Re: factoring with generics

[loftus@wpllabs.UUCP (William Loftus)]

In article <45441@ut-emx.uucp> hasan@ut-emx.uucp (David A. Hasan) writes:
>In article <45358@ut-emx.uucp> I wrote:
>
>>PACKAGE p IS
>>   TYPE someType IS ...;
>>   FUNCTION f1(arg1,arg2 : IN someType) RETURN someType;
>>   FUNCTION f2(arg1,arg2 : IN someType) RETURN someType;
>>END p;
>
>[ where <f1> & <f2> differ only in one subprogram call ]
>
>>It occurs to me that it might be wise to implement these
>>as instances of one generic function (<g_f>, say) which takes the 
>>crucial differing subprogram as a generic parameter.  
>>To do this, I could:
>
>>   1) put the GENERIC DECLARATION and BODY of <g_f> in the body of <p>.
>>      In this case, the implementations of <f1> & <f2> would 
>>      instantiate <g_f>.
>
>>      But of course, this is hogwash:  generic instantiations
>>      are *declarations* and cannot be used to define the
>>      implementation.  So, as an alternative...
>
>Or *is* it hogwash?  This technique is used by Habermann & Perry
>in their book "Ada for Experienced Programmers".  In fact, the
>LRM seems to indicate that a generic instantiation can be used
>as the implementation of a subprogram.  (I reached this
>conclusion by threading my way through the BNF -- a technique
>I often use when I can't find an explicit discussion of my
>problems.)
>
>The DEC compiler objects to the presence of generic
>instantiations such as
>
>   FUNCTION f1 is NEW g_f( ... );
>
>on the basis that it constitutes a redclaration of <f1> as it
>appears in the package spec.
>
>Anyone know whether this is legal or not?

It is not legal.  A generic instantiation introduces both a spec and body
(see 12.2(1) and 12.2(2)).  A generic instantiation of a procedure in a
package body introduces a spec and body for that routine into the package
body.  If there is a specification of a homograph in the package
specification then there is a redeclaring of the specification of the
subprogram in the package body, and therefore is illegal.  However, this
has been submitted by ALWG as a consideration for Ada 9x.

Also notice that you cannot use a renames solely as an implemenation,
either.
--
William Loftus                   (215) 668 3661
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