thisExePath purity

[Marc Schütz via Digitalmars-d-learn]

On Tuesday, 20 September 2016 at 04:17:21 UTC, crimaniak wrote:
> 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.

Have a look at `std.concurrency.initOnce`:
https://dlang.org/phobos/std_concurrency.html#.initOnce

But you will still need to use assumePure() for calling 
`thisExePath`, and it might do other things that are impure...