2 Appendix (examples)
2.1 Example directories
The examples are in‘/usr/share/swi-prolog/doc/packages/examples/protobufs`,
which is part of the swi-prolog-doc package. Each directory
has a README. To run these, you will need to copy them to a
writeable directory.
bootstrap- This contains the filecommon.mkfile, which is used by thedemoandinteropMakefiles.- demo - the
testgoal runs some examples. - interop - used for testing interoperability with C++ and Python
Additional tests are in test
\_protobufs.pl.See also Addressbook example (section 1.3.3).
The protobuf compiler (
protoc) uses two protobuf formats to communicate with the plugin: - plugin.proto
- descriptor.proto The Prolog code that were generated from these, as part of the bootstrap process, are:
- plugin
\_pb.pl - descriptor
\_pb.pl
2.2 Example: A Simple XML Like Structure
This is an example of using the low-level interface for implementing a domain-specific language that maps to protobufs.
In this example we demonstrate managing a recursive structure like
XML. The structure shown in xml_proto/1
below, is similar to the structure returned by load_xml_file/2,
which is part of the SGML library. We supply three message_sequence
decorators: kv_pair, xml_element, and aux_xml_element.
These are treated as first class host types.
:- multifile protobufs:message_sequence//3.
protobufs:message_sequence(Type, Tag, Value) -->
{ my_message_sequence(Type, Value, Proto) },
protobufs:message_sequence(embedded, Tag, Proto), !.
%
% On encode, the value type determines the tag. And on decode
% the tag to determines the value type.
%
guard(Type, Value) :-
( nonvar(Value) -> is_of_type(Type, Value); true ).
my_message_sequence(kv_pair, Key=Value, Proto) :-
Proto = protobuf([atom(30, Key), X]),
( ( guard(integer, Value), X = integer(31, Value) )
; ( guard(float, Value), X = double(32, Value) )
; ( guard(atom, Value), X = atom(33, Value)) ).
my_message_sequence(xml_element,
element(Name, Attributes, Contents), Proto) :-
Proto = protobuf([ atom(21, Name),
repeated(22, kv_pair(Attributes)),
repeated(23, aux_xml_element(Contents))]).
my_message_sequence(aux_xml_element, Contents, Proto) :-
Contents = element(_Name, _Attributes, _ElementContents),
Proto = protobuf([xml_element(40, Contents)]).
my_message_sequence(aux_xml_element, Contents, Proto) :-
Proto = protobuf([atom(43, Contents)]).
xml_proto([element(space1,
[foo='1', bar='2'],
[fum,
bar,
element(space2,
[fum=3.1415, bum= -14],
['more stuff for you']),
element(space2b,
[],
[this, is, embedded, also]),
to,
you])]).
test_xml(X, Y) :-
Proto = protobuf([repeated(20, xml_element(X))]),
protobuf_message(Proto, Y).
% And test it:
?- xml_proto(X), test_xml(X,Y), test_xml(Z,Y), Z == X.
X = Z,
Z = [element(space1,
[foo='1', bar='2'],
[fum,
bar,
element(space2,
[fum=3.1415, bum= -14],
['more stuff for you']
),
element(space2b,
[],
[this, is|...]
),
to,
you])],
Y = [162, 1, 193, 1, 170, 1, 6, 115, 112|...],
A protobuf description that is compatible with the above wire stream follows:
message kv_pair {
required string key = 30;
optional sint64 int_value = 31;
optional double float_value = 32;
optional string atom_value = 33;
}
message aux_xml_element {
optional string atom = 43;
optional xml_element element = 40;
}
message xml_element {
required string name = 21;
repeated kv_pair attributes = 22;
repeated aux_xml_element contents = 23;
}
message XMLFile {
repeated xml_element elements = 20;
}
Verify the wire stream using the protobuf compiler's decoder:
$ protoc --decode=XMLFile pb_vector.proto <tmp98.tmp
elements {
name: "space1"
attributes {
key: "foo"
atom_value: "1"
}
attributes {
key: "bar"
atom_value: "2"
}
contents {
atom: "fum"
}
contents {
atom: "bar"
}
contents {
element {
name: "space2"
attributes {
key: "fum"
float_value: 3.1415
}
attributes {
key: "bum"
int_value: -14
}
contents {
atom: "more stuff for you"
}
}
}
contents {
element {
name: "space2b"
contents {
atom: "this"
}
contents {
atom: "is"
}
contents {
atom: "embedded"
}
contents {
atom: "also"
}
}
}
contents {
atom: "to"
}
contents {
atom: "you"
}
}
2.3 Example: Vectors of Numbers
This is an example of using the low-level interface.
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, [encoding(octet),type(binary)])
~> close(S),
format(S, '~s', [WireStream]), !.
testv1(V) :-
read_file_to_codes('tmp99.tmp', Codes, [encoding(octet),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 the wire stream using the protobuf compiler's decoder:
$ 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
2.4 Example: Heterogeneous Collections
This is an example of using the low-level interface.
The following example shows how one can specify a Protocol Buffer message that can deal with variable-length, unstructured bags of numbers:
compound_protobuf(complex(Real, Img), group(12, [double(1, Real), double(2, Img)])).
compound_protobuf(float(Val), float(13, Val)).
compound_protobuf(double(Val), double(14, Val)).
compound_protobuf((Num rdiv Den), group(15, [integer(1, Num), integer(2, Den)])).
compound_protobuf(integer(Val), integer(16, Val)).
protobuf_bag([], []).
protobuf_bag([ Type | More], WireCodes) :-
compound_protobuf(Type, X),
Proto = protobuf([embedded(1, protobuf([X]))]),
protobuf_message(Proto, WireCodes, WireCodes1),
protobuf_bag(More, WireCodes1), !.
Use it as follows:
?- protobuf_bag([complex(2,3), complex(4,5),
complex(6,7), 355 rdiv -113, integer(11)], X).
X = [10, 20, 99, 9, 0, 0, 0, 0, 0|...].
?- protobuf_bag(Y, $X).
Y = [complex(2.0, 3.0), complex(4.0, 5.0),
complex(6.0, 7.0), 355 rdiv -113, integer(11)].
A protobuf description that is compatible with the above wire stream follows:
message compound_protobuf {
optional group Complex = 12 {
required double real = 1;
required double img = 2;
};
optional group Fraction = 15 {
required sint64 num = 1;
required sint64 den = 2;
};
optional float float = 13;
optional double double = 14;
optional sint32 integer = 16;
}
message protobuf_bag {
repeated compound_protobuf bag = 1;
Verify the wire stream using the protobuf compiler's decoder:
$ protoc --decode=protobuf_bag pb_vector.proto <tmp96.tmp
bag {
Complex {
real: 2
img: 3
}
}
bag {
Complex {
real: 4
img: 5
}
}
bag {
Complex {
real: 6
img: 7
}
}
bag {
Fraction {
num: 355
den: -113
}
}
bag {
integer: 11
}