2.2 Example: Vectors of Numbers

In the Prolog client:

vector_type(double(_List), 2).
vector_type(float(_List), 3).
vector_type(integer(_List), 4).
vector_type(integer64(_List), 5).
vector_type(integer32(_List), 6).
vector_type(unsigned(_List), 7).
vector_type(codes(_List), 8).
vector_type(atom(_List), 9).
vector_type(string(_List), 10).

vector(Type, B):-
        vector_type(Type, Tag),

        Proto = protobuf([ repeated(Tag, Type) ]),

        protobuf_message(Proto, B).

A protobuf description that is compatible with the above wire stream follows:

  message Vector {
  repeated double double_values     = 2;
  repeated float float_values	    = 3;
  repeated sint32 integer_values    = 4;
  repeated fixed64 integer64_values = 5;
  repeated fixed32 integer32_values = 6;
  repeated uint32 unsigned_values   = 7;
  repeated bytes bytes_values       = 8;
  repeated string atom_values	    = 9;
  repeated string string_values     = 10;
  }

A typical application might consist of an abstract adapter class along with a collection of concrete subclasses that refine an abstract behavior in order to hide the interaction with the underlying protobuf interpreter. An example of such a class written in C++ may be found in the demos.

On the Prolog side:

  :- meta_predicate ~>(0,0).
  :- op(950, xfy, ~>).

  ~>(P, Q) :-
        setup_call_cleanup(P, (true; fail), assertion(Q)).

  write_as_proto(Vector) :-
        vector(Vector, Wirestream),

        open('tmp99.tmp', write, S, [type(binary)])
          ~> close(S),

        format(S, '~s', [Wirestream]), !.

  testv1(V) :-
        read_file_to_codes('tmp99.tmp', Codes, [type(binary)]),

        vector(V, Codes).

Run the Prolog side:

?- X is pi,
   write_as_proto(double([-2.2212, -7.6675, X, 0,
                          1.77e-9, 2.54e222])).
X = 3.14159.

?- testv1(Vector).
Vector = double([-2.2212, -7.6675, 3.14159, 0.0,
                 1.77e-09, 2.54e+222])
?-

Verify using the protobuf compiler:

$ protoc --decode=Vector pb-vector.proto <tmp99.tmp
double_values: -2.2212
double_values: -7.6675
double_values: 3.1415926535897931
double_values: 0
double_values: 1.77e-09
double_values: 2.5400000000000002e+222