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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Top 10 most common hard skills listed on resumes...
Date: Mon, 26 Aug 2024 12:30:55 -0700
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Ben Bacarisse <ben@bsb.me.uk> writes:
> Bart <bc@freeuk.com> writes:
>> BLISS is a rather strange language. For something supposedly low level than
>> C, it doesn't have 'goto'.
>>
>> It is also typeless.
>>
>> There is also a key feature that sets it apart from most HLLs: usually if
>> you declare a variable A, then you can access A's value just by writing A;
>> its address is automatically dereferenced.
>
> Not always.  This is where left- and right-evaluation came in.  On the
> left of an assignment A denotes a "place" to receive a value.  On the
> right, it denotes a value obtained from a place.  CPL used the terms and
> C got them via BCPL's documentation.  Viewed like this, BLISS just makes
> "evaluation" a universal concept.

As I recall, the terms "lvalue" and "rvalue" originated with CPL.  The
'l' and 'r' suggest the left and right sides of an assignment.

Disclaimer: I have a couple of CPL documents, and I don't see the terms
"lvalue" and "rvalue" in a quick look.  The PDFs are not searchable.  If
someone has better information, please post it.  Wikipedia does say that
the notion of "l-values" and "r-values" was introduced by CPL.

An expression could be "evaluated for its lvalue", which means
determining what object it designates, or "evaluted for its rvalue",
which C just calls evaluating the expression.  An expression like 2+2
that does not designate an object does not have an lvalue.

So given `int foo = 42;`, in CPL terms evaluating `foo` for its lvalue
yields the identity of that object, and evaluating `foo` for its rvalue
yields the value 42.

C changed the meanings, so that a C lvalue is a kind of expression, not
the result of evaluating an expression.  C doesn't use the term "rvalue"
except in one footnote: "What is sometimes called "rvalue" is in this
document described as the "value of an expression"."

C has implicit *lvalue conversion* which converts an lvalue (expression)
to the value stored in the designated object.  Apparently BLISS requires
this conversion to be done explicitly.  (I don't hate the idea.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */