How can I have more information in a Predicate_Failure?

Question

I want to be able to include information about my type in the Predicate_Failure message. Here's what I have that works:

subtype Norm is Float range 0.0..1.0;
type Component is (Red, Green, Blue, Yellow, White);

type Proto_Color is array(Component'Range) of Norm;

function Is_Valid(This : Proto_Color) return Boolean is
    Positive_Count : Natural := 0;
    Sum : Norm := 0.0;
begin
    for I in Component'Range loop
        if This(I) > 0.001 then 
            Positive_Count := Positive_Count + 1;
            Sum := Sum + This(I);
        end if;
    end loop;
    if Positive_Count = 0 or Positive_Count > 2 then
        return False;
    end if;
    if This(Red) > 0.001 and then This(Green) > 0.001 then
        return False;
    end if;
    if This(Yellow) > 0.001 and then This(Blue) > 0.001 then
        return False;
    end if;
    return Sum > 0.999 and then Sum < 1.001;
end Is_Valid;

Invalid_Color : exception;

type Color is new Proto_Color with Dynamic_Predicate => Is_Valid(Proto_Color(Color)), 
    Predicate_Failure => raise Invalid_Color;

Note that the Proto_Color type only exists so I can add that Dynamic_Predicate. I couldn't find a way to declare that Is_Valid function on a type that wasn't complete yet.

What I want to do, because all this tells me is that it failed, is have some more information about the type displayed along with the exception. But I can't get it to accept a function call on the Predicate_Failure:

function To_String(This : Proto_Color) return String is
begin
    return "Red => " & Norm'Image(This(Red)) 
        & ", Green => " & Norm'Image(This(Green)) 
        & ", Blue => " & Norm'Image(This(Blue)) 
        & ", Yellow => " & Norm'Image(This(Yellow)) 
        & ", White => " & Norm'Image(This(White));
end To_String;

type Color is new Proto_Color with Dynamic_Predicate => Is_Valid(Proto_Color(Color)), 
    Predicate_Failure => raise Invalid_Color with To_String(Proto_Color'(Color));

The error I get with this is:

expected type "Proto_Color" defined at line [line number]
found type "Color" defined at line [line number]

Without the tick that makes it a qualified expression, it tells me:

the argument of conversion cannot be aggregate
use a qualified expression instead

Answer 1

What’s needed is judicious use of subtypes, and realisation that you can call a function in a predicate before writing the spec (can’t immediately see where this is discussed in the ARM).

This works (GCC 13.2.0, 14.0.1):

procedure Devsman is

   package Colors is
      subtype Norm is Float range 0.0 .. 1.0;
      type Component is (Red, Green, Blue, Yellow, White);

      type Proto_Color is array (Component'Range) of Norm;

      Invalid_Color : exception;

      subtype Color is Proto_Color with
        Dynamic_Predicate => Is_Valid (Color),
        Predicate_Failure => raise Invalid_Color with To_String (Color);

      function Is_Valid (This : Proto_Color) return Boolean;
      function To_String (This : Proto_Color) return String;

    end Colors;

   package body Colors is

      function Is_Valid (This : Proto_Color) return Boolean is
         Positive_Count : Natural := 0;
         Sum            : Norm    := 0.0;
      begin
         for I in Component'Range loop
            if This (I) > 0.001 then
               Positive_Count := Positive_Count + 1;
               Sum            := Sum + This (I);
            end if;
         end loop;
         if Positive_Count = 0 or Positive_Count > 2 then
            return False;
         end if;
         if This (Red) > 0.001 and then This (Green) > 0.001 then
            return False;
         end if;
         if This (Yellow) > 0.001 and then This (Blue) > 0.001 then
            return False;
         end if;
         return Sum > 0.999 and then Sum < 1.001;
      end Is_Valid;

      function To_String (This : Proto_Color) return String is
      begin
         return
           "Red => " & Norm'Image (This (Red)) & ", Green => " &
           Norm'Image (This (Green)) & ", Blue => " &
           Norm'Image (This (Blue)) & ", Yellow => " &
           Norm'Image (This (Yellow)) & ", White => " &
           Norm'Image (This (White));
      end To_String;

   end Colors;

   C : Colors.Color;
begin
   C := (0.0, 0.0, 0.0, 0.0, 0.0);
end Devsman;

I have to admit to a certain amount of hacking to arrive at this :-(