RFC: patch statement

[Minty Fresh via Digitalmars-d]

On Monday, 3 April 2017 at 11:16:57 UTC, Dejan Lekic wrote:
> I know people her do not like to see proposals that change (add 
> stuff to) the language. However, I strongly feel that for the 
> testing purposes D should provide means to patch any object (no 
> matter whether it is final or not!). Therefore I wonder what 
> people think of adding a `patch(obj) {}` or perhaps change the 
> semantics of the `with(obj) {}` so unittest writers can modify 
> the object and set values.
>
> The patch keyword would work ONLY inside unittest {} blocks AND 
> inside functions annotated with @test annotation.
>
> Imagine we have:
>
> int myFun(Person person) { /* some logic here */ }
>
> unittest {
>   auto p = new Person() /* does not really matter which 
> constructor we use */
>   patch(p) {
>     // here we can modify ANY attribute, no matter whether it 
> is private or public
>     p.fname = "Nikola"
>     p.sname = "Tesla"
>   }
>   auto res = myFun(p)
>   // do some assertions here
> }
>
> Similarly:
>
> @test
> void test_myFun() {
>   // same code as in the unittest above.
> }
>
> I do not even know if patch() {} statement is even possible, 
> that is the whole point of writing this, so people can 
> enlighten me... :)
>
> As I said in the introduction paragraph, for this purpose the 
> semantics of the with statement could be changed, but I prefer 
> a different keyword (patch) to be honest.

It looks like what you're trying to do is set up object mocks for 
unit testing.
In general, I find that well designed libraries provide such 
tools for testing, either in the form of factory functions or 
some other means of constructing mocks for test builds. I try to 
follow such patterns myself.

Getting back to the immediate subject:
You can already grant write access to whatever attributes with a 
bit of conditional compilation. Notably, defining accessors that 
exist only for unittest builds.

You could even go so far as to define a generalized one,
ie.
version(unittest)
{
     void patch(string attr, T)(Person p, T value)
     {
         __traits(getMember, p, attr) = value;
     }
}

And then, elsewhere:

// Given p is some Person.
p.patch!"fname" = "Nikola";
p.patch!"sname" = "Tesla";

So long as this is defined in the same module as the type, it'll 
be able to access protected and private fields.