How do I make this predicate work? extractedKnowledge(SourceList,ExtractedList)

I have this predicate that I can't seem to get to work. The predicate should be used the following way: You give the predicate a list of things (KnowledgeList), and an uninst开发者_开发问答antiated variable (ExtractedList). The predicate then should proceed filling ExtractedList. To fill ExtractedList it iterates over the items of KnowledgeList, builds a new list of extracted things, and appends this list to ExtractedList.

I know I should probably use recursion to solve this, but I feel stumped at the moment.

extractedKnowledge(KnowledgeList, ExtractedList) :-
  list(KnowledgeList),
  ExtractedList = [],
  length(KnowledgeList,ListLength),
  for(X,1,ListLength),
  nth(X,KnowledgeList,ListElement),
  ...?

Since you unify ExtractedList which [], it will always be empty. You should indeed use recursion. Here's a skeletal recursive program to get you started:

% base case: we can only extract 0 items from 0 items
extracted_knowledge([],[]).
% recursive case
extracted_knowledge([Item|Knowledge], Extracted) :-
    extracted_knowledge(Knowledge, Extracted0),
    % build Extracted from Extracted0 by adding Item,
    % if it needs to be extracted