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digitalmars.D - preview in: it seems that return type auto solves scope variable

reply Dmytro Katyukha <firemage.dima gmail.com> writes:
Hi,

Not sure it is bug, or feature, or what is going on in examples 
below (with `-preview=in` enabled).

For example, we have following code snippet:

```d
import std.stdio;
import std.path;

struct Path {
     private string _path;

      safe pure nothrow this(in string path) {
         _path = path.dup;
     }

      safe pure nothrow this(in string[] segments...) {
         this(std.path.buildNormalizedPath(segments));
     }

     /// Check if current path is inside other path
      safe bool isInside(in Path other) const {
         import std.algorithm: startsWith;
         return this.toAbsolute.segments.startsWith(
             other.toAbsolute.segments);
     }

      safe Path toAbsolute() const {
         return Path(
             std.path.buildNormalizedPath(
                 std.path.absolutePath(_path.dup.expandTilde)));
     }

      safe pure auto segments() const {
         return std.path.pathSplitter(_path);
     }

}

unittest {
     assert(Path("my", "dir", "42").isInside(Path("my", "dir")) == 
true);
     assert(Path("my", "dir", "42").isInside(Path("oth", "dir")) 
== false);
}

void main()
{
     auto p = Path(".");
     auto h = Path("~");
     auto r = p.isInside(h);
}
```

Without `-preview=in` option, this code sample works just fine.
But when i enable `-preview=in` (or just manualy replace `in` 
with `scope const` in `isInside` method) it fails with following 
error:

```
source/app.d(19,13): Error: scope variable `other` assigned to 
non-scope parameter `this` calling `toAbsolute`
```

So, the first question is: where in this code assignment to 
`this` happens?

But, what is more strange, is that if i change return type for 
method `toAbsolute` to `auto`, error disappears:

```diff
-     safe Path toAbsolute() const {
+     safe auto toAbsolute() const {
```

So, the second question is: what happens in this case?

PS: What is the status of preview `in`? Do it have sense to take 
it into account?
Feb 09
parent reply Dennis <dkorpel gmail.com> writes:
On Thursday, 9 February 2023 at 17:42:00 UTC, Dmytro Katyukha 
wrote:

 So, the first question is: where in this code assignment to 
 `this` happens?


When calling a member function like `other.toAbsolute`, you're 
assigning `other` to the implicit `this` parameter, as if you are 
calling `toAbsolute(this: other)`.


 So, the second question is: what happens in this case?


Functions with `auto` return get attributes inferred, just like 
template functions. The compiler infers `scope` based on the 
implementation of `toAbsolute`.
Feb 09
parent reply Dmytro Katyukha <firemage.dima gmail.com> writes:
On Thursday, 9 February 2023 at 17:59:39 UTC, Dennis wrote:

 On Thursday, 9 February 2023 at 17:42:00 UTC, Dmytro Katyukha 
 wrote:

 So, the first question is: where in this code assignment to 
 `this` happens?


 When calling a member function like `other.toAbsolute`, you're 
 assigning `other` to the implicit `this` parameter, as if you 
 are calling `toAbsolute(this: other)`.


 So, the second question is: what happens in this case?


 Functions with `auto` return get attributes inferred, just like 
 template functions. The compiler infers `scope` based on the 
 implementation of `toAbsolute`.


Hi, thank you. Got it)

So, the next questions:
- Do it have sense to explicitly annotate all (most) member 
methods of a struct with `scope` attribute? Or it is better to 
use `auto` everywhere?.
- What drawbacks of annotating all (most) parameters as scope? Is 
it correct approach?
- What about `in` parameter attribute? Why it is not used wide in 
Phobos?

How to correctly call method from standard lib (for example 
[expandTilde](https://dlang.org/phobos/std_path.html#expandTilde)) that does
not annotated param with `scope` attribute?

For example:

```d
 safe unittest {
     scope string p1 = "~/projects/d";
     scope string p2 = expandTilde(p1);
     assert(p1 != p2);
}
```

In this case, following error raised:

```
source/app.d(41,35): Error: scope variable `p1` assigned to 
non-scope parameter `inputPath` calling `expandTilde`

```

Is it correct, to use `.dup` method?

```diff
   safe unittest {
      scope string p1 = "~/projects/d";
-    scope string p2 = expandTilde(p1);
+    scope string p2 = expandTilde(p1.dup);
      assert(p1 != p2);
  }
```
Feb 09
parent tsbockman <thomas.bockman gmail.com> writes:
On Thursday, 9 February 2023 at 21:28:54 UTC, Dmytro Katyukha 
wrote:

 So, the next questions:
 - Do it have sense to explicitly annotate all (most) member 
 methods of a struct with `scope` attribute?
 ...
 - What drawbacks of annotating all (most) parameters as scope?


`scope` restricts what a function's implementation can do, while 
giving more freedom to the caller. (Mostly, this is the freedom 
to allocate less things with the GC.)

This is generally a good thing, provided that you understand when 
`scope` and `return scope` apply well enough not to be slowed 
down or confused by them too much.

Of course, sometimes you will actually have a good reason to 
escape a reference to a global or whatever, and `scope` doesn't 
do anything for types without indirections, so obviously you 
shouldn't put `scope` *everwhere*.


 Or it is better to use `auto` everywhere?.


Whether it is better to explicitly write out attributes and 
types, or have the compiler infer them via `auto` and/or 
templatization, depends on whether you want the compiler to give 
you an error message if you change the implementation in a way 
that alters the API, and how you value explicit documentation of 
the API versus keeping your code concise.

I recommend being as explicit as practical for public APIs, and 
for any aspect of any API that is fundamental to its purpose. 
(For example, a function intended to be used in CTFE should be 
explicitly `pure`.)

Just be careful not to over-constrain template APIs without a 
good reason.


 - What about `in` parameter attribute? Why it is not used wide 
 in Phobos?


Most of Phobos was written before the semantics of `in` or 
`scope` were finalized. That's why there is `-preview=in` and 
`-dip1000`, because them actually working is a somewhat new, 
work-in-progress feature.


 How to correctly call method from standard lib (for example 
 [expandTilde](https://dlang.org/phobos/std_path.html#expandTilde)) that does
not annotated param with `scope` attribute?


If there is no obvious valid reason for the function to be 
escaping one of its parameters, it's likely a bug in Phobos - a 
bit of code that hasn't been fully updated yet for `-dip1000`. 
This is especially likely to be the case if the parameter in 
question is not `immutable`.


 For example:

 ```d
  safe unittest {
     scope string p1 = "~/projects/d";
     scope string p2 = expandTilde(p1);
     assert(p1 != p2);
 }
 ```

 In this case, following error raised:

 ```
 source/app.d(41,35): Error: scope variable `p1` assigned to 
 non-scope parameter `inputPath` calling `expandTilde`

 ```

 Is it correct, to use `.dup` method?

 ```diff
   safe unittest {
      scope string p1 = "~/projects/d";
 -    scope string p2 = expandTilde(p1);
 +    scope string p2 = expandTilde(p1.dup);
      assert(p1 != p2);
  }
 ```


Either use `.dup`, `.idup`, or don't make `p1` `scope` in the 
first place.

The last option is preferred if you know you would otherwise need 
to `.dup` the majority of the time. Also, the compiler is 
generally pretty good about inferring `scope` where needed for 
local stack variables, so `scope` should mostly only appear 
explicitly when receiving `scope` parameters, not when sending 
them.
Feb 09