Ada beginner Stack program - stack

Question

Basically, I have 2 files ( .adb and .ads). I am totally new to Ada and also how to compile 2 files. The program is of a basic stack implementation. I got this compile error when I compiled the .adb file.
$ gcc -c test_adt_stack.adb
abstract_char_stack.ads:22:01: end of file expected, file can have only one compilation unit
The 2 files I have are:
abstract_char_stack.ads
-----------------------------------------------------------
package Abstract_Char_Stack is
type Stack_Type is private;
procedure Push(Stack : in out Stack_Type;
Item : in Character);
procedure Pop (Stack : in out Stack_Type;
Char : out Character);
private
type Space_Type is array(1..8) of Character;
type Stack_Type is record
Space : Space_Type;
Index : Natural := 0;
end record;
end Abstract_Char_Stack;
-----------------------------------------------------------
package body Abstract_Char_Stack is
----------------------------------------------
procedure Push(Stack : in out Stack_Type;
Item : in Character) is
begin
Stack.Index := Stack.Index + 1;
Stack.Space(Stack.Index) := Item;
end Push;
--------------------------------------------
procedure Pop (Stack : in out Stack_Type;
Char : out Character) is
begin
Char := Stack.Space(Stack.Index);
Stack.Index := Stack.Index - 1;
end Pop;
--------------------------------------------
end Abstract_Char_Stack;
and the other one is test_adt_stack.adb
-----------------------------------------------------------
with Ada.Text_IO; use Ada.Text_IO;
with Abstract_Char_Stack; use Abstract_Char_Stack;
procedure Test_ADT_Stack is
S1 : Stack_Type;
S2 : Stack_Type;
Ch : Character;
begin
Push(S1,'H'); Push(S1,'E');
Push(S1,'L'); Push(S1,'L');
Push(S1,'O'); -- S1 holds O,L,L,E,H
for I in 1..5 loop
Pop(S1, Ch);
Put(Ch); -- displays OLLEH
Push(S2,Ch);
end loop; -- S2 holds H,E,L,L,O
New_Line;
Put_Line("Order is reversed");
for I in 1..5 loop
Pop(S2, Ch);
Put(Ch); -- displays HELLO
end loop;
end Test_ADT_Stack;
-----------------------------------------------------------
What am I doing wrong? I just want to have it compile and display what it's supposed to do. This was a study the program kind of assignment. But I can't make it compile or don't know if I am doing it right.

Answer 1

The problem is that GNAT [and FSF GNAT is what GCC uses, IIRC] does not allow multiple compilation-units in a single file. (This is due to how they manage the library, but that is perhaps a bit too detailed for a beginner to worry about.)
Solution, each of these needs its own file:
Abstract_Char_Stack Specification (abstract_char_stack.ads)
Abstract_Char_Stack Body (abstract_char_stack.adb)
Test_ADT_Stack [procedure] body (test_adt_stack.adb)

Answer 2

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
-----------------------------------------------------------
procedure Test_ADT_Stack is
enter code here
package Abstract_Char_Stack is
type Stack_Type is private;
procedure Push(Stack : in out Stack_Type;
Item : in Character);
procedure Pop (Stack : in out Stack_Type;
Char : out Character);
private
type Space_Type is array(1..8) of Character;
type Stack_Type is record
Space : Space_Type;
Index : Natural := 0;
end record;
end Abstract_Char_Stack;
use Test_ADT_Stack.Abstract_Char_Stack ;
S1 : Stack_Type;
S2 : Stack_Type;
Ch : Character;
-----------------------------------------------------------
package body Abstract_Char_Stack is
----------------------------------------------
procedure Push(Stack : in out Stack_Type;
Item : in Character) is
begin
Stack.Index := Stack.Index + 1;
Stack.Space(Stack.Index) := Item;
end Push;
--------------------------------------------
procedure Pop (Stack : in out Stack_Type;
Char : out Character) is
begin
Char := Stack.Space(Stack.Index);
Stack.Index := Stack.Index - 1;
end Pop;
--------------------------------------------
end Abstract_Char_Stack;
-----------------------------------------------------------
begin
Push(S1,'H'); Push(S1,'E');
Push(S1,'L'); Push(S1,'L');
Push(S1,'O'); -- S1 holds O,L,L,E,H
for I in 1..5 loop
Pop(S1, Ch);
Put(Ch); -- displays OLLEH
Push(S2,Ch);
end loop; -- S2 holds H,E,L,L,O
New_Line;
Put_Line("Order is reversed");
for I in 1..5 loop
Pop(S2, Ch);
Put(Ch); -- displays HELLO
end loop;
end Test_ADT_Stack;
-----------------------------------------------------------