Hi all,
I tried to create some MVP for syntax, and put in on Wiki (as Akiva suggested).
Everything is on table and can be discussed. Here please find link to it
https://github.com/rsmogura/swift/wiki/Attributes-Proposal and below there is a
copy of page
What do you think about defining for MVP, which outcome should be minimal
consensus about syntax, as follow:
- parsing minimised attribute declaration, enabled by command line feature
switch
- producing some AST
- validate attribute tree
After this we can start thinking how use attributes, store those and mirror (I
believe there is a plenty of options)?
Best regards,
Radek
PS. I noticed we switched thread
Attribute Declaration Syntax
Attribute can be declared similar to class or struct, however as attribute is
not class, nor struct and it's differently treated on runtime, it's a separate
language construction. The example attribute is presented below:
@target(AttributeTarget.PROPERTY)
@available(AttributeAvailibility.RUNTIME)
@attribute JSONProperty {
var name:String?;
var readable:Bool? = true;
var writeable:Bool? = true;
}
<https://github.com/rsmogura/swift/wiki/Attributes-Proposal/_edit#formal-syntax>Formal
syntax
TBD Formal syntax description
<https://github.com/rsmogura/swift/wiki/Attributes-Proposal/_edit#attribute-types-properties>Attribute
types properties
The attribute supports following types for its properties:
Int
Double
Bool
String
Enum values
Arrays of above
Later, when there will be consensus how to store attributes, and how the set
properties, list of supported types can be extended
<https://github.com/rsmogura/swift/wiki/Attributes-Proposal/_edit#attribute-target>Attribute
target
Attribute can be described with specifier determining on which language
elements attribute can be used. Following enum defines possible target
enum AttributeTarget {
case ATTRIBUTE
case CLASS
case STRUCT
case INITIALIZER
case METHOD
case ENUM
case ENUM_VALUE
case PROPERTY
}
It's compile time error if attribute is used on different elements than it was
declared.
<https://github.com/rsmogura/swift/wiki/Attributes-Proposal/_edit#attribute-destination>Attribute
destination
Attribute destination describes where attribute will be available - if it's
should be present in source, during compilation or during runtime. Some
attributes do not have to be visible on runtime, ie. external tools can mark
specific pieces of code to find those faster. The deprecated or availability
attributes should be available during compilation time, so compiler can report
appropriate warnings or error, but have no meaning on runtime.
Attribute destination is described by following enum:
enum AttributeAvailibility {
case SOURCE
case COMPILE
case RUNTIME
}
> On 26 Dec 2015, at 20:31, Radosław Smogura via swift-evolution
> <[email protected]> wrote:
>
>
>> On 26 Dec 2015, at 19:57, JOSE MARIA GOMEZ CAMA <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Dear all,
>>
>> Let me answer inlined.
>>
>>> El 26/12/2015, a las 19:15, Radosław Smogura via swift-evolution
>>> <[email protected] <mailto:[email protected]>> escribió:
>>>
>>> Hi Dimitir,
>>>
>>> Thanks for your questions, as you pointed me to deeper considerations and
>>> better description of idea.
>>>
>>>> On 26 Dec 2015, at 16:20, Dmitri Gribenko <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>>
>>>> On Sat, Dec 26, 2015 at 3:59 PM, Radosław Smogura <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>>> So,
>>>>>
>>>>> What do you think about syntax like this:
>>>>>
>>>>> // Declaration
>>>>> @AttributeUsage(RUNTIME) //Can be SOURCE - attribute not emitted into
>>>>> binary
>>>>> @AttributeTarget(PROPERTY) //Other CLASS, ENUM, METHOD, INIT
>>>>> @attribute JSONProperty {
>>>>> var name:String!;
>>>>> var serializable:Bool? = true;
>>>>> var deserializable:Bool? = true;
>>>>> }
>>>>
>>>> Is this a new declaration kind? Do you think we could make it work
>>>> with existing language constructs instead, like structs? Any
>>>> downsides to that?
>>> It’s really good concern and I have to describe attribute concept in more
>>> details. The _implementation_ of attributes would and should use existing
>>> constructs like structs or classes - I prefer classes as it will allow
>>> attributes to subclass some common class (Attribute?), so in future new
>>> functionality can be added.
>>>
>>> Let’s consider attributes, and marking struct with @Attrbute.
>>>
>>> On runtime attributes will be passed as references or value objects (just
>>> to keep contract programming) and assignment to attribute properties should
>>> be prohibited:
>>> - if attribute is bound to type, than attribute would be nothing more than
>>> extension of static context of that type,
>>> - it’s error prone for developers, as someone could change i.e. JSON
>>> property name.
>>>
>>> Attributes are metadata, and do not require methods (however methods could
>>> be interesting and innovative addition). So attribute’s properties should
>>> be declared with let, instead of var, to prevent modification of those, but
>>> from other hand we would like to set default values for some properties, I
>>> just think that declaration like this would be confusing:
>>> @Attribute
>>> struct JSONProperty {
>>> /* … */
>>> let deserializable:Bool? = true;
>>> }
>>>
>>> Additionally having attributes as a new construct will give us more
>>> flexibility on changing syntax and adding new features in future.
>>>
>>> I think this implies that attribute can’t be considered as struct (on
>>> language level), and should be treated as new language construct.
>>
>> I do not agree that attributes should only provide metadata. I think they
>> shall also provide means, like in Java or Python, to modify the behavior in
>> runtime. For this, mirroring would be needed.
> My bad, I think I unnesessary used a word ‘metadata’ word. It may be
> confusing.
>
> You are right it is something we would like to achieve, but attribute alone
> is disconnected from mean. Mean (with some exceptions), similar like in Java,
> should be defined by library / container which inspects annotations on given
> object.
>
>>
>>>>> // Usage
>>>>> @JSONProperty(name=“id”)
>>>>> var id:Int;
>>>>>
>>>>>
>>>>> Attributes should be module aware (actual attributes should be redefined
>>>>> as Swift attribute, for beginning still can be hardcoded).
>>>>
>>>> Sorry, I'm not sure what this means.
>>> Attributes should be in module. So modules can define attributes with same
>>> name. Right now attributes are global.
>>>>
>>>>> The attribute’s name have to be unique in module. It’s compile time error
>>>>> if attribute, class, enum, struct, etc has same name.
>>>>>
>>>>> Attribute’s properties are read only and can’t be assigned - so during
>>>>> reflection no-one would change ‘shared’ values.
>>>>
>>>> I think using structs and requiring that attributes are value types
>>>> would solve both issues.
>>> I think attributes should be treated as new language construct, regardless
>>> how those will be implemented on runtime - it can be struct or class.
>>
>> I do not see why it should be a new construct, in Java it is, but in case of
>> Python it is not. Another option would be to define the attribute @attribute
>> that would identify a class or a struct as an attribute.
>>
>>>>> Attribute’s properties can only support basic types: primitives, strings,
>>>>> types, and other attributes (optional).
>>>>
>>>> Well, there is no such thing as "basic types" in Swift. Strings,
>>>> integers etc. are defined in the standard library and the compiler
>>>> does not know anything special about them.
>>> By basic type I meant a types which can be serialised to format which will
>>> not change in future. Numbers, strings, type names, arrays of those can be
>>> stored in binaries without risks of backward incompatibility, objects and
>>> structs are more fragile. I think next answer will clarify more.
>>
>> I think this is always the case when you have a compiled language, you
>> always store information as a binary when you have a library.
> I wanted to explain different topic here. In generally representation of
> primitive types do not change over time, which is in oposition to composite
> types where properties can be added or removed or it order can be changed.
>
>>>>> When declaring attributes, properties can be set to constant values
>>>>> (static let) or enum values, however the final value is stored in binary,
>>>>> not a reference to it.
>>>>
>>>> Again, given that strings are defined in the standard library, and
>>>> that the language does not have a notion of a constant expression, I'm
>>>> not sure how this would work. I'm not saying it can't, I'm just
>>>> saying you need to introduce a lot of new language concepts and
>>>> compiler machinery.
>>> It’s another good question. By const I mean virtual concept of static let
>>> declaration. My motivation is to keep attributes as static metadata.
>>>
>>> Let’s consider three levels of attributes:
>>> - source - those attributes are present only in source file and are not put
>>> in binary, such attributes can be used to mark pieces of code for external
>>> tools like visual UI designers, UMLs, etc
>>> - compile - those attributes are present in output binary, but have not
>>> meaning on runtime - ie. @Deprecated,
>>> - runtime - those attributes are present on runtime, and can be inspected
>>> by libraries to affect execution, ie.: @JSONProperty, @NSManaged
>>>
>>> My other concern is related to attribute storage (let’s skip implementation
>>> of this). Suppose you want to centralise some values, ie. messages with
>>> following pice of code
>>> CommonMessages {
>>> static var genericError = “Something went wrong :(“
>>> }
>>>
>>> And you want to bound a property to message when validation fails.
>>> @ValidationMessage(message=CommonMessages.genericError)
>>> var id:Int;
>>>
>>> - For runtime referencing variables can be confusing, as some one seeing
>>> above notion can think that changing genericError will change message on
>>> runtime (annotations are static, so it will not work),
>>> - For compilation level attributes compiler has limited possibilities to
>>> evaluate what genericMessage is, as there is no application runtime (it
>>> will be compiled), so only simple traversals through source can be
>>> performed. Even for const it can be hard right now.
>>>
>>> The other question arises here, suppose CommonMessages are in external
>>> library,. What should happen if it changes, should a genericError be
>>> referenced or it’s value copied and stored in binary? Referencing could be
>>> quite interesting topic.
>>>
>>> We can consider allowing simple or more complicated expressions in
>>> attribute properties
>>> @Size(width=20+50+sqrt(12))
>>> As long as those refer static context.
>>>
>>>>> The compiler has build in support for core attributes, which can affect
>>>>> code generation. Compiler may perform additional checks on core
>>>>> attributes.
>>>>
>>>> OK.
>>>>
>>>> Another question is, how would you inspect attributes at runtime?
>>> I would like to focus on syntax, semantic and parsing. This will require
>>> answering question how to store attributes (and I would like to move this
>>> to separate chain, as it will be long discussion). From the other hand,
>>> some questions still should be answered, ie.
>>>
>>> How attribute should behave in such a case:
>>> @ValidationMessage(message=CommonMessages.genericError)
>>> var id:Int;
>>>
>>> Should genericValue be copied or referenced (see above consideration), this
>>> implies what and how to store.
>> Probably, in this case it would be good to have a look on Java annotations.
>> This can provide some hints on how to store the compiled information.
> In Java compiler traverses references and stores copy of calculated values.
> The types of value is limited. The internal representation of annotation
> element value can be found here :)
> https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.16.1
> <https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.16.1>
>
> And we can create something more. I like Java, but I have to admit that some
> concepts could be improved.
>>>> Dmitri
>>>>
>>>> --
>>>> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
>>>> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <[email protected]
>>>> <mailto:[email protected]>>*/
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> [email protected] <mailto:[email protected]>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>>
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