only to the year, month, or date are assumed to be UTC values with unknown local offset.

null.timestamp                   // A null timestamp value

2007-02-23T12:14Z                // Seconds are optional, but local offset is not
2007-02-23T12:14:33.079-08:00    // A timestamp with millisecond precision and PST local time
2007-02-23T20:14:33.079Z         // The same instant in UTC ("zero" or "zulu")
2007-02-23T20:14:33.079+00:00    // The same instant, with explicit local offset
2007-02-23T20:14:33.079-00:00    // The same instant, with unknown local offset

2007-01-01T00:00-00:00           // Happy New Year in UTC, unknown local offset
2007-01-01                       // The same instant, with days precision, unknown local offset
2007-01-01T                      //    The same value, different syntax.
2007-01T                         // The same instant, with months precision, unknown local offset
2007T                            // The same instant, with years precision, unknown local offset

2007-02-23                       // A day, unknown local offset 
2007-02-23T00:00Z                // The same instant, but more precise and in UTC
2007-02-23T00:00+00:00           // An equivalent format for the same value
2007-02-23T00:00:00-00:00        // The same instant, with seconds precision

2007                             // Not a timestamp, but an int
2007-01                          // ERROR: Must end with 'T' if not whole-day precision, this results as an invalid-numeric-stopper error
2007-02-23T20:14:33.Z            // ERROR: Must have at least one digit precision after decimal point.

Zero and negative dates are not valid, so the earliest instant in time that can be represented as a timestamp is Jan 01, 0001. As per the W3C note, leap seconds cannot be represented.

Two timestamps are only equivalent if they represent the same instant with the same offset and precision. This means that the following are not equivalent:

2000T                            // January 1st 2000, year precision, unknown local offset
2000-01-01T00:00:00Z             // January 1st 2000, second precision, UTC
2000-01-01T00:00:00.000Z         // January 1st 2000, millisecond precision, UTC
2000-01-01T00:00:00.000-00:00    // January 1st 2000, millisecond precision, negative zero local offset

In the text notation, timestamp values must be followed by one of the fifteen numeric stop-characters: {}[](),\"\'\ \t\n\r\v\f.


Ion string values are Unicode character sequences of arbitrary length.

In the text format, strings are delimited by double-quotes and follow C/Java backslash-escape conventions (see below). The binary format always uses UTF-8 encoding.

null.string            // A null string value
""                     // An empty string value
" my string "          // A normal string
"\""                   // Contains one double-quote character
"\uABCD"               // Contains one unicode character

xml::"<e a='v'>c</e>"  // String with type annotation 'xml'

Long Strings

The text format supports an alternate syntax for “long strings”, including those that break across lines. Sequences bounded by three single-quotes (‘’’) can cross multiple lines and still count as a valid, single string. In addition, any number of adjacent triple-quoted strings are concatenated into a single value. The concatenation happens within the Ion text parser and is neither detectable via the data model nor applicable to the binary format. Note that comments are always treated as whitespace, so concatenation still occurs when a comment falls between two long strings.

( '''hello '''     // Sexp with one element
  '''world!'''  )

("hello world!")   // The exact same sexp value

// This Ion value is a string containing three newlines. The serialized
// form's first newline is escaped into nothingness.
The first line of the string.
This is the second line of the string,
and this is the third line.

Escape Characters

The Ion text format supports escape sequences only within quoted strings and symbols. Ion supports most of the escape sequences defined by C++, Java, and JSON.

The following sequences are allowed:

Unicode Code Point Ion Escape Meaning
U+0000 \0 NUL
U+0007 \a alert BEL
U+0008 \b backspace BS
U+0009 \t horizontal tab HT
U+000A \n linefeed LF
U+000B \v vertical tab VT
U+000C \f form feed FF
U+000D \r carriage return CR
U+0022 \" double quote
U+0027 \' single quote
U+002F \/ forward slash
U+003F \? question mark
U+005C \\ backslash
nothing \NL escaped NL expands to nothing
U+00HH \xHH 2-digit hexadecimal Unicode code point
U+HHHH \uHHHH 4-digit hexadecimal Unicode code point
U+HHHHHHHH \UHHHHHHHH 8-digit hexadecimal Unicode code point

Any other sequence following a backslash is an error.

Note that Ion does not support the following escape sequences:


Symbols are much like strings, in that they are Unicode character sequences. The primary difference is the intended semantics: symbols represent semantic identifiers as opposed to textual literal values. Symbols are case sensitive.

In the text format, symbols are delimited by single-quotes and use the same escape characters.

A subset of symbols called identifiers can be denoted in text without single-quotes. An identifier is a sequence of ASCII letters, digits, or the characters $ (dollar sign) or _ (underscore), not starting with a digit.

null.symbol  // A null symbol value
'myVar2'     // A symbol
myVar2       // The same symbol
myvar2       // A different symbol
'hi ho'      // Symbol requiring quotes
'\'ahoy\''   // A symbol with embedded quotes
''           // The empty symbol

Within S-expressions, the rules for unquoted symbols include another set of tokens: operators. An operator is an unquoted sequence of one or more of the following nineteen ASCII characters: !#%&*+-./;<=>?@^`|~ Operators and identifiers can be juxtaposed without whitespace:

( 'x' '+' 'y' )  // S-expression with three symbols
( x + y )        // The same three symbols
(x+y)            // The same three symbols
(a==b&&c==d)     // S-expression with seven symbols

Note that the data model does not distinguish between identifiers, operators, or other symbols, and that – as always – the binary format does not retain whitespace.

See Ion Symbols for more details about symbol representations and symbol tables.


The blob type allows embedding of arbitrary raw binary data. Ion treats such data as a single (though often very large) value. It does no processing of such data other than passing it through intact.

In the text format, blob values are denoted as RFC 4648-compliant Base64 text within two pairs of curly braces.

When parsing blob text, an error must be raised if the data:

Within blob values, whitespace is ignored. Comments within blobs are not supported: the / character is always considered part of the Base64 data and the * is invalid.

// A null blob value

// A valid blob value with zero padding characters.

// A valid blob value with one required padding character.
{{ VG8gaW5maW5pdHkuLi4gYW5kIGJleW9uZCE= }}

// ERROR: Incorrect number of padding characters.
{{ VG8gaW5maW5pdHkuLi4gYW5kIGJleW9uZCE== }}

// ERROR: Padding character within the data.
{{ VG8gaW5maW5pdHku=Li4gYW5kIGJleW9uZCE= }}

// A valid blob value with two required padding characters.
{{ dHdvIHBhZGRpbmcgY2hhcmFjdGVycw== }}

// ERROR: Invalid character within the data.
{{ dHdvIHBhZGRpbmc_gY2hhcmFjdGVycw= }}


The clob type is similar to blob in that it holds uninterpreted binary data. The difference is that the content is expected to be text, so we use a text notation that’s more readable than Base64.

In the text format, clob values use similar syntax to blob, but the data between braces must be one string. The string may only contain legal 7-bit ASCII characters, using the same escaping syntax as string and symbol values. This guarantees that the value can be transmitted unscathed while remaining generally readable (at least for western language text). Either form of comment within a clob is invalid.

Strings and Clobs gives details on the subtle, but profound, differences between Ion strings and clobs.

null.clob  // A null clob value

{{ "This is a CLOB of text." }}

shift_jis ::
  '''Another clob with user-defined encoding, '''
  '''this time on multiple lines.'''

  // ERROR
  "comments not allowed"

Note that the shift_jis type annotation above is, like all type annotations, application-defined. Ion does not interpret or validate that symbol; that’s left to the application.

Container Types

Ion defines three container types: structures, lists, and S-expressions. These types are defined recursively and may contain values of any Ion type.


Structures are unordered collections of name/value pairs. The names are symbol tokens, and the values are unrestricted. Each name/value pair is called a field.

When two fields in the same struct have the same name we say there are “repeated names” or (somewhat misleadingly) “repeated fields”. Implementations must preserve all such fields, i.e., they may not discard fields that have repeated names. However, implementations may reorder fields (the binary format identifies structs that are sorted by symbolID), so certain operations may lead to nondeterministic behavior.

In the text format, structures are wrapped by curly braces, with a colon between each name and value, and a comma between the fields. For the purposes of JSON compatibility, it’s also legal to use strings for field names, but they are converted to symbol tokens by the parser.

null.struct                         // A null struct value
{ }                                 // An empty struct value
{ first : "Tom" , last: "Riddle" }  // Structure with two fields
{"first":"Tom","last":"Riddle"}     // The same value with confusing style
{center:{x:1.0, y:12.5}, radius:3}  // Nested struct
{ x:1, }                            // Trailing comma is legal in Ion (unlike JSON)
{ "":42 }                           // A struct value containing a field with an empty name
{ x:1, }                 // WARNING: repeated name 'x' leads to undefined behavior
{ x:1, , }                          // ERROR: missing field between commas

Note that field names are symbol tokens, not symbol values, and thus may not be annotated. The value of a field may be annotated like any other value. Syntactically the field name comes first, then annotations, then the content.

{ annotation:: field_name: value }     // ERROR
{ field_name: annotation:: value }     // Okay


Lists are ordered collections of values. The contents of the list are heterogeneous (that is, each element can have a different type). Homogeneous lists are not supported by the core type system, but may be imposed by schema validation tools.

In the text format, lists are bounded by square brackets and elements are separated by commas.

null.list         // A null list value
[]                // An empty list value
[1, 2, 3]         // List of three ints
[ 1 , two ]       // List of an int and a symbol
[a , [b]]         // Nested list
[ 1.2, ]          // Trailing comma is legal in Ion (unlike JSON)
[ 1, , 2 ]        // ERROR: missing element between commas


An S-expression (or symbolic expression) is much like a list in that it’s an ordered collection of values. However, the notation aligns with Lisp syntax to connote use of application semantics like function calls or programming-language statements. As such, correct interpretation requires a higher-level context other than the raw Ion parser and data model.

In the text format, S-expressions are bounded by parentheses. S-expressions also allow unquoted operator symbols, in addition to the unquoted identifier symbols allowed everywhere.

null.sexp         // A null S-expression value
()                // An empty expression value
(cons 1 2)        // S-expression of three values
([hello][there])  // S-expression containing two lists

(a+-b)  ( 'a' '+-' 'b' )    // Equivalent; three symbols
(a.b;)  ( 'a' '.' 'b' ';')  // Equivalent; four symbols

Note that comments are allowed within S-expressions and have higher precedence than operators, therefore // and /* denote the start of comment blocks. Users are advised to avoid them as operators, though they can be used when escaped with single quotes:

(a/* word */b)       // An S-expression with two symbols and a comment
(a '/*' word '*/' b) // An S-expression with five symbols

Although Ion S-expressions use a syntax similar to Lisp expressions, Ion does not define their interpretation or any semantics at all, beyond the pure sequence-of-values data model indicated above.

Type Annotations

Any Ion value can include one or more annotation symbols denoting the semantics of the content. This can be used to:

When multiple annotations are present, the Ion processor will maintain their order. Duplicate annotation symbols are allowed but discouraged.

In the text format, type annotations are denoted by a non-null symbol token and double-colons preceding any value. Multiple annotations on the same value are separated by double-colons:

int32::12                                // Suggests 32 bits as end-user type
degrees::'celsius'::100                  // You can have multiple annotaions on a value
'my.custom.type' :: { x : 12 , y : -1 }  // Gives a struct a user-defined type

{ field: some_annotation::value }        // Field's name must precede annotations of its value

jpeg :: {{ ... }}                        // Indicates the blob contains jpeg data
bool ::                         // A very misleading annotation on the integer null
'' :: 1                                  // An empty annotation
null.symbol :: 1                         // ERROR: type annotation cannot be null 

Except for a small number of predefined system annotations, Ion itself neither defines nor validates such annotations; that behavior is left to applications or tools (such as schema validators).

It’s important to understand that annotations are symbol tokens, not symbol values. That means they do not have annotations themselves. In particular, the text a::c is a single value consisting of three textual tokens (a symbol, a double-colon, and another symbol); the first symbol token is an annotation on the value, and the second is the content of the value.