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digitalmars.D.learn - What is the state of scope function parameter?

reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
(More correctly, "scope storage class".)

   https://dlang.org/spec/function.html#Parameter

still says

   scope: references in the parameter cannot be escaped
          (e.g. assigned to a global variable). Ignored for
          parameters with no references

However, it doesn't behave that way. For example, my example here 
currently is a lie because there is no compilation error with 2.073.2:

 
http://ddili.org/ders/d.en/function_parameters.html#ix_function_parameters.scope

What's the truth? How would you change the text there?

Thank you,
Ali
Mar 28 2017
next sibling parent Jonathan M Davis via Digitalmars-d-learn writes:
On Tuesday, March 28, 2017 22:15:33 Ali ehreli via Digitalmars-d-learn 
wrote:

 (More correctly, "scope storage class".)

    https://dlang.org/spec/function.html#Parameter

 still says

    scope: references in the parameter cannot be escaped
           (e.g. assigned to a global variable). Ignored for
           parameters with no references

 However, it doesn't behave that way. For example, my example here
 currently is a lie because there is no compilation error with 2.073.2:


 http://ddili.org/ders/d.en/function_parameters.html#ix_function_parameters
 .scope

 What's the truth? How would you change the text there?

 Thank you,
 Ali


Not sure when that was changed, since that's not what the spec used to say
on scope. IIRC, it used to be that it did not say anything about references
and was _very_ vague. But unless something major has changed, _all_ scoped
does at this point is make it so that a delegate will not allocate a
closure. I'm not sure that it's even properly checked for escaping (though
it may be). But scoped has zero effect on other types.

Now, I believe that Walter has been working on stuff that would change that,
but I don't think that any of that has made it in yet except maybe with a
compiler switch intended to separate out the changes until they're ready.

- Jonathan M Davis
Mar 29 2017
prev sibling next sibling parent Petar Kirov [ZombineDev] <petar.p.kirov gmail.com> writes:
On Wednesday, 29 March 2017 at 05:15:33 UTC, Ali Çehreli wrote:

 (More correctly, "scope storage class".)

   https://dlang.org/spec/function.html#Parameter

 still says

   scope: references in the parameter cannot be escaped
          (e.g. assigned to a global variable). Ignored for
          parameters with no references

 However, it doesn't behave that way. For example, my example 
 here currently is a lie because there is no compilation error 
 with 2.073.2:


 http://ddili.org/ders/d.en/function_parameters.html#ix_function_parameters.scope

 What's the truth? How would you change the text there?

 Thank you,
 Ali


The truth is that almost non of the scope checks are on, unless 
you compile with -dip1000:
$ source ~/dlang/dmd-2.073.2/activate
$ dmd ddili_scope_test1.d
$ echo $?
0
$ dmd -dip1000 ddili_scope_test1.d
ddili_scope_test1.d(5): Error: scope variable parameter may not 
be returned

Now that's one error, out of two expected, so what's going on 
here? As DIP1000 mentions 
(https://github.com/dlang/DIPs/blob/master/DIPs/DIP1000.md#safe):


 Errors for scope violations are only reported in  safe code.


So if when I changed the code to:

int[] globalSlice;

int[] foo(scope int[] parameter)  safe {
     globalSlice = parameter;    // ← compilation ERROR
     return parameter;           // ← compilation ERROR
}

void main() {
     int[] slice = [ 10, 20 ];
     int[] result = foo(slice);
}

I got:
$ dmd -dip1000 ddili_scope_test1.d
ddili_scope_test1.d(4): Error: scope variable parameter assigned 
to non-scope globalSlice
ddili_scope_test1.d(5): Error: scope variable parameter may not 
be returned

Just as expected.
Mar 29 2017
prev sibling next sibling parent Petar Kirov [ZombineDev] <petar.p.kirov gmail.com> writes:
On Wednesday, 29 March 2017 at 05:15:33 UTC, Ali Çehreli wrote:

 [..] How would you change the text there?


scope: references in the parameter cannot be escaped (e.g. 
assigned
        to a global variable) in  safe code when compiled with 
-dip1000.
        Ignored for parameters with no references.
Mar 29 2017
prev sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Wednesday, 29 March 2017 at 05:15:33 UTC, Ali Çehreli wrote:

   scope: references in the parameter cannot be escaped
          (e.g. assigned to a global variable). Ignored for
          parameters with no references

 However, it doesn't behave that way. For example, my example 
 here currently is a lie because there is no compilation error 
 with 2.073.2:


If you break the rules, even if the compiler doesn't actually 
catch it, you are still writing illegal code and subject to 
runtime undefined behavior and/or compilation errors in future 
versions.

The compiler DOES use the scope attribute to optimize out heap 
allocations in some cases now, which means if you use in 
improperly you are liable for memory corruption.

So maybe it should say "must not" instead of "cannot" since you 
CAN, it is just broken if you do.
Mar 29 2017