-- filename: listn.adb 11:40AM  9/7/95

with gnat.io;
use gnat.IO;
package body listn is

function cons(x:in element; toList : in pointer) return pointer is
  L: pointer;
begin
  L := new listnode;
  L.data := x;
  L.next := toList;
  return L;
end cons;

function first(ofList: in pointer) return element is
begin
  return ofList.data;
end first;

function rest(ofList: in pointer) return pointer is
 norest : exception;
begin
  if ofList = null
  then raise norest;
  else return ofList.next;
  end if;
exception
  when norest => put_line("tried to take rest of empty list");
		 return null;
end rest;

function Empty return pointer is
begin
  return null;
end Empty;

function isEmpty(L: in pointer) return boolean is
begin
  return L=null;
end isEmpty;

function IsEqual(L1,L2: in pointer) return boolean is
begin
     if L1=null then return L2=null;
     elsif L1.data=L2.data
        then return IsEqual(L1.next, L2.next);
        else return FALSE;
     end if;
end IsEqual;

procedure insert(value:in element; 
                 afterX, inList: in out pointer) is
  t: pointer;
begin
  t := new listnode;
  t.data := value;
  if inList = null
    then inList := t;            -- inserting into the empty list
         t.next := null;
    else t.next := afterX.next;  -- insert after afterX   
         afterX.next := t;
  end if;
end insert;

procedure setFirst(L: in out pointer; NewV: in Element) is
begin
  L.data := NewV;
end setFirst;

procedure setRest(L: in out pointer; NewR: in pointer) is
begin
  L.next := NewR;
end setRest;

procedure delete(x, precededByY, inList: in out pointer) is
begin
  if precededByY = null
    then inList := inList.next;    -- deleting first element
    else precededByY.next := x.next;
  end if;
-- dispose(x); or free(x); if you choose to define it and do so
end delete;

end listn;