Just requiring explicit assignment before first use feels like the superior approach to automatic initialization, regardless of whether the automatic initialization is with 0 or with NaN.
The trouble with it is a bug I've seen often. People will get an error message about an "uninitialized variable". Then they go into "just get the compiler to shut up" mode, amd pick "0" as the initializer. Then, the program compiles and runs, and silently produces the wrong answer. Code reviews will simply pass over the "0" initializer, as it looks right.
With default NaN initialization, the programmer is more likely to stop and think about it, not just insert 0.
Another issue with it is:
float x = 0.0;
setFloat(&x);
void setFloat(float* px) { *px = 3.0; }
int x = void;
To find the source of a NaN, it helps to know that every operation that has a NaN as an operand produces a NaN as a result. So if you see a NaN in the output, you can work backwards to where it originated.
That's not true. The minimum/maximum functions (fmin and fminimum_num variants, but not the fminimum one) treat NaN inputs as not-present, so return the non-NaN value if there is one. Similarly, hypot also treats NaN inputs as not-present. pow and compoundn will ignore NaN exponents if the base is 1.
But I contend it's more useful (and interesting) to think about the idea with your own mind instead of tallying up the perceived authority of its supporters and relying on trust. It was also somewhat rude to suggest that the OP had not given their idea much thought. This is a forum for discussion, isn't it?
1. just insert '= 0;' to get it to compile
2. insert '= 0;' and then be puzzled by an initialization further along in the code
3. see the '= 0;' and wonder why the programmer did that as 0 was not a valid value for it
A goal of D is to be able to make code more understandable. Forcing a vacuous initialization on the programmer is not conducive to that.
An assignment is required at some point before the first read, not in the declaration. It tracks assignments and usages, and it flags a compiler error if you read a variable before assigning to it for the first time. A variable that hasn't been assigned cannot be read.
It means you can do "int a;" and then later in the function do "a = 5;" and the compiler guarantees that you never read the variable before the assignment in any path through the function. You cannot do "int a;" and then read from it; that's a compile-time error.
It does not mean you have to assign something in the declaration. We never need "vacuous" initializations, and this solution works on all types. Indeed, we avoid vacuous initializations so that the compiler will catch use-before-assign bugs at compile time. The situation you described doesn't happen in C#. Our C# variables become readable on their first assignment, not their declaration; the declaration merely sets the scope. There's no need for a state where it's initialized to an invalid value before receiving the first intended assignment, because in C# the variable is completely inaccessible during that time.
An analogous thing happens in D:
int x;
x = 5;
In any event, I don't think Walter needed any help here. He is an HN veteran and always willing to discuss the points. Every programming language designer loves an opportunity to discuss their language with interested people! There's almost never a truly right answer in language design, just various tradeoffs.
You might note my original comment included softening elements ("perhaps", "not saying you're wrong"). In general if you look at all my comments you'll see I'm not a rude person, I'm pretty agreeable in general.
I was (trying to) make a meta point rather than a point about the specific technical issue. I agree (again!) that the last word has not been said on this issue or on any other issue where tradeoffs need to be weighed.
I read Walter's comment and thought "Wow, that's a surprising, clever and innovative idea, I'm impressed". And I just didn't enjoy someone bluntly saying, in effect. "No you're wrong, you shouldn't do it like that". It's as simple as that really. I know blunt exchanges of views are normal for programmers and engineers, I don't have to like it every time.
Finally, I know Walter Bright is no shrinking violet and he definitely doesn't need me to defend him!
Like not allowing macros in D, or version algebra.