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April 03, 2017 RFC: patch statement | ||||
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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. |
April 03, 2017 Re: RFC: patch statement | ||||
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Posted in reply to Dejan Lekic | 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.
We can already do that. Proof of concept:
// =======================
module bar;
class A {
private int n;
public int GetN() {
return n;
}
}
// =======================
module foo;
import bar;
void main() {
import std.stdio : writeln;
auto a = new A();
assert(a.GetN() == 0);
with (patch(a)) { // Look ma, magic!
n = 3;
}
assert(a.GetN() == 3);
}
auto patch(T)(T value) {
return Patch!T(value);
}
struct Patch(T) {
T _payload;
mixin PatchFields!T;
}
mixin template PatchFields(T, int __n = -1) {
static if (__n == -1) {
mixin PatchFields!(T, T.tupleof.length-1);
} else {
enum name = __traits(identifier, T.tupleof[__n]);
mixin("auto "~name~"(U)(U value) {
_payload.tupleof[__n] = value;
}");
static if (__n > 0) {
mixin PatchFields!(T, __n-1);
}
}
}
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April 03, 2017 Re: RFC: patch statement | ||||
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Posted in reply to Dejan Lekic | 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.
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