% Term-tool builtins for inspecting, constructing, rendering, and validating
% structured terms.
%
% Each report/2 answer demonstrates one meta-programming operation over ordinary
% Eyepl terms: functor/3 and arg/3 inspect shape, =../2 can decompose or rebuild
% a compound, term_string/2 renders a term, and forall/2 validates all edge
% weights.
%
% This is a good reference example when writing rules that need to treat terms as
% data rather than only as predicate calls.
%% goal: report(X0, X1)


edge(a, b, 3).
edge(b, c, 4).

report(shape, shape(Name, Arity)) :-
  functor(edge(a, b, 3), Name, Arity).

report(second_argument, Node) :-
  arg(2, edge(a, b, 3), Node).

report(parts, parts(Name, Args)) :-
  (edge(a, b, 3) =.. [Name | Args]).

report(rebuilt, Term) :-
  (Term =.. [edge | [c, d, 5]]).

report(rendered, Text) :-
  term_string(edge(a, [b, c]), Text).

report(all_weights_positive, yes) :-
  \+ nonpositive_edge.
nonpositive_edge :-
  edge(_from, _to, Weight),
  \+ (Weight > 0).