Lazy functions, lazy arrays

[John Colvin via Digitalmars-d-learn]

On Friday, 20 March 2015 at 14:20:16 UTC, John Colvin wrote:
> 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.

I made a mistake about the static variable and thread-local 
storage.

immutable(int)[] factorials()() @property
{
     static immutable int[N] results = memoizeFactorials(N);
     return results[];
}

is the correct way to do it if you have to.

Still, it's totally not worth doing for such a short array.