push a list to Stack as one element without brackets

Question

I have list of elements in a list like [1,2,+] and I want to push them as a one element onto a stack. I can do that by putting them between square brackets but this will make brackets appear in the output. For Example, I want to push the elements of the list [1,2,+] onto a stack:

stack([1,2,+],S,Y).

Where stack is:

stack(T,S,[T|S]).

The problem is that if I push more expressions onto the stack, they will have nested brackets. For example, I would get [[+,1,2],[*,3,4]], but I want [+,1,2,*,3,4]. How can I accomplish this?

Answer 1

You can 'flatten' the list:

| ?- List = [[+,1,2],[*,3,4]], flatten(List, FlatList).

List = [[+,1,2],[*,3,4]]
FlatList = [+,1,2,*,3,4]

Prolog interpreters often include a list library which will have a flatten predicate, but here is one implementation (from SWI-Prolog's lists library):

flatten(List, FlatList) :-
flatten(List, [], FlatList0), !,
FlatList = FlatList0.

flatten(Var, Tl, [Var|Tl]) :-
  var(Var), !.
flatten([], Tl, Tl) :- !.
flatten([Hd|Tl], Tail, List) :- !,
  flatten(Hd, FlatHeadTail, List), 
  flatten(Tl, Tail, FlatHeadTail).
flatten(NonList, Tl, [NonList|Tl]).

Answer 2

I don't completely understand your question and what is the overall goal, but maybe you want something like this.

stack(el(X, Y, Z), StackTail, [X, Y, Z | StackTail]).

If your stack elements are all triples, then do not represent them as lists of three elements. This is not space efficient. Rather, represent them as terms el/3.

Also, I understand that you do not want your stack to be a list of complex terms, but a list of atomic terms. The above definition of stack/3 will unravel the el/3 term when pushing and will build it when popping.

Answer 3

Adding two more rules to stack should do the trick.

As this looks a lot like homework, I won't give a listing, but you will need a new rule where the first argument is explicitly a list, the items of which are recursively prepended to the existing stack.

If you've written member/2 and append/2, you should have no problem with this.