alt Hacker News> Also, imagine a situation where the line of code actually lists three different variables, but all three of them are passed in by address. It quickly becomes impossible for the compiler to know you violated the spec by reusing the same variable.
OK. What is the value of a spec to which compliance is impossible?
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> OK. What is the value of a spec to which compliance is impossible?
It lets you tell people you have a spec? It makes it easy for compiler developers to dismiss bug reports with "your code violated the spec"?
Welcome to C.
But more seriously it's the job of the program to not do undefined things.
>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++;
a = (b = b + 1);
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);
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;
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;
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.
The compiler does comply to the spec. It's the program that fails to comply with the spec. It's definitely possible to write programs that have no undefined behavior.