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From: nebbe@lglsun.epfl.ch (Robb Nebbe)
Newsgroups: comp.lang.ada
Subject: Re: Ada.strings.bounded problems?
Date: 23 Jan 1995 13:37:35 GMT
Organization: Ecole Polytechnique Federale de Lausanne
Sender: nebbe@lglsun3.epfl.ch (Robb Nebbe)
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Date: 1995-01-23T13:37:35+00:00
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dewar@cs.nyu.edu (Robert Dewar) writes:
|> It is important to realize the hit on shared generics. THe design of Ada 95
|> has been carefully done to preserve the practicality of using shared
|> generics, as is currently done by at least two implementations. In particular,
|> suppose we have a simple generic with some discrete types as arguments. It
|> seems unfortunate (both in terms of implementation complexity, and in 
|> efficiency) to require all equality operations to be done with thunks.

And of course redefining equality on discrete types is a very common
programming idiom. (For all those people who can't put up with the
overhead of comparing those unused bits :-) This is a strawman. It would
have made more sense (to me) to exclude discrete types, where the
reemergence of primitive operations is much less likely to cause a
problem and provide clean semantics for record types.

What probably bothers Magnus and others is that the justifications for
not providing semantics that are consistent with tagged types are all
pragmatic. Clearly the semantics as they are defined, which allow the
reemergence of predefined equality, are not desirable (or even justifyable)
from a language design point of view; rather, they must be justified
from on implementation point of view or other pragmatic concerns such
as upwards compatability. (which are certainly legitimate)

Maybe the cost of not providing consistant semantics between tagged
and untagged record types will exceed the cost of introducing an
upward incompatability, maybe not. It is a really tough call right
now. Especially since there really isn't any data, just speculation,
to base the decision on.

Robb Nebbe