Interresting, possibly buggy behavior in GNAT generics w/ expression function.

[Shark8 <onewingedshark@gmail.com>]

Lately I've been interested in perhaps getting Ada as a script-tag language, much like JavaScript. I figured I'd need a DOM library or somesuch and was looking into XMLAda (which, oddly fails compile on DOTNET-Gnat and seems oblivious to --target=JVM) so I was tinkering with it in native-code hoping to be able to get to a point where I could seek-out and alter attributes [for a node] on a DOM-Document and was surprised to find there's no functionality for altering attributes via XMLAda DOM-package (at least nothing obvious).

So I decided to play around and see about my own DOM-library (which I would rather not do, as there are several DOM-ish Ada packages about) and found this interesting tidbit: behavior changes when the parameter of image is changed from positive to natural. {Actually it only happens when Attribute_Values is instantiated with Integer[-type].}
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With Ada.Containers.Indefinite_Vectors, Ada.Strings.Fixed;
Use  Ada.Containers;

Generic
    Attribute_Name : String;
    Type Attribute_Values is (<>);

    With Function Img(Value : Attribute_Values) Return String is Attribute_Values'Image;
    With Package Units is New Indefinite_Vectors(
		Index_Type => Positive,
		Element_Type => String );

    Attribute_Units : Units.Vector:= Units.Empty_Vector;

Package Generic_Attribute_Set is

    Type Attribute(Has_Units : Boolean:= Attribute_Units.Length /= 0) is record
	Value : Attribute_Values;
	case Has_Units is
	When True  => Unit : Positive;
	When False => Null;
	end case;
    end record;

    -- The 'Image Attribute, when applied to numeric types, leaves a leading
    -- space for non-negative numbers (for uniform columnuar display); this is
    -- undesirable for our use, so we wrap it in a call to trim.
    Function Image(Value : Attribute_Values) Return String is
	( Ada.Strings.Fixed.Trim(Img(Value), Side => Ada.Strings.Left) );
    
    -- Getting the string stored in the position for out units-vector is a
    -- simple index into the proper position in the vector containing the units.
    Function Image(Value : Positive) Return String is
      ( --if Value not in positive then "" else 
        Units.Constant_Reference( Attribute_Units, Value ) );
    
    Function To_String( Attr : Attribute ) Return String is
      ( Attribute_Name & "=""" & 
	 Image(Attr.Value) & (if Attr.Has_Units then Image(Attr.Unit) else "") &
	 '"'
      );
    
End Generic_Attribute_Set;
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With
Generic_Attribute_Set,
Ada.Streams,
Ada.Text_IO.Text_Streams,
Ada.Containers.Indefinite_Vectors;

Procedure Test is
    Package Units is New Ada.Containers.Indefinite_Vectors(
		Index_Type => Positive,
		Element_Type => String );

    Use Units;
    Screen : Units.Vector:= Vector'("en" & "ex") & Vector'("px" & "%");
    
    Type J is (Alpha, Beta, 'K', Fred);
    
    Package Margins is new Generic_Attribute_Set(
	Attribute_Name   => "margin-left",
	Attribute_Values => Integer,
	Units            => Units,
	Attribute_Units  => Screen);
Begin
    Declare
      Use Margins;
      K : Attribute;
    begin
	K.Value:= 88; --J'Succ(Beta);
	K.Unit:= 4;
	Ada.Text_IO.Put_Line( To_String(K) );
    end;
End Test;
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As I understand it this shouldn't *ever* happen because in the generic there's supposed to be no knowledge of what the parameters are actually instantiated with.