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FIDL grammar

Modified BNF rules

This is the grammar for FIDL source files. The grammar is expressed in a modified BNF format.

A nonterminal symbol matches a sequence of other symbols, delimited by commas.

nonterminal = list , of , symbols ;

Some symbols are terminals, which are either in all caps or are in double quotes.

another-nonterminal = THESE , ARE , TERMINALS , AND , SO , IS , "this" ;

Alternation is expressed with a pipe.

choice = this | that | the-other ;

An option (zero or one) is expressed with parentheses.

optional = ( maybe , these ) , but , definitely , these ;

Repetition (zero or more) is expressed with parentheses and a star.

zero-or-more = ( list-part )* ;

Repetition (one or more) is expressed with parentheses and a plus.

one-or-more = ( list-part )+ ;

Tokens

Whitespace and comments are ignored during lexing, and thus not present in the following grammar.

Comments can start with two slashes ('//') or three slashes ('///'), or they can be embodied within a [Doc] attribute. The three-slash variant and the [Doc] attribute behave the same way, propagating the comments to the generated target.

The fidldoc tool processes comments propagated into the JSON-based Intermediate Representation (IR) to generate reference documentation pages for FIDL files.

See the FIDL style guide for more details on comments.

Grammar

file is the starting symbol.

file = library-header , ( using-list ) , declaration-list ;

library-header = ( attribute-list ) , "library" , compound-identifier , ";" ;

using-list = ( using , ";" )* ;

using = "using" , compound-identifier , ( "as" , IDENTIFIER ) ;

compound-identifier = IDENTIFIER ( "." , IDENTIFIER )* ;

declaration-list = ( declaration , ";" )* ;

declaration = const-declaration | layout-declaration | protocol-declaration
            | type-alias-declaration | resource-declaration | service-declaration ;

const-declaration = ( attribute-list ) , "const" , IDENTIFIER , type-constructor , "=" , constant ;

layout-declaration = ( attribute-list ) , "type" , IDENTIFIER , "=" , inline-layout ; [NOTE 1]

inline-layout = ( attribute-list ) , ( declaration-modifiers )* , layout-kind , ( subtype ) ,
                layout-body ;

declaration-modifiers = "flexible" | "strict" | "resource" ; [NOTE 2]

subtype = ":" , type-constructor ; [NOTE 3]

layout-kind = "struct" | "bits" | "enum" | "union" | "table" ;

layout-body = value-layout | struct-layout | ordinal-layout ;

value-layout = "{" , ( value-layout-member , ";" )+ , "}" ;

value-layout-member = ( attribute-list ) , IDENTIFIER , "=" , value-layout-member-value ;

value-layout-member-value = IDENTIFIER | literal ; [NOTE 4]

struct-layout =  "{" , ( struct-layout-member, ";" )* , "}" ;

struct-layout-member = ( attribute-list ) , member-field , ( "=" , constant ) ;

ordinal-layout =  "{" , ( ordinal-layout-member , ";" )* , "}" ; [NOTE 5]

ordinal-layout-member = ( attribute-list ) , ordinal , ":" , ordinal-layout-member-body ; [NOTE 6]

ordinal-layout-member-body = member-field | "reserved" ;

protocol-declaration = ( attribute-list ) , "protocol" , IDENTIFIER ,
                       "{" , ( protocol-member , ";" )*  , "}" ;

protocol-member = protocol-method | protocol-event | protocol-compose ;

protocol-method = ( attribute-list ) , IDENTIFIER , parameter-list,
                  ( "->" , parameter-list , ( "error" type-constructor ) ) ; [NOTE 7]

protocol-event = ( attribute-list ) , "->" , IDENTIFIER , parameter-list ;

parameter-list = "(" , ( type-constructor ) , ")" ; [NOTE 8]

protocol-compose = "compose" , compound-identifier ;

type-alias-declaration = ( attribute-list ) , "alias" , IDENTIFIER ,  "=" , type-constructor ;

resource-declaration = ( attribute-list ) , "resource_definition" , IDENTIFIER , ":",
                       "uint32" , "{" , resource-properties ,  "}" ;

resource-properties = "properties" , "{" , ( member-field  , ";" )* , "}" , ";"

service-declaration = ( attribute-list ) , "service" , IDENTIFIER , "{" ,
                      ( service-member , ";" )* , "}" ;

service-member = ( attribute-list ) , member-field ; [NOTE 9]

member-field = IDENTIFIER , type-constructor ;

attribute-list = attribute | attribute , "," , attribute-list ;

attribute = "@", IDENTIFIER , ( "(" , constant | attribute-args, ")" ) ;

attribute-args = attribute-arg | attribute-arg, "," attribute-args ;

attribute-arg = IDENTIFIER , "=" , constant ;

type-constructor = layout , ( "<" , layout-parameters , ">" ) , ( ":" type-constraints ) ;

layout = compound-identifier | inline-layout ;

layout-parameters = layout-parameter | layout-parameter , "," , layout-parameters ;

layout-parameter = type-constructor | constant ;

type-constraints = type-constraint | "<" type-constraint-list ">" ;

type-constraint-list = type-constraint | type-constraint , "," , type-constraint-list ;

type-constraint = constant ;

constant = compound-identifier | literal ;

ordinal = NUMERIC-LITERAL ;

literal = STRING-LITERAL | NUMERIC-LITERAL | "true" | "false" ;

NOTE 1

Attributes for an anonymous layout introduction can be placed in one of two locations:

  • before the type keyword, or
  • as part of the inline-layout.

Placing attributes in both locations for a single layout definition is not allowed by the compiler.

NOTE 2

The grammar allows ( declaration-modifiers )* on all declarations, but the compiler limits this as follows:

  • A modifier cannot occur twice on the same declaration.
  • The flexible and strict modifiers cannot be used together.
  • The flexible and strict modifiers can only be used when the layout-kind is bits, enum, or union.
  • The resource modifier can only be when the layout-kind is struct, table, or union.

NOTE 3

The grammar allows ( subtype ) on all declarations, but the compiler limits this to only be allowed when the layout-kind is bits or enum.

Further, subtype allows the more liberal type-constructor in the grammar, but the compiler limits this to signed or unsigned integer types (see primitives) for enums and unsigned integer types for bits.

NOTE 4

The value-layout-member-value allows the more liberal literal in the grammar, but the compiler limits this to:

  • A NUMERIC-LITERAL in the context of an enum;
  • A NUMERIC-LITERAL, which must be a power of two, in the context of a bits.

NOTE 5

The ordinal-layout grammar allows any number of members, but unions specifically must at least one non-reserved member.

NOTE 6

Attributes cannot be placed on a reserved member.

Also, though ordinals can be any numeric literal, the compiler enforces that the specified ordinals for any union or table cover a contiguous range starting from 1.

NOTE 7

The protocol-method error stanza allows the more liberal type-constructor in the grammar, but the compiler limits this to an int32, uint32, or an enum thereof.

NOTE 8

The parameter-list allows the more liberal type-constructor in the grammar, but the compiler only supports layouts that are non-empty structs.

NOTE 9

The service-member allows the more liberal type-constructor in the grammar, but the compiler limits this to protocols.