New feature proposal: "initialization scope"

[Kenji Hara]

2013/5/16 TommiT <tommitissari at hotmail.com>

> I'd like to make it easier to initialize function local immutable/const
> data. Here's the type of problem I'd like to alleviate:
>
> const string[100] int__str;
> const int[string] str__int;
>
> for (int i = 0; i < 100; ++i)
> {
>     auto str = to!string(i);
>     int__str[i] = str; // ERROR: Can't modify const
>     str__int[str] = i; // ERROR: Can't modify const
> }
>
> In short, I want to initialize two different const variables at once (in
> the same loop or other block). If I needed to initialize only one const
> variable, I could use a lambda:
>
> const string[100] int__str = {
>     string[100] tmp;
>     // ... init tmp ...
>     return tmp;
> }();
>
> ...But I can't see any easy solution for initializing two or more const
> variables at the same time.
>
> Here's my proposal: "initialization scope". You'd use it like this:
>
> initialization {
>     const string[100] int__str;
>     const int[string] str__int;
>
>     for (int i = 0; i < 100; ++i)
>     {
>         auto str = to!string(i);
>         int__str[i] = str; // OK
>         str__int[str] = i; // OK
>     }
> }
>
> string s = int__str[42]; // OK
> int__str[42] = "43" // ERROR: Can't modify const
>
> As you can see, 'initialization scope' would be a scope that is not a
> lexical scope (like static if), it merely makes all const and immutable
> variables created in that scope modifiable inside that scope but not after
> it.
>

Pure delegate and implicit conversion for unique expression should work.

import std.conv;
void main()
{
    immutable string[100] int__str = () pure {
        string[100] tmp;
        debug printf("&tmp[0] = %p\n", &tmp[0]);        // [1]
        for (int i = 0; i < 100; ++i)
        {
            auto str = to!string(i);
            // BUG: pure function '__lambda1' cannot call impure function
'to'
            tmp[i] = str;
        }
        return tmp; // BUG: NRVO doesn't work
    }();
    debug printf("&int__str[0] = %p\n", &int__str[0]);  // [2]
}

Compiler bug: Currently [1] and [2] does not print same address.
Phobos bug: Currently std.conv.to is not pure.

Kenji Hara
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