Ada functions, accessing stack - ada

Question

Very new with Ada, this is my first time coding with it. Very lost. Any tips, and direction would be great.
Ada question:
I am trying to make: the function Top (S : Stack) return Item_Type, which returns the top item on the stack or raises the Underflow exception, to the generic unbounded stack package.
The function I added for this is at the bottom of this code block.
Current errors:
invalid use of subtype mark in expression or call
actual for "From" must be a variable
invalid use of subtype mark in expression or call
package body Unbound_Stack is
type Cell is record
Item : Item_Type;
Next : Stack;
end record;
procedure Push (Item : in Item_Type; Onto : in out Stack) is
begin
Onto := new Cell'(Item => Item, Next => Onto); -- '
end Push;
procedure Pop (Item : out Item_Type; From : in out Stack) is
begin
if Is_Empty(From) then
raise Underflow;
else
Item := From.Item;
From := From.Next;
end if;
end Pop;
function Is_Empty (S : Stack) return Boolean is
begin
return S = null;
end Is_Empty;
--added this code, and then had errors!
function Top (S : Stack) return Item_Type is
begin
--Raise the underflow
if Is_Empty(S) then
raise Underflow;
else
--or return top item from the stack, call pop
Pop (Item_Type, from => S);--I think I should pull from the stack w/pop
end if;
return Item_Type;
end Top;
end Unbound_Stack;

Answer 1

You're passing in a type (Item_Type) into Pop. Instead you need to declare a variable of type Item_Type and use that.
e.g.
function Top (S : Stack) return Item_Type is
Popped_Item : Item_Type;
begin
...
and then the call to Pop becomes:
Pop (Item => Popped_Item, From => S)

Answer 2

You have two error messages referring to this line:
Pop (Item_Type, from => S);--I think I should pull from the stack w/pop
The first one is pointing at Item_Type and says "invalid use of subtype mark in expression or call".
This means that you are using the name of a type in a place where that isn't allowed. Actual parameters to subprograms can never be types. You need to use (depending on the parameter direction) a variable or an expression for the actual parameter.