Hi,
I think it'd be good to have an annotated grammar of the Qml language in the
documentation. The current way of teaching by example is great for new users.
It isn't very good as a quick reference, a reliable way to resolve corner
cases or to figure out whether something is valid Qml.
To help with that, I've extracted the grammar from qdeclarativejs.g and
rewritten it to be easier on the eyes. While doing so, I came across a few
issues that I've commented on inline:
QmlProgram:
QmlImport* QmlObjectDefinition
QmlImport:
'import' QmlImportId QmlImportVersion ('as' QmlIdentifier)?
'import' StringLiteral QmlImportVersion? ('as' QmlIdentifier)?
QmlImportId:
IdentifierName ('.' IdentifierName)*
The implemented grammar just says "QmlImportId: MemberExpression" and has an
extra check to verify it's either a string literal or a series of
FieldMemberExpressions in the parsing code. I think this should be explicit in
the documentation. It may also be worthwhile to change the grammar like this
to allow using reserved words for qualified imports.
QmlImportVersion:
DecimalIntegerLiteral '.' DecimalDigits
The implemented grammar uses NumericLiteral here. Being more explicit shows
that 1.5e9 is not a valid import version more clearly.
QmlQualifiedId:
QmlIdentifier ('.' QmlIdentifier)*
This also used to be a MemberExpression.
QmlObjectDefinition:
QmlQualifiedId '{' QmlObjectMember* '}'
This isn't related to the grammar, but I think reusing an object definition to
group property bindings isn't great. It can easily confuse new users and
complicates documenting the Qml syntax. A QmlObjectDefinition defines a new
object - except when it groups property bindings instead!
QmlObjectMember:
QmlPropertyDeclaration
QmlPropertyBinding
QmlObjectDefinition
QmlSignalDeclaration
FunctionDeclaration
Omitted VarDeclaration.
QmlPropertyDeclaration:
'property' QmlPropertyType QmlIdentifier (':' QmlPropertyRhs)?
'property' 'list' '<' QmlQualifiedId '>' (':' '[' QmlArrayMemberList? ']')?
'default' 'property' 'alias' QmlIdentifier ':' QmlPropertyRhs
This is different from the implemented grammar in several ways.
1. It allows any right-hand side that's valid in a property binding as the
initializer of a declaration. While separating the two may make sense to
programmers, what's the harm in making
property int foo : 3*width
be equivalent to the following:
property int foo
foo: 3*width
2. I changed the list type from list<QmlIdentifier> to list<QmlQualifiedId> to
allow lists of types that have been imported into a namespace. This seems to
be an oversight in the current implementation.
3. is there any use for 'default' properties that don't have 'alias' type? I
couldn't think of any and altered the grammar accordingly.
I also omitted 'readonly' properties, they're not implemented. This could be
simplified further if list types became regular QmlPropertyTypes (i.e. also
usable in a SignalParameterList and 'default' properties).
QmlPropertyType:
QmlIdentifier
I dropped ReservedWord and 'var'. The latter is deprecated and no Qml types
seem to be reserved words.
QmlPropertyBinding:
QmlQualifiedId ':' QmlPropertyRhs
QmlQualifiedId ':' '[' QmlArrayMemberList ']'
QmlQualifiedId 'on' QmlObjectDefinition
QmlPropertyRhs:
IfStatement
ExpressionStatement
EmptyStatement
Block
QmlObjectDefinition
QmlArrayMemberList:
QmlObjectDefinition (';' QmlArrayMemberList)?
QmlSignalDeclaration:
'signal' QmlIdentifier ('(' QmlSignalParameterList? ')')?
QmlSignalParameterList:
QmlPropertyType IdentifierName (',' QmlSignalParameterList)?
QmlIdentifier:
Identifier except for 'import' 'as' 'on' 'signal' 'property'
'import' is redundant as it is reserved word anyway. There's currently no need
to reserve 'list'.
I'll open tickets for these bugs/suggestions in a few days, depending on the
feedback.
Cheers,
Christian
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