[Moims-sc] Binary encoding of lists

[Serge Lacourte]

Dear WG members,

we discussed several optimizations proposed by Stefan for the binary 
encoding. One of them concerns the encoding of lists. I understood the 
proposal as the possible encoding of a list holding elements of a single 
concrete type with a single encoding of the type instead of repeating 
the encoding of this type for all the elements of the list. This 
bothered me, and I try to discuss it more formally here.

I explain in the following my understanding of the MAL requirements. I 
would like to have your opinion on this.

There is no formal API of an encoding. An informal API of the encoding 
could be to convert a MAL data into an array of bytes(*1), and the 
reverse. I would go one step further, asserting that the decoding of the 
array of bytes which results from the encoding of a MAL data should be 
the exact copy of the original MAL data, including its exact MAL type.

With MAL v1 this was clear and obvious. However MALv2 introduces grey 
cases that have probably not been cleared enough, and I believe that 
some ambiguity may remain. The heterogeneous list is such a case.

The MAL specifies Abstract and Concrete types. An operation may declare 
an Abstract type in an operation parameter. At runtime, this parameter 
will be filled with a Concrete type that "derives" from the Abstract 
type. The "derivation" rules is clear for Composite types, it is less 
clear for List types. The MAL specification never tells the Abstract or 
Concrete type of a List. It does define List as a way to define a MAL 
type from the Abstract or Concrete type of the list elements. Il also 
defines the concepts of Homogeneous and Heterogeneous lists, both 
concepts being assigned a Short Form Part. It never formally defines 
what List types are Abstract or Concrete. My understanding is the 
following :

  * a List of Concrete element type is a Concrete type. Its Short Form
    Part is the negative value of the SFP of the element type.
  * an Homogeneous List is a global naming for all the lists defined
    from a Concrete element type
  * a List of Abstract element type is an Abstract type
  * an Heterogeneous List is a concrete type. It may hold elements of
    any MAL type. It is given 0 as SFP.
  * the statement 4.2.6.1.6 "If the type of the List is Abstract, then
    the List shall be a Heterogeneous List." is ambiguous. It could be
    interpreted as : If a service specification specifies a List type
    from an abstract element type as the parameter of a service
    operation, then, at runtime, an Heterogeneous List shall be used.

However this interpretation of statement 4.2.6.1.6 is clearly too 
restrictive. If concrete composite type C extends abstract composite 
type A, then List<C> can be used in place of a List<A> declared 
parameter. And list<C> is an Homegeneous List. There are then 2 options 
matching a List<A> declared parameter :

 1. Homogeneous List<C>, where C extends A
 2. Heterogeneous List holding only elements which extend A (runtime check)

In the case 1 above, the parameter will obviously be decoded as a 
List<C>. The question we have to answer now, which is not considered in 
the MAL specification, is : in the case 2 above, where an heterogeneous 
list is provided, should it always be decoded as an heterogeneous list, 
even if all the enclosed elements are of the same type C? My answer is 
yes, it should. This is why I reacted to the encoding proposal to allow 
encoding it as a List<C>. I agree that this optimization is still 
possible, as long as we agree that the parameter is eventually converted 
into an heterogeneous list. This is possible as the encoder knows the 
declared type of the operation.

Serge

(*1) with the split binary, this is not exactly true. It is true for the 
overall encoding of the MAL message, however the encoding usually refers 
to the encoding of each individual MAL data type in the MAL message 
body. The encoding of the MAL header is defined in the transport, with a 
specific encoding which may not follow the encoding of the body. So the 
split binary encoding transforms a MAL data into 2 separate arrays of 
bytes, one for the included booleans, and another for all other included 
data types.

-- 
Serge Lacourte
Directeur general
ScalAgent Distributed Technologies SA
tel. +33 4 76 29 79 81
mobile. +33 6 86 47 41 06
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