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From: ark@ritcv.UUCP (Alan Kaminsky)
Newsgroups: comp.lang.ada
Subject: Re: What's wrong with this picture?
Message-ID: <961@ritcv.UUCP>
Date: 24 Oct 88 13:58:47 GMT
Article-I.D.: ritcv.961
References: <8810211257.AA07315@savax.Sanders.COM>
Organization: Rochester Institute of Technology, Rochester, NY
List-Id: <comp.lang.ada>

> What's wrong with this picture.  The below package Data does not compile.  
> It looks fine to me, but there must be something I'm missing.  Does 
> anyone see what I've overlooked.  The with'd source, Data package, error 
> listing and generic are all listed below.
> . . .
> --:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
> --Data.Ada
> --:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
> with Single_List_Utility;
> with Dynamic_String_Definition;
> use  Dynamic_String_Definition;
> 
> package Data is new Single_List_Utility(Element_Type => 
>    Dynamic_String(Maximum_Length => 132) );

Summarizing the original posting:  We are trying to instantiate a generic
package, Single_List_Utility, which has a single generic formal parameter,
Element_Type, that was declared to be a private type.  In the instantiation,
we are trying to match this generic formal parameter with an actual type,
Dynamic_String, that has a discriminant, Maximum_Length.  We are trying to
supply a value for the discriminant as we instantiate the generic package.
The goal is to instantiate a version of Single_List_Utility that works on
Dynamic_Strings of length 132.  The compiler does not like this generic
instantiation.

The problem is simply one of Ada syntax.  A generic_actual_parameter must
be a type_mark [ALRM 12.3] if the actual parameter is a data type.  A
type_mark must be a type_name or subtype_name [ALRM 3.3.2].  Thus, the
generic actual parameter must be simply a name, not a name followed by a
discriminant.

Here is how to get what we want:

	with Single_List_Utility;
	with Dynamic_String_Definition;
	use  Dynamic_String_Definition;
	
	subtype String_132 is Dynamic_String (Maximum_Length => 132);

	package Data is new Single_List_Utility
		(Element_Type => String_132);

-- 
Alan Kaminsky                           P. O. Box 9887
School of Computer Science              Rochester, NY  14623
Rochester Institute of Technology       716-475-5255
ark@cs.rit.edu