Fixed size array initialization

[rjframe]

On Sat, 10 Feb 2018 10:55:30 +0000, rumbu wrote:

> I know that according to language spec
> (https://dlang.org/spec/arrays.html#static-init-static) you can skip
> declaring all your elements in a fixed size array.
> 
> I'm just recovering from a bug which took me one day to discover because
> of this.
> 
> I have a large static initialized array, let's say int[155], and I
> forgot to declare the last element:
> 
> int[155] myarray = [
>    a,
>    b,
>    c,
>    ...
>    //forgot to declare the 155th element
> ];
> 
> I took for granted that the compiler will warn me about the fact that my
> number of elements doesn't match the array declaration but I was wrong.
> 
> Does it worth to fill an enhancement on this, or this is intended
> behavior?

If you separate initialization to a static this, you'll get a compile 
error:

```
immutable uint256[78] pow10_256;

static this() {
    // Error: mismatched array lengths, 78 and 2
    pow10_256 = [
        uint256(1UL),
        uint256(10UL)
    ];
}
```

Or if you know that no element should be the object's init value, you 
could do a static foreach to validate that at compile-time.

You could also generate the elements at compile-time (this could use some 
improvement, and should be generalized so a single function can generate 
fillers for your other arrays as well):

```
immutable uint256[78] pow10_256;

static this() {
    // Uncomment this to view the generated code. Helpful for debugging.
    //pragma(msg, GenPow10_256Initializer);
    mixin(GenPow10_256Initializer);
}

static string GenPow10_256Initializer() {
    import std.range : repeat;
    import std.conv : text;

    string code = "pow10_256 = [\n";
    foreach(i; 0..78) {
        code ~= `    uint256("1` ~ '0'.repeat(i).text ~ `")` ~ ",\n";
    }
    code = code[0..$-2]; // Remove trailing comma.
    code ~= "\n];";
    return code;
}
```