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Recon is object notation with attributes, like if JSON and XML had a baby. Attributes facilitate uniformly disambiguating polymorphic structures. And first-class language extensions for selectors, expressions, and functions make Recon a highly expressive format for domain specific languages. @swim/recon is part of the @swim/core framework.

Language Overview

Recon combines the simplicity of JSON with the expressiveness of XML. As shown in the example below, Recon looks a bit like a hybrid of the two. Yet Recon is deceptively simple: the grammar for Recon is scarcely larger than the grammar for JSON. And this underlying uniformity makes Recon more expressive, and more consistent to work with, than either XML or JSON.

@html {
  @head {
    @title "Greetings"
  @body {
    @h1 "Introduction"
    @p [I have @a(href:"https://en.wikipedia.org/wiki/Markup_language")[markup syntax]
        for when you need it.  But I'm not a text chauvinist.  I'm a structured object
        notation first and foremost.  The numbers {1, 2, 3} are parsed as numbers,
        not strings.  Any my attributes make it easy to define, embed, and
        disambiguate microformats and domain specific languages.]
    @p [Need a microformat for time?  You'll find it falls out naturally after
        {{10 @minutes}} of using Recon.  Need to build a DSL for real-time GUI
        widgets?  Recon helps you do so cleanly and concisely, like this:]
    @pie {
      title: "Events"
      linkStats: @link(host: "warp://traffic.swim.services", node: "swim:meta:mesh", lane: "linkStats", type: value)
      @slice {
        value: $max(0.1, $rate($linkStats.downMessageCount))
        label: @text($percent($value, $total))
        legend: @text([Down ({$round($value)}/s)])
        innerRadius: 10 + 7.5 * $value / $max($value) @pct
        outerRadius: 20 + 7.5 * $value / $max($value) @pct
      @slice {
        value: $max(0.1, $rate($linkStats.upMessageCount))
        label: @text($percent($value, $total))
        legend: @text([Up ({$round($value)}/s)])
        innerRadius: 10 + 7.5 * $value / $max($value) @pct
        outerRadius: 20 + 7.5 * $value / $max($value) @pct

The name Recon is shorthand for Record notation. Record Notation has six primitive data types: text, data, num, bool extant, and absent; and one aggregate data type: record. Read on to learn about the underlying structure of the language.

Text Values

Text values take one of two forms: a quoted string, or an unquoted identifier.


Data Values

Binary data encodes as a leading '%' symbol, followed by a base64 literal.


Num Values

Numbers serialize as decimal literals.


Bool Values

Booleans are represented by the true and false identifiers.


Extant Values

Extant symbolizes a thing that is defined, but which has no specific value. Extant is represented by an empty token where a value is expected.

foo: # value of foo slot is extant
@bar # value of bar attr is extant

Absent Values

Absent represents something that does not exist. Its only direct representation in Record Notation is an empty document.

Record Values

Record Notation has a single aggregate data type, called record. Records play the combined role of array and associative array. Think of a record as a partially keyed list—a sequence where some items may have keys, and other items may lack keys. An array is a record in which no items have keys. An associative array is a record in which every item has a key. An object is a record where every item has a text key.

The example below contains a record with two ordered items, first a "subject" field with value "Greetings", then the unkeyed string "Hello, Earthlings!".

{ subject: "Greetings", "Hello, Earthlings!" }

Items in a record are separated by a single comma, a single semicolon, or one or new lines. Newline separated records provide a clean syntax for pretty-printed documents.

  subject: "Re: Greetings"
  "Hi Martians!"

Records support arbitrary values as slot keys.

  @planet Jupiter: {}
  @god Jupiter: {}


Top-level documents can omit the curly braces around their root record. The content of a record, sans curly braces, is called a block. When a block contains only a single item, the value of the block reduces to the value of the item it contains. The example block below is equivalent to the example record with curly braces above.

subject: "Re: Greetings"
"Hi Martians!"


Square brackets denote markup. Markup offers an inverted syntax for records, with values embedded in text, as opposed to text embedded in records. Markup looks like this:

[Hello, @em[world]!]

Markup is really just syntactic sugar for records. The above example expresses the exact same structure as the example below.

{ "Hello, "; @em "world"; "!" }

Curly braces splice blocks into markup, lifting the enclosed block into the markup's record. The following records are equivalent.

[Answer: {42}.]
{ "Answer", 42, "." }

Square brackets lift nested markup into the enclosing record. Make sure to backslash escape square brackets if you want to include them verbatim.

[Say [what]?]
{ "Say ", "what", "?"}
[Say \[what\]?]
{ "Say [what]?" }

Sequential attributes within markup don't chain; each markup-embedded attribute inserts a nested record.

{ "http", @colon, @slash, @slash }

Attributes in markup can prefix curly brace enclosed blocks, as well as nested markup.

[Goals: @select(max:2){fast,good,cheap}.]
{ "Goals: ", @select(max:2){fast,good,cheap}, "." }

Beware that whitespace inside markup is significant. There can be no whitespace between markup-embedded attributes and the blocks they're intended to modify. Notice how the single space added to the example below completely changes its meaning, when compared to the previous example.

[Goals: @select(max:2) {fast,good,cheap}.]
{ "Goals: ", @select(max:2), " ", {fast,good,cheap}, "." }


The @ sigil introduces an attribute. Attributes call out key fields of a record. The markup [Hello, @em[world]!] further reduces to the form below.

  "Hello, "

Note that the @em field above has no explicit value. Recon models unspecified–but existent–values as extant. We say that the record @em[world] has an extant attribute named em.

Of course, attributes can have specific associated values too. Place attribute parameters in parentheses following the attribute's name.


The above attributes are structurally equivalent to:


Attribute parentheses enclose a block, meaning attribute values construct an implicit record when needed. An example, with its desugared equivalent:

@img(src: "tesseract.png", width: 10, height: 10, depth: 10, time: -1)
  "@img": {
    src: "tesseract.png"
    width: 10
    height: 10
    depth: 10
    time: -1

Attributes modify adjacent values. Modified values interpolate into the record formed by their adjacent attributes. Here are some examples of values with prefix, postfix, and circumfix attributes:

@duration 30
30 @seconds
@duration 30 @seconds
@relative @duration 30 @seconds

The above attribute expressions desugar to the following records:

{ "@duration":, 30 }
{ 30, "@seconds": }
{ "@duration":, 30, "@seconds": }
{ "@relative":, "@duration":, 30, "@seconds": }

Modified records flatten into the record formed by their adjacent attributes. So @point{x:0,y:0}, reduces to {"@point":,x:0,y:0}, not {"@point":,{x:0,y:0}}.







API Overview

@swim/recon uses @swim/structure as its default abstract syntax tree. Refer to the documentation for @swim/structure to learn how to manipulate parsed Recon structures.

The Recon factory class provides static methods for parsing and serializing Recon structures. Use Recon.parse to parse a @swim/structure Value from a string:

Recon.parse("[Welcome @a(href:'index.html')@em[home].]");

Use Recon.toString to serialize a structured Value to a Recon string:

Recon.toString(Record.of("a", Slot.of("b", 2), "c"));
// "{a,b:2,c}"

Use "Recon.toBlockString" to write a top-level block when serializing documents:

Recon.toBlockString(Record.of("a", Slot.of("b", 2), "c"));
// "a,b:2,c"

@swim/recon also adds extension methods to @swim/structure to make parsing and serializing Recon more convenient.

The Value.parseRecon static method is an alias for Recon.parse.

Value.parseRecon("[Hello, @em[world]!]");

The Item.toRecon and Item.toReconBlock instance methods delegate to Recon.toString and Recon.toBlockString, respectively.

Record.of("Hello, ", Record.of(Attr.of("em"), "world"), "!").toRecon();
// "[Hello, @em[world]!]"

Invoke Item.toAny to convert parsed Recon structures to plain old JavaScript objects:

Value.parseRecon("1, 2, 3").toAny();
// [1, 2, 3]
Value.parseRecon("a: 1, b: 2, c: 3").toAny();
// {"a": 1, "b": 2, "c": 3}
Value.parseRecon("[Hello, @em[world]!]").toAny();
// ["Hello, ",{"@em":null,"$1":"world"},"!"]

Recon also provides methods to get composable @swim/codec Parsers and Writers for low-level parsing and serialization needs, such as incremental parsing out of arbitrary Input streams.

The exported ReconParser and ReconWriter classes can be extended to directly generate and serialize alternative syntax trees, or to extend the Recon language itself by overriding parse methods. ReconStructureParser and ReconStructureWriter provide the standard @swim/structure-based ReconParser and ReconWriter implementations, with full support for Recon selectors, expressions, and functions.

Language Grammar

Record Notation Grammar

Record Notation is the name of the minimal grammar for parsing Recon structured values.

SP ::= #x20 | #x9

NL ::= #xA | #xD

WS ::= SP | NL

Char ::= [#x1-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]

NameStartChar ::=
  [A-Z] | "_" | [a-z] |
  [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] |
  [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] |
  [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] |
  [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]

NameChar ::=  NameStartChar | '-' | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]

MarkupChar ::= Char - ('\\' | '@' | '{' | '}' | '[' | ']')

StringChar ::= Char - ('"' | '\\' | '@' | '{' | '}' | '[' | ']' | '\b' | '\f' | '\n' | '\r' | '\t')

CharEscape ::= '\\' ('"' | '\\' | '/' | '@' | '{' | '}' | '[' | ']' | 'b' | 'f' | 'n' | 'r' | 't')

Base64Char ::= [A-Za-z0-9+/]

Block ::= WS* Slots WS*

Attr ::= '@' (Ident | String) ('(' Block ')')?

Slots ::= Slot SP* ((',' | ';' | NL) WS* Slots)?

Slot ::= BlockItem (SP* ':' SP* BlockItem?)?

BlockItem ::= BlockExpression SP* (Attr SP* BlockItem?)? | Attr SP* BlockItem? | Comment

InlineItem ::= Attr (Record | Markup)? | Record | Markup

Literal ::= Record | Markup | Data | Ident | String | Num | Bool | Selector

Record ::= '{' Block '}'

Markup ::= '[' (MarkupChar* | CharEscape | InlineItem)* ']'

Data ::= '%' (Base64Char{4})* (Base64Char Base64Char ((Base64Char '=') | ('=' '=')))?

Ident ::= NameStartChar NameChar*

String ::= ('"' (StringChar* | CharEscape)* '"') | ('\'' (StringChar* | CharEscape)* '\'')

Num ::= '-'? (([1-9] [0-9]*) | [0-9]) ('.' [0-9]+)? (('E' | 'e') ('+' | '-')? [0-9]+)?

Bool ::= 'true' | 'false'

Comment ::= '#' [^\n]*

# Extended by Recon Selectors grammar.
Selector ::=

# Extended by Recon Expressions grammar.
BlockExpression ::= Literal

Recon Selectors Grammar

Recon Selectors extends the Record Notation grammar to support parsing first-class selector expressions.

# Redefinition of Record Notation non-terminal.
Selector ::= '$' (Literal | '*:' | ':*' | '*' | '**' | '#' Integer | Filter)
             ('.' (Literal | '*:' | ':*' | '*' | '**') | '#' Integer | Filter | '(' Block ')')*

Filter ::= '[' BlockExpression ']'

Recon Expressions Grammar

Recon Expressions extends the Record Notation grammar to support logical, bitwise, algebraic, and functional expressions.

# Redefinition of Record Notation non-terminal.
BlockExpression ::= LambdaFunc

LambdaFunc ::= ConditionalOperator (SP* '=>' SP* ConditionalOperator)?

ConditionalOperator ::= OrOperator SP* ('?' SP* ConditionalOperator SP* ':' SP* ConditionalOperator)?

OrOperator ::= AndOperator SP* ('||' SP* AndOperator)*

AndOperator ::= BitwiseOrOperator SP* ('&&' SP* BitwiseOrOperator)*

BitwiseOrOperator ::= BitwiseXorOperator SP* ('|' SP* BitwiseXorOperator)*

BitwiseXorOperator ::= BitwiseAndOperator SP* ('^' SP* BitwiseAndOperator)*

BitwiseAndOperator ::= ComparisonOperator SP* ('&' SP* ComparisonOperator)*

ComparisonOperator ::= AttrExpression SP* (('<' | '<=' | '==' | '>=' | '>') SP* AttrExpression)?

AttrExpression ::= AdditiveOperator SP* (Attr SP* AttrExpression?)? | Attr SP* AttrExpression?

AdditiveOperator ::= MultiplicativeOperator SP* (('+' | '-') SP* MultiplicativeOperator)*

MultiplicativeOperator ::= PrefixOperator SP* (('*' | '/' | '%') SP* PrefixOperator)*

PrefixOperator ::= InvokeOperator SP* | ('!' | '~' | '-' | '+') SP* PrefixOperator

InvokeOperator ::= Primary ('(' Block ')')*

Primary ::= Literal | '(' BlockExpression (',' BlockExpression)* ')'



For an npm-managed project, npm install @swim/recon to make it a dependency. TypeScript sources will be installed into node_modules/@swim/recon/main. Transpiled JavaScript and TypeScript definition files install into node_modules/@swim/recon/lib/main. And a pre-built UMD script can be found in node_modules/@swim/recon/dist/main/swim-recon.js.


Browser applications can load swim-core.js, which comes bundled with the @swim/recon library, directly from the swimOS CDN.

<!-- Development -->
<script src="https://cdn.swimos.org/js/latest/swim-core.js"></script>
<!-- Production -->
<script src="https://cdn.swimos.org/js/latest/swim-core.min.js"></script>

Alternatively, the standalone swim-system.js script may be loaded from the swimOS CDN, which bundles @swim/recon together with all other @swim/system libraries.

<!-- Development -->
<script src="https://cdn.swimos.org/js/latest/swim-system.js"></script>
<!-- Production -->
<script src="https://cdn.swimos.org/js/latest/swim-system.min.js"></script>



@swim/recon can be imported as an ES6 module from TypeScript and other ES6-compatible environments.

import * as recon from "@swim/recon";


@swim/recon can also be used as a CommonJS module in Node.js applications.

var recon = require("@swim/recon");


When loaded by a web browser, the swim-core.js script adds all @swim/recon library exports to the global swim namespace.

The swim-system.js script also adds all @swim/recon library exports to the global swim namespace, making it a drop-in replacement for swim-core.js when additional @swim/system libraries are needed.


npm i @swim/recon

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