Append element to beginning of list in Prolog

Question

How do I append an element to the beginning of a list in Prolog? I need the end result to be called like so:

pushFront(8, [3, 1], NewList). % NewList is now [8, 3, 1].

I have tried to implement it as follows:

pushFront(Item, [], [Item|_]). %Problematic
pushFront(Item, [OldDequeH|OldDequeT], NewDeque) :-
    leftPush(OldDequeH, OldDequeT, [Item|NewDeque]).

But it does not work, and I'm out of ideas tbh. Can anyone describe what is wrong with my implementation and what changes it needs to work properly?

Answer 1

To add an element at the beginning of a list, just use list notation:

pushFront(Item, List, [Item|List]).

The list representation uses internally the cons functor ( . ), so a list [b,c,d] is just syntactic sugar for '.'(b,'.'(c, '.'(d, []))) .

This representation allows you to add an item at the front just by wrapping another cons functor, ie if you want to a add an item a at the front of a list L you would wrap a '.'(a, L) , which we usually write simply as [a|L] .