Pre-processor for rules

[vasily-kartashov]

When loading a file with an example rule:

:- response_body(endpoint, [json]).
endpoint([message('Hello, World!')]).

I'd like to automatically replace it with the following rule (based on the presence of response_body "annotation"):

endpoint(Response) :-
     original_endpoint(Response0), // keeping the original goal
     term_json(Response0, Payload),
     Response = [headers(['Content-Type': 'application/json']), payload(Payload)].

What is the best way to achieve it?

[gciatto]

The first line you wrote its a directive, which is not properly a rule.

Both rules and directives are clauses.

A theory is a sequence of clauses, essentially.

So the best way to do what you propose is to parse the theory, selecting the directive and manipulating it from java, thus crating a new theory having a rule instead of a directory

[gciatto]

What do you mean by "term expansion"?

However, I don't know what you have tried to perform such a kind of transformation, but here are my 50 cents.

The transformation involves

1. the renaming of some predicates occurring into the theory and
2. the addition of some clauses.

Step 2 is straightforward, given the assertZ method of theories.
Step 1 is what is giving you troubles, I bet.

The immutable design of 2P's terms and classes is intended, and the same is true for the difficulty in modifying terms and clauses. The preferred way to do so is to rewrite terms for scratch, possibly while varying something, e.g. a functor.

The best way to apply a whole transformation to a term/clause is to exploit the notion of TermVisitor.
Here's an example: https://github.com/tuProlog/2p-kt/blob/feature/examples/examples/src/test/java/it/unibo/tuprolog/examples/TermVisitorExamplesJvm.java

Essentially one must create a visitor aimed at renaming a single clause if it matches a particular patter, or leave the clause unaffected otherwise. Then, one must apply such visitor to each clause in the original theory via the Term.accept method.
This should apply the renaming to each clause.
The new list of clauses should then be used to create a new theory, to which further clauses may be optionally added.

[vasily-kartashov]

The visitor pattern makes perfect sense, I was hoping there is something like this. Thank you.

FYI, this is term_expansion predicate from SWI: https://www.swi-prolog.org/pldoc/doc_for?object=term_expansion/2

[gciatto]

Yeah the visitor pattern is cool, we use it a lot behind the scenes to perform such kinds of manipulations :)

No, term_expansion is not supported ATM.

[vasily-kartashov]

@gciatto-unibo Thank you again for the example! So far I find the overall API design logical and consistent and your help is very valuable.

[gciatto]

Thank you mate. Very appreciated.

Thank you too for giving me the opportunity of writing these examples