RE: [vhdl-200x-ft] Revised FT16: context declarations and clauses

Tristan,

> 1) Note that to use IEEE_STD_CONTEXT, you have to write:
> library IEEE; context IEEE.IEEE_STD_CONTEXT; Maybe we should
> go further with only: context IEEE.IEEE_STD_CONTEXT;

The problem is that you need to have a denotation for the name IEEE. The
library clause introduces such a denotation. I guess you could have a
special rule for context references that assumes the prefix of a selected
name denotes a library, but I'd be very reluctant to do that.

Perhaps a simpler solution that would work in 99% of cases would be to add
IEEE to the impicit library clause. That is, each design unit would
implicitly have

  library STD, IEEE, WORK; use STD.STANDARD.all;

in their context clause. What do you think?

> 2) The 'context' reserved word can either start a
> context_declaration or a context_reference. This is not very good.
> Suggestion:
> use context xxx;
> or
> with context xxx;

What is your concern? Is it the need for look-ahead in parsing a design
unit to see if you have a context reference or a context declaration? Or is
it a human factors concern?

We already have other cases of a reserved word staring two different (though
related) constructs. Some that come to mind are:

  procedure .....;
  procedure ..... is ...

  function .....;
  function ..... is ...

  attribute a : T;
  attribute a of ... is ...;

  group g is ...;
  group x : g (...);

I think the proposed syntaxes for context declarations and context
references are sufficiently distinct, both for a parser and for a human
reader, not to need extra keywords.

> 3)
> context_reference ::=
> context selected_name { , selected_name } ;
> Should be:
> context_reference ::=
> context name { , name } ;
>
> ie, you should accept:
> library IEEE;
> use IEEE.all;
> context IEEE_STD_CONTEXT;

Yes, you're right. I was modeling the syntax on that of use clauses, which
do need selected names. I'll revise the doc to:

    context_reference ::=
         context <i>context_</i>name { , <i>context_</i>name } ;

> 4) Use of STD.STANDARD.all should indeed be forced, otherwise very odd
> things have to be precised.

Indeed!

> 5) A major (?) problem with the context clauses, as they are
> written in VHDL LRM is that you don't easily know which
> design units you are using. Ie, if your methodology is not to
> use 'ieee.std_logic_arith' (for example), then it is
> difficult to check your designs do not use it. Ada has 'with
> clauses'. In C, if you want to use feature of stdio, you
> have to include it. VHDL is really more permissive. It is a
> problem for VHDL users ?
> Suggestion: add a 'with clause'.

You are right that dependencies between units are implicit. Clause 11.4 of
the LRM describes this in terms of references from one unit to another
introducing a dependency. Ada makes the dependencies explicit through its
"with" clauses.

I would suggest that this is an orthogonal issue to that of context
declarations (the topic of FT18). Hence, it should properly be addressed as
a separate proposal. However, I don't see much hope for introducing such a
feature. The stable door was open in 1987 and the horse bolted long ago!
All legacy models rely on the dependency semantics as they currently stand.
Perhaps someone who was involved in the original language design might be
able to recall the rationale?

Cheers,

PA

--
Dr. Peter J. Ashenden                        peter@ashenden.com.au
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