Ada - Accessibility check raised within a procedure - ada

Question

I previously asked a question regarding accessibility checks being raised in Ada, which #Brian Drummond was kind enough to awnser. The accessibility check was in a function, now I have a similair problem within a procedure; any guidance as to why this is would be greatly appreciated.
The code I am working on has been taken from here: https://github.com/raph-amiard/ada-synth-lib
The code in main file below is from the the Simple_Sine example which can be found here:
https://github.com/raph-amiard/ada-synth-lib/blob/master/examples/simple_sine.adb
My main file looks like this:
with Write_To_Stdout;
with Command; use Command;
with Effects; use Effects;
with Sound_Gen_Interfaces; use Sound_Gen_Interfaces;
with Utils; use Utils;
procedure main is
pragma Suppress (Accessibility_Check);
BPM : Natural := 15;
Notes : Notes_Array :=
To_Seq_Notes ((C, G, F, G, C, G, F, A, C, G, F, G, C, G, F, G), 400, 4);
function Simple_Synth
(S : access Simple_Sequencer; Tune : Integer := 0; Decay : Integer)
return access Mixer
is
(Create_Mixer
((0 => (Create_Sine (Create_Pitch_Gen (Tune, S)), 0.5)),
Env => Create_ADSR (5, 50, Decay, 0.5, S)));
Volume : Float := 0.9;
Decay : Integer := 800;
Seq : access Simple_Sequencer;
Sine_Gen : access Mixer;
Main : constant access Mixer := Create_Mixer (No_Generators);
begin
for I in -3 .. 1 loop
Seq := Create_Sequencer (16, BPM, 1, Notes);
Sine_Gen := Simple_Synth (Seq, I * 12, Decay);
Main.Add_Generator (Sine_Gen, Volume);
BPM := BPM * 2;
Volume := Volume / 1.8;
Decay := Decay / 2;
end loop;
Write_To_Stdout (Main);
end main;
The error that's raised is this:
raised PROGRAM_ERROR : sound_gen_interfaces.adb:20 accessibility check failed
It is raised during a call to this procedure:
-- Register_Note_Generator --
-----------------------------
procedure Register_Simulation_Listener
(N : access I_Simulation_Listener'Class) is
begin
Simulation_Listeners (Simulation_Listeners_Nb) := N;
Simulation_Listeners_Nb := Simulation_Listeners_Nb + 1;
end Register_Simulation_Listener;
Which is line 20 of the code below:
with Ada.Containers.Vectors;
package body Sound_Gen_Interfaces is
package PA_Vectors
is new Ada.Containers.Vectors (Natural, Params_Scope);
Params_Aggregators : PA_Vectors.Vector;
function Current_FPA return Params_Scope is
(Params_Aggregators.Last_Element);
-----------------------------
-- Register_Note_Generator --
-----------------------------
procedure Register_Simulation_Listener
(N : access I_Simulation_Listener'Class) is
begin
Simulation_Listeners (Simulation_Listeners_Nb) := N;
Simulation_Listeners_Nb := Simulation_Listeners_Nb + 1;
end Register_Simulation_Listener;
---------------
-- Next_Step --
---------------
procedure Next_Steps is
begin
for I in 0 .. Simulation_Listeners_Nb - 1 loop
Simulation_Listeners (I).Next_Step;
end loop;
end Next_Steps;
----------------
-- Base_Reset --
----------------
procedure Base_Reset (Self : in out Generator) is
begin
null;
end Base_Reset;
--------------------
-- Reset_Not_Null --
--------------------
procedure Reset_Not_Null (Self : Generator_Access) is
begin
if Self /= null then
Self.Reset;
end if;
end Reset_Not_Null;
--------------------
-- Reset_Not_Null --
--------------------
procedure Reset_Not_Null (Self : Note_Generator_Access) is
begin
if Self /= null then
Self.Reset;
end if;
end Reset_Not_Null;
--------------------------
-- Compute_Fixed_Params --
--------------------------
procedure Compute_Params (Self : in out Generator) is
procedure Internal (Self : in out Generator'Class);
procedure Internal (Self : in out Generator'Class) is
begin
for C of Self.Children loop
if C /= null then
if C.Is_Param then
Add_To_Current (C);
end if;
Internal (C.all);
end if;
end loop;
end Internal;
begin
Self.Parameters := new Params_Scope_Type;
Enter (Self.Parameters);
Internal (Self);
Leave (Self.Parameters);
end Compute_Params;
-----------
-- Enter --
-----------
procedure Enter (F : Params_Scope) is
begin
Params_Aggregators.Append (F);
end Enter;
-----------
-- Leave --
-----------
procedure Leave (F : Params_Scope) is
begin
pragma Assert (F = Current_FPA);
Params_Aggregators.Delete_Last;
end Leave;
--------------------
-- Add_To_Current --
--------------------
procedure Add_To_Current (G : Generator_Access) is
use Ada.Containers;
begin
if Params_Aggregators.Length > 0 then
Current_FPA.Generators.Append (G);
end if;
end Add_To_Current;
------------------
-- All_Children --
------------------
function All_Children
(Self : in out Generator) return Generator_Array
is
function All_Children_Internal
(G : Generator_Access) return Generator_Array
is
(G.All_Children) with Inline_Always;
function Is_Null (G : Generator_Access) return Boolean
is (G /= null) with Inline_Always;
function Cat_Arrays
is new Generator_Arrays.Id_Flat_Map_Gen (All_Children_Internal);
function Filter_Null is new Generator_Arrays.Filter_Gen (Is_Null);
S : Generator'Class := Self;
use Generator_Arrays;
begin
return Filter_Null (S.Children & Cat_Arrays (Filter_Null (S.Children)));
end All_Children;
----------------
-- Get_Params --
----------------
function Get_Params
(Self : in out Generator) return Generator_Arrays.Array_Type
is
use Generator_Arrays;
function Internal
(G : Generator_Access) return Generator_Arrays.Array_Type
is
(if G.Parameters /= null
then Generator_Arrays.To_Array (G.Parameters.Generators)
else Generator_Arrays.Empty_Array) with Inline_Always;
function Cat_Arrays
is new Generator_Arrays.Id_Flat_Map_Gen (Internal);
begin
return Internal (Self'Unrestricted_Access)
& Cat_Arrays (Self.All_Children);
end Get_Params;
----------------------
-- Set_Scaled_Value --
----------------------
procedure Set_Scaled_Value
(Self : in out Generator'Class; I : Natural; Val : Scaled_Value_T)
is
V : Float :=
(if Self.Get_Scale (I) = Exp
then Exp8_Transfer (Float (Val)) else Float (Val));
Max : constant Float := Self.Get_Max_Value (I);
Min : constant Float := Self.Get_Min_Value (I);
begin
V := V * (Max - Min) + Min;
Self.Set_Value (I, V);
end Set_Scaled_Value;
end Sound_Gen_Interfaces;
Any help as to why this is happening would be greatly appreciated.
Thank you

Answer 1

What you're seeing here is the result of (over-)using anonymous access types (discussed in ARM 3.10.2, informally known as the “Heart of Darkness” amongst the maintainers of Ada).
I don't think there's a simple way around this (aside from using -gnatp, as we found earlier, to suppress all checks; though perhaps you might have luck with
pragma Suppress (Accessibility_Check);
in the units where there's a problem).
I managed to get a build without Program_Errors with a fairly brutal hack, changing the anonymous access I_Simulation_Listener'Class to the named Simulation_Listener_Access throughout and, for example,
function Create_Simple_Command
(On_Period, Off_Period : Sample_Period;
Note : Note_T) return access Simple_Command'Class
is
begin
return N : constant access Simple_Command'Class
:= new Simple_Command'(Note => Note,
Buffer => <>,
On_Period => On_Period,
Off_Period => Off_Period,
Current_P => 0)
do
Register_Simulation_Listener (N);
end return;
end Create_Simple_Command;
to
function Create_Simple_Command
(On_Period, Off_Period : Sample_Period;
Note : Note_T) return access Simple_Command'Class
is
Command : constant Simulation_Listener_Access
:= new Simple_Command'(Note => Note,
Buffer => <>,
On_Period => On_Period,
Off_Period => Off_Period,
Current_P => 0);
begin
Register_Simulation_Listener (Command);
return Simple_Command (Command.all)'Access;
end Create_Simple_Command;
Ideally I'd have thought about having Create_Simple_Command returning a named access type too.
You can see where I got to at Github.