Re: operation can be dispatching in only one type

["G.B." <bauhaus@futureapps.invalid>]

On 01.12.15 14:56, Dmitry A. Kazakov wrote:
> On Tue, 1 Dec 2015 12:19:02 +0100, G.B. wrote:
>
>> On 01.12.15 09:46, Dmitry A. Kazakov wrote:
>
>>> There is no reason why indexing or record member access
>>> should be less efficient when a user-defined implementation allowed, but
>>> not actually used.
>>
>> Sounds a bit like user defined aspects of compilation?
>
> It is not an aspect. Aspect is a view, characteristic, feature:
>
> http://www.thefreedictionary.com/aspect
>
> It means that aspect may not change the semantics.
>
> User-defined operation is exactly the opposite. It does not change the
> view. The object is still viewed as an array or a record. It is the
> semantics that gets changed.

I take it that by "semantics" you mean the simplest meaning
of this word, viz. the operations that happen to become generated
for a ".X" function, say, as a consequence of compilation?

If, however, a user may define what "rec.x" will do, then
this changes an aspect (sic) of compilation: what was previously
defined by the LRM (or would appear to be defined by the
language) does now seem to flow from the job of the programmer,
and becomes a property of the program, as it redefines certain
occurrences of ".".

The compiler now needs to ensure something different when
translating "rec.x".


>> What
>> guarantees would the compiler be able to generate that user
>> defined mechanics will work at the same level of assurance
>> as that of "regular" records?
>
> Generated guaranties? You lost me here.

The current Ada language makes the compiler emit code for "Rec.X"
that is going to have a somewhat predictable effect. It does so
driven by what it the LRM says. That much is a guaranteed outcome.

If users are allowed to change the semantics of "rec.x", then
what level of assurance do we get? We'd take away a basic building
block from the set of unequivocally predictable building blocks.

Is it hubris or are there hidden powers in overloading what now
are non-operators?