Re: FW: [sv-bc] [Fwd: Issues with IEEE 1364-2005]

What is meant by the "top-level" of conditionals?

Why should it matter to a user what the original "purpose" of an
operator was?

-- Brad

-----Original Message-----
From: Rishiyur Nikhil [mailto:nikhil@bluespec.com] 
Sent: Tuesday, August 15, 2006 10:09 AM
To: Brad Pierce; sv-bc@eda-stds.org
Cc: michael.burns@freescale.com; wadams@freescale.com
Subject: Re: FW: [sv-bc] [Fwd: Issues with IEEE 1364-2005]

Yes, there is a deliberate reason why '&&&' can only appear in limited
syntactic contexts, why it is asymmetric (left to right) and why there
is no corresponding '|||' (I repeat below my explanation from yesterday
about this),

The motivation for '&&&' is not short-circuiting, but pattern-matching.
'&&&' is intended to be used only with pattern-matching in conditionals
(because of its variable-binding and scoping function), and is not
intended as a general-purpose binary logical operator.  True, the syntax
allows it to be used as a short-circuiting conjunction at the top-level
of conditionals, but that is an accidental consequence, and not the
purpose of the operator.

If we want a separate short-circuiting logical conjunction and
disjunction (which are orthogonal to pattern-matching and can be used in
any expression context), then that needs to be a separate discussion
(not involving '&&&').

Regards,

Nikhil
----------------------------------------------------------------
Repeating explanation from yesterday:

There are specific reasons for these language design choices.

'&&&' is not merely a conjunction operator, and its reason for existence
is not to introduce short-circuiting-- it is because it has a
variable-binding function unique to the pattern-matching facilities of
the language.

If the left-hand operand is matching pattern, then it can also introduce
and bind identifiers from the pattern in the left-hand operand, which
are then in scope and available in the right-hand operand and in the
'then' part of the conditional.  This is the reason
why:
  - it is only at the top-level of conditionals (if it was nested more
    deeply, what would be the scope of the pattern variables?)

  - it is assymmetric, left-to-right (the pattern variables in the
    right-hand operand only make sense if the left-hand match succeeds)

  - there is no corresponding '|||' (again, the pattern variables in the
    right-hand operand only make sense if the left-hand match succeeds)

----------------------------------------------------------------


Brad Pierce wrote:
> -----Non-member submission-----
> From: Will Adams [mailto:wadams@freescale.com]
> Sent: Tuesday, August 15, 2006 6:26 AM
> Subject: Re: [sv-bc] [Fwd: Issues with IEEE 1364-2005]
> 
> The `&&&' operator can only appear in limited syntactic contexts. The 
> following reasonable uses of conjunction are not allowed by the
syntax.
> 
>    c = a &&& b ;
>    if ( ! ( a &&& b ) )
> 
> The second of these is a problem because there is no `|||' short 
> circuiting disjunction, and the syntax does not allow this operation 
> to be expressed with `!' and `&&&'.
> 
> If `&&' is not required to have short-circuit evaluation, and `&&&' is

> suggested as an alternative for cases where short-circuiting is 
> desired, we have a situation where a familiar operator has unfamiliar 
> semantics, and the familiar semantics are only available in limited 
> contexts from an unfamiliar operator.
> 
> will
> 
> 
> Brad Pierce wrote:
>>> It sounds like '&&&' is not appropriate to use as a general-purpose
>> short-circuit
>>> logical AND.
>> Because &&& allows the
>>
>>      expression 'matches' pattern &&& ...
>>
>> syntax, it can do *more* than a general-purpose short-circuit logical

>> AND.  How does its greater generality make it inappropriate for a 
>> more
> 
>> restrictive purpose?
>>
>> Regardless of the original reasons for introducing
>>
>>     if (expression &&& expression)
>>
>> it behaves exactly like C users have come to expect from
>>
>>     if (expression && expression)
>>
>> .
>>
>> -- Brad
>>
>> -----Original Message-----
>> From: Steven Sharp [mailto:sharp@cadence.com]
>> Sent: Monday, August 14, 2006 2:43 PM
>> To: Brad.Pierce@synopsys.COM; nikhil@bluespec.com
>> Cc: wadams@freescale.com; sv-bc@eda-stds.org; 
>> michael.burns@freescale.com
>> Subject: Re: [sv-bc] [Fwd: Issues with IEEE 1364-2005]
>>
>>
>>> From: "Rishiyur Nikhil" <nikhil@bluespec.com> '&&&' is not merely a 
>>> conjunction operator, and its reason for existence is not to 
>>> introduce short-circuiting-- it is because it has
> 
>>> a
>>> variable-binding function unique to the pattern-matching facilities 
>>> of the language.
>> Thanks for the explanation.  It sounds like '&&&' is not appropriate 
>> to use as a general-purpose short-circuit logical AND.
>>
>> Steven Sharp
>> sharp@cadence.com
>>
>>
>