Mixing <const> with <comptime>

[stefanos82]

Based on #80 comment I made about the generated C code, I've got curious to see how C++ would behave with constexpr in this case, as assigning <const> to <const> resembles this behavior.

In C++ we can do the following:

#include <iostream>

static const int x = 1;
static constexpr int a = x;

int main()
{
    std::cout << a << '\n'; // outputs '1'
}

and works as expected.

I have tried the same in Nelua and it does not work...

local x <const> = 1
local a <comptime> = x
print(a) -- I was hoping to get '1' as my output

So, how exactly <const> getting assigned to another <const> works behind the scenes during preprocessing procedure?

[edubart]

This is not possible, because <const> is a runtime constant, while <comptime> is a compile-time constant. The compiler does not store or process values about runtime constants, even if it's visible, so they cannot be mixed, for this case to work both should be <comptime> instead.

So, how exactly getting assigned to another works behind the scenes during preprocessing procedure?

The <const> just serves one purpose, to disallow variable assignment throwing a compile error on any assignment attempt, other than that, it behaves just as usual non-const variable. Their values are unknown at compile-time (like runtime variables values are unknown).

[stefanos82]

This is not possible, because <const> is a runtime constant, while <comptime> is a compile-time constant. The compiler does not store or process values about runtime constants, even if it's visible, so they cannot be mixed, for this case to work both should be <comptime> instead.

When you say "runtime constant" here, you mean Nelua's runtime or executable's runtime process?

[edubart]

I mean executable's runtime process.