Lazy functions, lazy arrays

[John Colvin via Digitalmars-d-learn]

On Friday, 20 March 2015 at 13:35:10 UTC, Dennis Ritchie wrote:
>> Use case?
>
> No. I need to be able to make an array "factorials" is not 
> evaluated, if I don't.
>
> import std.stdio;
>
> enum N = 15;
>
> static int[] factorials = memoizeFactorials(N); // lazy array? 
> :)
>
> int[] memoizeFactorials(int n)
> {
>     if (!__ctfe) {
>         // Make sure that this function is never called at run 
> time
>         assert(false);
>     }
>
>     int[] result = new int[n];
>
>     result[0] = 1;
>
>     foreach (i; 1 .. n) {
>         result[i] = result[i - 1] * i;
>     }
>
>     return result;
> }
>
> void main()
> {
> 	writeln(factorials[10]);
> }

Why? To make a smaller executable? For faster compilation?

This can work

auto factorials()() @property
{
     //if we're here, factorials are used somewhere

     //generate them at compile-time.
     enum int[N] resultsE = memoizeFactorials(N);
     //put them in thread-local array on first access
     static resultsS = resultsE;

     return resultsS[];
}

void main()
{
     writeln(factorials[10]);
}

or much easier and simpler, but different:

auto factorials()(int n)
{
     //generate them at compile-time.
     enum int[N] results = memoizeFactorials(N);

     return results[n];
}

void main()
{
     writeln(factorials(10));
}

However, none of this is a good idea at all. There are only 13 
factorials (0 through 12) that fit in an int, so it's such a 
small array that you might as well write

enum int[N] factorials = memoizeFactorials(N);

and be done.