[datum] initial public push (IPP)

[Vicente Romero]

Hi all,

We have just made public the development of data classes [1]. The 
development will continue in the "datum" branch in the amber repo [2]. 
Data classes allow for more compact class declarations. Basically for 
classes for which the state of the class can be declared in the class 
header. The basic, informal, data class syntax is:

   __datum Name(Fields) { BODY } or for a body-less data class:
     __datum Name(Fields);

here Fields is a list of fields, and each field can be:

[@Annos] [ __nonfinal ] type name

*Note:* __datum and __nonfinal are just place holders, the final keyword 
to be used is still to be defined

The fields are implicitly lifted onto fields of the class, with the 
default accessibility of "package final", unless noted as non-final. Any 
additional field declarations in the body are prohibited. Non-abstract 
data classes are final. Data classes can be generic and can implement 
arbitrary interfaces. Every non-abstract data class acquires:

  - a public constructor that takes fields in the order specified by the 
field list, and initializes all fields;
  - public getters for each field, whose name is same as the field;
  - public equals(), hashCode(), and toString().

If the user provides any of the above members explicitly, it will be 
used instead of the default defined by the compiler. Abstract data 
classes are permitted; but they get only a public constructor.  By 
default the field access of abstract data classes is protected and they 
are final.  In addition, data classes can extend abstract data classes.  
Also, data classes with no parent class have the new class 
java.lang.DataClass as its parent.  Extension looks like:

abstract __datum X(int x);
     __datum Y(int x, int y) extends X(int x);

that is, the fields of X must be a proper prefix (arity, name, and 
types) of Y's fields. For example the compiler won't accept this 
declaration for Bad data class:

     abstract __datum Sup(int x, int y);
     __datum Bad(int x, int y, int z) extends Sup(x, z); // field name 
mismatch should be Sup(x, y)

The current implementation doesn't allow data classes with an empty list 
of fields, so:

     __datum D2();  // compiler error

we can relax this constraint in the future if there are sensible use 
cases for which it makes sense to declare an empty data class.

Please try it out and get back to us with your feedback!

Thanks,
Vicente

PS. this code is under active development and bugs are being chased out 
but still lurking, thanks a lot in advance for any bug report!

[1] http://hg.openjdk.java.net/amber/amber/rev/04260073c6bd
[2] http://hg.openjdk.java.net/amber/amber