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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: Top 10 most common hard skills listed on resumes...
Date: Thu, 29 Aug 2024 13:35:47 +0100
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Bart <bc@freeuk.com> writes:

> On 29/08/2024 00:43, Ben Bacarisse wrote:
>> Bart <bc@freeuk.com> writes:
>> 
>>> On 28/08/2024 15:57, Ben Bacarisse wrote:
>>>> Bart <bc@freeuk.com> writes:
>>>>
>>>>> On 28/08/2024 00:49, Ben Bacarisse wrote:
>>>>
>>>>>> Indeed, and BLISS is not like that.  I had hoped to shed some light on
>>>>>> why there is some logic to BLISS's rather idiosyncratic design.
>>>>>>
>>>>>>> Given a declaration like 'int A' then:
>>>>>>>
>>>>>>>                               BLISS   C
>>>>>>>
>>>>>>> Read or write A's value   .A       A
>>>>>> I don't think that's right.  To change the value at address A (what I
>>>>>> think you mean by "write A's value") you write
>>>>>>      A = 42;
>>>>>> in BLISS.  And to add one to the value at address A you write
>>>>>>      A = .A + 1;
>>>>>
>>>>> OK. That's just makes it more bizarre than I'd thought.
>>>> Curious.  It's what makes it consistent, though it is definitely an
>>>> uncommon approach.
>>>>
>>>>> The example I saw
>>>>> included these lines:
>>>>>
>>>>>       GETNUM(X);            ! returns a value via X
>>>>>       Y = STEP(.X);
>>>>>       PUTNUM(.Y)
>>>>>
>>>>> So in an rvalue context:  X reads its address; while .X reads its
>>>>> value.
>>>> The whole point is to remove the two contexts.  A variable name is
>>>> /always/ an lvalue (which is why it can be assigned).  C has an implicit
>>>> lvalue to rvalue conversion in the contexts you have come to expect it.
>>>> BLISS does not.  You always need a dot to convert to an rvalue.
>>>
>>> This is the kind of thing I meant a few posts back. You don't need to take
>>> A (which refers to some place where you can store values), and tell it to
>>> fetch that value. Most HLLs will do that without being told.
>>>
>>> (My point was that that was a distinguishing feature of HLLs, which is
>>> missing in Forth for example.)
>> We are talking at cross purposes then.  I was not addressing anything
>> about your view of what makes an HLL.
>> 
>>>>> But in an lvalue one:     Y writes its value; .Y may not be defined
>>>>>
>>>>> It looks asymmetric. C like most languages is symmetric, you write 'A = A'
>>>>> with the same syntax on both sides.
>>>> Since assignment is inherently asymmetric (you can't write 3 = A but you
>>>> can write A = 3) C's syntactic symmetry hides a semantic difference.
>>>> What is needed on the two sides is not the same.
>>>
>>> I would argue that it is exactly the same.
>> How do you argue that, given that A=3 is allowed and 3=A is not?
>
> I explained that. LHS and RHS can be identical terms for assignment in
> pretty much every aspect, but there are extra constraints on the LHS.

So you use "exactly the same" to mean "exactly the same except for the
differences".  I don't think this point needs any more discussion, do
you?

> In the case of :
>
>    (c?a:b) = (z?x:y);
>
> C won't allow it, but some other languages will.
>
> Remember that the programmer can only express their intentions in the form
> of syntax.
>
>> ...
>>>>> I assume that in BLISS, A = A is legal, but does something odd like copy
>>>>> A's address into itself.
>>>> What's odd about that?  And why call is a copy operation?  Do you think
>>>> of A = 42 as a copy operation?  BLISS is a low-level system language.
>>>
>>> Why do you mean by call?
>> Typo.  I meant to write... And why call /it/ a copy-operation?  Do you
>> think of A = 42 as a copy operation?
>
> If '=' means assignment, then what else is it?

That's was my question.  You called it (in BLISS) a "copy" operation.
Why did you use that term rather than just saying that "A = A assigns
that address of A to the location A".  I'm trying to find out if your
use of the word copy rather than assign is interesting in some way.

> I don't know why you're always so contradictory. Is it a game trying to
> catch me out on some pendanty? It seems to be popular here.

I wanted to explain how BLISS gets rid of the lvalue/rvalue distinction
because you seemed to have misunderstood it.

> This subthread started with me asking which HLL goes between Assembly and
> C, if C was supposedly mid-level. I don't know how it got on discussing
> what exactly assignment means.

Because, unlike you, I want to understand you before commenting.  It was
a trivial question (made more complex by a typo if mine, for which I'm
sorry).  You found BLISS's meaning of A = A to be "odd" and you
explained the "odd" by using the word "copy" rather than "assigns".  I
just wanted to know if there was more behind your use of the word.

I don't think there is anything that needs further explanation because I
think you just said "copy" when "assigns" would have done.

-- 
Ben.