Prolog append to head

Question

I am learning Prolog right now and I am trying to write a predicate newhead/3 that simply appends the second parameter to the first parameter that represents a list.

So newhead([1,2],3,R) should yield R = [3,1,2].

I wrote the following and I am confused as to what this error message says as well as why the logic of my code does not seem to be correct.

newhead([H|T],E,R) :-
    L is [H|T],
    R is [E|L].

Or:

newhead(L,E,R) :- 
    R is [E|L].

Also doesn't work. This seems like it should be a pretty trivial operation but I can't believe that it could need recursion.

I appreciate any help.

Answer 1

You can simply write:

new_head(Tail, Head, [Head| Tail]).

Answer 2

You can use the predicate 'append/3'.

apped([3],[1,2],R).
R = [3,1,2]

or

append([X],[1,2],R), X=3.
R = [3,1,2]

Answer 3

I am sure that you already found your answer :) but I want to explain my answer too.

My understanding: first parameter is list and aim is the make the second argument head of the list.

Here is code:

new([],F,[F]).  % if your list is empty.
new([X|XS],F,[F,X|XS]):-
 new(XS,F,YS). 

Output:

?- new([2,3],1,R).
R = [1, 2, 3].

?- new([],1,R).
R = [1].

?- new([2,3],[1],R).
R = [[1], 2, 3].