Fixed size array initialization

[rumbu]

On Sunday, 11 February 2018 at 14:06:32 UTC, rjframe wrote:
> On Sat, 10 Feb 2018 10:55:30 +0000, rumbu wrote:
>
> 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)
>     ];
> }
> ```

Yes, this was in fact the idea of 
https://github.com/dlang/phobos/pull/4936
Didn't like it, sorry.

>
> 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.

Nice idea too, but this will significantly increase compilation 
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;
> }
> ```

As I said in my previous comments, this was the initial approach 
for all the arrays there (I'm a very lazy person, believe me) but 
the compiler complained with a nice "Out of memory" error, that's 
why I ended writing the array elements by hand.