Struct Embedding in Prolog

Here’s the translated code and explanation in Prolog, formatted in Markdown suitable for Hugo:

Prolog doesn’t have a direct equivalent to struct embedding, but we can achieve similar functionality using compound terms and modules. Here’s an example that demonstrates a similar concept:

% Define a base structure
base(Num) :- number(Num).

% Define a method for the base structure
describe_base(Base, Description) :-
    Base =.. [base, Num],
    format(atom(Description), 'base with num=~w', [Num]).

% Define a container structure that includes the base
container(Base, Str) :- 
    Base =.. [base, _],
    atom(Str).

% Define a method for the container structure
describe_container(Container, Description) :-
    Container =.. [container, Base, _],
    describe_base(Base, Description).

% Main predicate
main :-
    % Create a container with a base
    Container = container(base(1), 'some name'),
    
    % Access the base's field directly
    Container =.. [container, base(Num), Str],
    format('co={num: ~w, str: ~w}~n', [Num, Str]),
    
    % Access the full path using the compound term
    Container =.. [container, Base, _],
    Base =.. [base, BaseNum],
    format('also num: ~w~n', [BaseNum]),
    
    % Use the describe method
    describe_container(Container, Description),
    format('describe: ~w~n', [Description]),
    
    % Demonstrate interface-like behavior
    describer(Container, Description),
    format('describer: ~w~n', [Description]).

% Define a "interface-like" predicate
describer(Object, Description) :-
    (container(Base, _) = Object ->
        describe_container(Object, Description)
    ;   base(_) = Object ->
        describe_base(Object, Description)
    ).

% Run the main predicate
:- main.

In this Prolog example, we’ve created structures and predicates that mimic the behavior of the original code:

We define a base structure using a compound term, and a describe_base predicate to represent its method.
We create a container structure that includes the base as its first argument, similar to embedding.
The describe_container predicate demonstrates how the container can use the base’s method.
In the main predicate, we show how to create and use these structures, accessing fields and calling methods.
The describer predicate demonstrates an interface-like behavior, where it can work with both base and container structures.

To run this program, you would typically save it in a file (e.g., struct_embedding.pl) and then run it using a Prolog interpreter:

$ swipl -s struct_embedding.pl
co={num: 1, str: some name}
also num: 1
describe: base with num=1
describer: base with num=1

This example demonstrates how Prolog can achieve similar functionality to struct embedding using its own language constructs. While it’s not a direct translation, it captures the essence of the original code’s behavior in a Prolog-idiomatic way.

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Struct Embedding in Prolog