Re: Ada generics are bad

[Brian Rogoff <bpr@shell5.ba.best.com>]

On 10 Apr 1998, Christopher Green wrote:
> In article <Er3npt.4sK@array.ca>, Glenden Lee <glee@array.ca> wrote:
> > ... complaints about generics leading to recompilation snipped ...
> 
> Some compilers (Apex is one) have an option for "function form" generics.
> This allows the generic body to be compiled once and reused for each
> instance of the generic.  Function form generics are often not quite as
> efficient in code speed (because they can't be inlined), and there are
> a few cases in which they can't be used at all.

I think this can be phrased very simply, at the risk of some
oversimplification. Generics which share code rely on some level of 
uniform representation, and hence pointers/access-types. This indirection 
allows one to get away with fewer recompiles. The same issue arises
without generics if you look at the representation information in the 
private section of a package spec. If the size of some data type changes
there than you have to recompile dependent packages. If you use the 
trick of making all datatypes pointers to things in the body, than you
shouldn't have to recompile. 

Ada compilers typically implement generics by macro expanding, ML
is typically like Lisp in that everything is a poinetr "under the hood".
Each has its advantages and disadvantages. One could argue that the 
method of implementation should be specified; I tend to write Ada as 
though macro expansion of generics is a given.

-- Brian

> With "function form" generics, updating and recompiling the generic body
> does not obsolete units that instantiate it or depend on instantiations,
> and it is possible to deliver libraries that include only object code
> for such bodies (this is a key concern for developers of commercial Ada
> software such as ourselves, who are wary of releasing more source code
> than we absolutely have to).
> -- 
> Chris Green                                  Email cgreen@atc.com
> Advanced Technology Center                   Phone (714) 583-9119
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> 
>