Prolog refactor if/else

Question

How do I refactor this mess of if/else conditions so that it's more readable?

 ( Reverse_Path = [[]] -> Child = [c(New_Score,G1,Pos),P]
 ; otherwise -> Child = [c(New_Score,G1,Pos),P|Reverse_Path]
 ),
 ( memberchk(Child,NewAgenda) -> addChildren(Others,Reverse_Path,Current,NewAgenda,Target,Result)
 ; otherwise -> 
 ( NewAgenda=[] -> BrandNewAgenda = [Child|NewAgenda]
 ; otherwise -> 
 (New_Score =< FP -> BrandNewAgenda = [Child|NewAgenda];
 otherwise -> append(NewAgenda,[Child],BrandNewAgenda)
 )
 ),
 )).

• Roland Illig: What is the purpose of the code, apart from being a mess?

Answer 1

Generally speaking, you can resolve if/else logic by using multiple clauses for the same predicate, as multiple clauses implicitely represent 'or' or 'case', e.g.:

foo([[]], Child) :-
 Child = [c(New_Score,G1,Pos),P].
foo(Reverse_Path, Child) :-
 Child = [c(New_Score,G1,Pos),P|Reverse_Path].
bar(Reverse_Path, Child, NewAgenda, BrandNewAgenda) :-
 memberchk(Child,NewAgenda),
 addChildren(Others,Reverse_Path,Current,NewAgenda,Target,BrandNewAgenda).
bar(Reverse_Path, Child, [], BrandNewAgenda) :-
 BrandNewAgenda = [Child].
bar(Reverse_Path, Child, NewAgenda, BrandNewAgenda) :-
 New_Score =< FP,
 BrandNewAgenda = [Child|NewAgenda].
bar(Reverse_Path, Child, NewAgenda, BrandNewAgenda) :-
 append(NewAgenda, [Child], BrandNewAgenda).
main(Reverse_Path, BrandNewAgenda) :-
 foo(Reverse_Path, Child),
 bar(Reverse_Path, Child, [], BrandNewAgenda).

This is just a rough sketch, there are a lot of loose ends here like variables Others, Target or Result (I guess you are juggling too many variables anyway), but I hope you get the gist of it:

• Decompose into multiple simpler predicates that encode alternatives through multiple clauses.
• Bind results to "out" variables in the clause head (like Child in foo/2 or BrandNewAgenda in bar/4).
• Compose the smaller predicates in a "main" predicate that glues them togehter.