[alterom]

>What is the value of a spec to which compliance is impossible?

Are you saying, what's the value of a language spec that allows undefined behavior, as C does?

Well, it's that it allows for compiler implementations that aren't too hard to implement and maintain.

It allows for a language that's close enough to hardware (and allows you to do programming on a low level), while still offering a reasonable amount of abstraction to be useful (and usable).

It's also difficult to define a formal system that won't have undefined expressions. Mathematics itself is full of them (in logic, "this sentence is a lie" has no truth value; you can't define the set of all sets, or a set of sets that don't contain themselves; etc).

That said, I think we've settled on a rather silly choice here with the "++" operator.

Personally, I'd do away with the ++ operator in either pre- or post- increment forms, or at least disallow it in arithmetic expressions.

The only thing having it realistically accomplished is saving a few characters when writing a for-loop in C.

Even for that it's not necessary.

The problem with it is that, unlike normal arithmetic operators, it both returns a value and assigns one, which means that you can assign values to several variables in a single arithmetic expression, as in

     a = b++;

...which C, in general, allows, as in:

     a = (b = b + 1);

The result of these two expressions, of course, is different.

Now, I have the following religious belief, and it's that arithmetic operators shouldn't have side effects. That's to say, assignment and evaluation should be separate.

So that when I write

     x = (arithmetic);

..I could be sure that the only outcome of this computation is changing the value of x.

Perhaps calling the function sqrt(x) would summon Cthulhu — I'll read the documentation for it to be sure. But in general, I'd hope that calling abs(x) wouldn't change the value of x to |x| in addition to returning it.

But K&R decided to have fun by saying that "x = 5;" is both an assignment and an expression with a value. Which allows one to write:

      x = y = z = 5;

as a parlor trick.

That's it, that's the only utility.

Instead of defining this as a special initialization syntax and otherwise disallowing it (as Pyhthon does), they went YOLO and made assignment an expression rather than a mere statement.

Which means that the very useful statement "increase the value of this variable by one" became two expressions with different values.

In an ideal world, the following would be equivalent, and would not evaluate to anything you can assign to a variable:

     ++x;
     x += 1;
     x = x + 1;

...while "x++" would not exist at all (or would be equivalent to ++x).

And that's how it is in Go. Thompson fixed the design mistake after 4-5 decades of it giving everyone headaches.

Sadly, C++, Java, C# all wanted to be "like C" in basic syntax, so we're stuck with puzzles like this to this day.

TL;DR: if you're asking "what's the value of the spec that makes assignment an expression", i.e. why is making "a = (b = c + d);" valid syntax a good idea, the answer is:

It isn't. It's a bad decision made in 1970s that modern languages like Go no longer support.

[thaumasiotes]

> Well, it's that it allows for compiler implementations that aren't too hard to implement and maintain.

> It allows for a language that's close enough to hardware (and allows you to do programming on a low level), while still offering a reasonable amount of abstraction to be useful (and usable).

I can see the first of these. The second appears to be untrue; if you removed the concept of undefined behavior from C, it wouldn't get farther away from the hardware.

Is that first point actually something that somebody wants? Who benefits from the idea that it's easy to write a "standards-compliant" compiler, because you are technically "standards-compliant" whether you comply with the standard or not?

At that point, you've given up on having a standard, and the interviewers Susam calls out, who say that the correct answer is whatever their compiler says it is, are correct in fact. Susam is the one who's wrong, for reading the standard.

You can run a language that way just fine. I had the impression that Perl was defined by a reference implementation. But it's the opposite of having a standard.

[alterom]

>The second appears to be untrue; if you removed the concept of undefined behavior from C, it wouldn't get farther away from the hardware

My understanding is that even common CPU instruction sets can have undefined behavior[1].

When C was written, the CPU architectures were more of a Wild West. It might have made sense to leave some parts up to the compiler authors on a particular architecture.

>Is that first point actually something that somebody wants?

When C was written — absolutely.

Portability of C code is almost taken for granted these days.

Things were different then. Portability was a big challenge.

All that said, this is my non-authoritative understanding of the reasons why it's a thing. Take it with a grain of salt.

>At that point, you've given up on having a standard

Sure. Just treat C as a family of languages which have a common standardized part.

Proprietary compiler extensions are/were common anyway, so that's not an unusual situation.

[1] https://www.os2museum.com/wp/undefined-isnt-unpredictable/

[reichstein]

Assigning to multiple variables in a single expression is fine and useful. Take

``` target[i++] = source1[j++] + source2[k++]; ``` That's idiomatic, it shows the intent to read and consume the value in a single expression. You can write it longer, but not more clearly.

It's only when you assign to the same variable multiple times, or read it after it was assigned, that it introduces ordering issues.

A single `i++` or `++i`/`i += 1` is safe and useful.

[alterom]

>A single `i++` or `++i`/`i += 1` is safe and useful

Sure, and you don't need the assignment to be an expression with a value for it to be useful.

>target[i++] = source1[j++] + source2[k++]; That's idiomatic

That's idiomatic to C for sure.

Also idiomatically horrible. Why are you using three index variables here?

>You can write it longer, but not more clearly.

    target[i] = source1[i] + source2[i];

    i++;

This is absolutely more clear to any sane person, and less prone to error.

You can't forget to increase one if the indices when all three are meant to go in lockstep.

It's longer by one semicolon, and requires far less cognitive overload to parse.

There's a reason why they did away with it in Go. What do you think that reason was if it's so useful?

[1718627440]

> Why are you using three index variables here?

> You can't forget to increase one if the indices when all three are meant to go in lockstep.

Obviously they are not in this example.

The next line might contain:

    i++; j *= 42; k = srandom (k), random ();

[alterom]

>Obviously they are not in this example

...of utter insanity which doesn't belong in any real world code.

It just keeps getting worse, and shows why it was a horrible idea to allow this in the first place.

>The next line might contain:

    i++; j *= 42; k = srandom (k), random ();

Then that's where you do the arithmetic.

You're already doing it there, why do you need to do an assignment inside the brackets?

(This was a rhetorical question. You don't).

[1718627440]

If you only have a single index, that you continue to increment, you don't need an index at all, you just invoke memcpy.

It is useful to distinguish between consuming an element and only jumping to it. So you would have an ptr[i++] for consuming the current token, but not, when you are switching to another token.

A grouping of index and array modification also provides clarity about the intention. It would read very annoying if that constantly would be split into two steps, and also provides more room for error.

It's obvious that you do prefer the stylistic choices Go made, but that doesn't mean everyone does.

[alterom]

>It is useful to distinguish between consuming an element and only jumping to it. So you would have an ptr[i++] for consuming the current token, but not, when you are switching to another token.

So, it's trivial to do it with a function. Or a macro.

    int adv(int *i){
        int t = *i;
        (*i) = t + 1;
        return t;
    }
    
    while(i < 10)
        printf("%d\n", adv(&i));

Not to mention, iterators and all that jazz.

The point is, you don't need assignment to return a value to have this.

Can you give a non-contrived example where you really need it?

>It's obvious that you do prefer the stylistic choices Go made, but that doesn't mean everyone does.

It's not that I "prefer stylistic choices of Go", it's that I hate to have undefined behavior in language spec which is easy to stumble into - the cost of the "stylistic choice" that C made doesn't make that choice justified.