• Olivier Prat (21/21) Mar 25 2023 I'm trying to call assumeSorted on a const(Array) using this code
• Olivier Prat (5/8) Mar 25 2023 In a similar fashion, a number of methods in SortedRange do not
• =?UTF-8?Q?Ali_=c3=87ehreli?= (8/17) Mar 25 2023 There are a number of interesting points in your original post but I
• Steven Schveighoffer (26/29) Mar 25 2023 It's because a Range keeps a copy of the array (Array is a reference
• Olivier Prat (6/33) Mar 26 2023 Thanks to both of you for the answers. It confirms my

Olivier Prat <o.prat free.fr> writes:

I'm trying to call assumeSorted on a const(Array) using this code 
snippet:

     void test(T)(const ref Array!T a)
     {
         auto b = a[].assumeSorted;
         writeln(b);
     }

     void main() {
         Array!int a = [1, 5, 7, 8, 15, 100];
         test(a);
     }

Unfortunately, I keep having a compilation error:
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(10871): Error:
cannot modify struct instance `result._input` of type
`RangeT!(const(Array!int))` because it contains `const` or `immutable` members
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(10918): Error:
cannot modify struct instance `result._input` of type
`RangeT!(const(Array!int))` because it contains `const` or `immutable` members
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(11500): Error:
template instance `std.range.SortedRange!(RangeT!(const(Array!int)), "a < b",
SortedRangeOptions.assumeSorted)` error instantiating
onlineapp.d(10):        instantiated from here: `assumeSorted!("a 
< b", RangeT!(const(Array!int)))`
onlineapp.d(17):        instantiated from here: `test!int`

Would someone explain the exact nature of this error as I fail to 
understand as how just telling that an Array ConstRange is 
assumed to be sorted actually modifies anything??

Olivier Prat <o.prat free.fr> writes:

On Saturday, 25 March 2023 at 13:45:36 UTC, Olivier Prat wrote:
 I'm trying to call assumeSorted on a const(Array) using this 
 code snippet:

 [...]

In a similar fashion, a number of methods in SortedRange do not 
compile if called on a const(SortedRange) or 
immutable(SortedRange), such as length, or front. Is this on 
purpose?

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 3/25/23 09:31, Olivier Prat wrote:
 On Saturday, 25 March 2023 at 13:45:36 UTC, Olivier Prat wrote:
 I'm trying to call assumeSorted on a const(Array) using this code 
 snippet:

 [...]

 
 In a similar fashion, a number of methods in SortedRange do not compile 
 if called on a const(SortedRange) or immutable(SortedRange), such as 
 length, or front. Is this on purpose?

There are a number of interesting points in your original post but I 
couldn't find time to answer all of those.

I can inject this for now: :) assumeSorted returns a range object. The 
concept of a const range object is flawed because by nature iteration of 
a range happens by mutating it: For example, popBack has to do that.

So, it's normal that any range algorithm will assume a non-const object.

Ali

Steven Schveighoffer <schveiguy gmail.com> writes:

On 3/25/23 9:45 AM, Olivier Prat wrote:

 Would someone explain the exact nature of this error as I fail to 
 understand as how just telling that an Array ConstRange is assumed to be 
 sorted actually modifies anything??

It's because a Range keeps a copy of the array (Array is a reference 
counted type). Since that is labeled `const`, then editing the Range is 
forbidden.

Inside SortedRange, it has this, which is causing the issue:

```d
     static if (isForwardRange!Range)
      property auto save()
     {
         // Avoid the constructor
         typeof(this) result = this;
         result._input = _input.save;
         return result;
     }
```

Overwriting the input isn't possible here, because it contains a const 
member.

Now, this possibly could be fixed with something like:

`return typeof(this)(_input.save);`

But it might just push the error to another place.

The whole thing is kind of ugly.

There is a note inside the Array Range which says it's trying to work 
around some old bug that is now marked as "works for me", so maybe that 
can be reexamined.

https://github.com/dlang/phobos/blob/17b1a11afd74f9f8a69af93d77d4548a557e1b89/std/container/array.d#L137

-Steve

Olivier Prat <o.prat free.fr> writes:

On Sunday, 26 March 2023 at 02:16:15 UTC, Steven Schveighoffer 
wrote:
 On 3/25/23 9:45 AM, Olivier Prat wrote:

 [...]

 It's because a Range keeps a copy of the array (Array is a 
 reference counted type). Since that is labeled `const`, then 
 editing the Range is forbidden.

 Inside SortedRange, it has this, which is causing the issue:

 ```d
     static if (isForwardRange!Range)
      property auto save()
     {
         // Avoid the constructor
         typeof(this) result = this;
         result._input = _input.save;
         return result;
     }
 ```

 Overwriting the input isn't possible here, because it contains 
 a const member.

 Now, this possibly could be fixed with something like:

 `return typeof(this)(_input.save);`

 But it might just push the error to another place.

 The whole thing is kind of ugly.

 There is a note inside the Array Range which says it's trying 
 to work around some old bug that is now marked as "works for 
 me", so maybe that can be reexamined.

 https://github.com/dlang/phobos/blob/17b1a11afd74f9f8a69af93d77d4548a557e1b89/std/container/array.d#L137

 -Steve

Thanks to both of you for the answers. It confirms my 
investigation and my workaround is to cast the const ranges into 
their non const versions, when I encounter these issues. Not very 
pretty.