thisExePath purity

[crimaniak via Digitalmars-d-learn]

Hi and thanks all!

On Tuesday, 20 September 2016 at 00:43:10 UTC, Jonathan M Davis 
wrote:

> immutable string executablePath;
>
> shared static this()
> {
>     import std.file : thisExePath();
>     executablePath = thisExePath();
> }

This code is good for my needs but I start to think about how to 
call thisExePath only if it is really used and come to this 
solution:

import std.traits: ReturnType, Parameters;

string staticMemoize(alias T, Parms = Parameters!T)() pure
{
     struct Holder(alias T)
     {
         static shared immutable ReturnType!T value;
         shared static this(){ value = T(Parms); }
     }

     return Holder!T.value;
}

unittest
{
     import std.file : thisExePath;
     assert(staticMemoize!thisExePath == thisExePath);
}

Something like this. Need to refine about input parameters, but I 
hope, idea is clear.
Unlike the function memoize from phobos staticMemoize really 
pure. And unlike proposed solution with ordinary variable 
staticMemoize is lazy, because no call - no instantiation.

https://dlang.org/library/std/functional/memoize.html