lazy variables

[Ali Çehreli]

On 10/17/2018 12:32 AM, aliak wrote:
> Hi,
> 
> Is there any notion of lazy vars in D (i see that there're parameters)?
> 
> i.e:
> 
> struct S {
>    //...
>    int y;
>    //...
> }
> 
> lazy S x = () {
>      // do some heavy stuff
> }();
> 
> if (condition) {
>    func(x.y); // heavy stuff evaluated here
> }
> 
> Cheers,
> - Ali
> 
> 
> 
> 

Not very clean but something like this:

import std.stdio;

struct LazyVar(alias exp) {
     alias T = typeof(exp());
     T value() {
         static bool initialized = false;
         static T val;

         if (!initialized) {
             val = exp();
             initialized = true;
         }
         return val;
     }

     alias value this;
}

LazyVar!(() {
     writeln("Doing heavy stuff");
     return 42;
}) a;

void main() {
     auto b = a.value;    // Must specify .value or
     int c = a;           // must specify type of value (int).
                          // Otherwise, b and c have type LazyVar!(...)

     // Some more usage
     b = c = a;

     assert(b == 42);
     assert(c == 42);
}

Ali