How to use user input variable as parameter for generic package?

Question

In Stack.adb I have specified two parameters (Size and Type). I want to create a stack that is of the exact data type that a user specifies within my multistack.adb file.

I can't seem to find a way to create a new package or instantiation of a stack with a user-defined variable for the stack type. Before I go on, code is below (To avoid a wall of code, I have taken out some unrelated lines):

Stack.adb :

GENERIC
   SIZE : Integer; --size of stack
   TYPE Item IS PRIVATE; --type of stack

---

multistack.adb :

WITH Ada.Text_Io; USE Ada.Text_Io;
WITH Stack;
PROCEDURE multistack IS
   PACKAGE Iio IS NEW Ada.Text_Io.Integer_Io(Integer); USE Iio;
   Type StackType IS (Int, Str, Char, Day);

   package stack_io is new Ada.Text_IO.Enumeration_IO(StackType); use stack_io;    

    package get_user_specs is
        function makestack return StackType;
    end get_user_specs;

   package body get_user_specs is 
        function makestack return StackType is
            s_type : StackType;
        begin
            put("What is the stack type?"); new_line;
            get(s_type);
            return s_type;
        end makestack;
    begin
        null;
    end get_user_specs;

   user_stack_type : StackType := get_user_specs.makestack;

   PACKAGE User_Stack IS NEW Stack(100, user_stack_type); use User_Stack;

BEGIN
    null;
END Multistack;

So, as you can tell by the code I have created the Data type for Stack types. I also created an Enumeration_IO package to be able to get the user input. The line I'm having specific trouble with is:

   PACKAGE User_Stack IS NEW Stack(100, user_stack_type); use User_Stack;

It is complaining about the fact that I'm attempting to use user_stack_type as the type. The specific error is expect valid subtype mark to instantiate "Item", then says that User_Stack is undefined.

I did a put(user_stack_type) just to test, and I can confirm that it does get the user specified data type. So why will it not allow me to create this package User_Stack?

Answer 1

In your fragment, user_stack_type is an object declaration, but a generic instantiation requires a subtype mark. One way to get the desired effect is to instantiate the generic in a nested scope once the chosen subtype is known:

if User_Stack_Type = Int then 
    declare
        package User_Stack is new Stack(100, Integer);
    begin
        Put_Line(Stack_Type'Image(User_Stack_Type));
        …
    end;
end if;