Posting here instead of learn because I think it uncovers a design flaw

void main(string[] args)
{
    import std.file : dirEntries, SpanMode;
    import std.stdio : writeln;
    foreach(file; dirEntries(args[1], SpanMode.depth))
        writeln(file.name);
}

Modify this program such that it will print "<file.name> access denied" instead of crashing with an exception whenever it hits a permissions problem. Remember that you might not even have permission to read the directory given in args[1]. Remember that access permissions can change at any time.

It can be done, but it is seriously ugly.

On Friday, 18 September 2015 at 11:35:45 UTC, John Colvin wrote:
> Posting here instead of learn because I think it uncovers a design flaw
>
> void main(string[] args)
> {
>     import std.file : dirEntries, SpanMode;
>     import std.stdio : writeln;
>     foreach(file; dirEntries(args[1], SpanMode.depth))
>         writeln(file.name);
> }
>
> Modify this program such that it will print "<file.name> access denied" instead of crashing with an exception whenever it hits a permissions problem. Remember that you might not even have permission to read the directory given in args[1]. Remember that access permissions can change at any time.
>
> It can be done, but it is seriously ugly.

http://dlang.org/phobos/std_exception.html#.handle

foreach(file; dirEntries(args[1], SpanMode.depth)
        .handle!(Exception, RangePrimitive.front, (e, r) => DirEntry())) {
    writeln(file.name);
}

change Exception to the Exception Type to handle and select the throwing range primitive (RangePrimitive). Then just supply a delegate that does the actual handling.
This will not break any range chain!

On Friday, 18 September 2015 at 11:35:45 UTC, John Colvin wrote:
> Posting here instead of learn because I think it uncovers a design flaw
>
> void main(string[] args)
> {
>     import std.file : dirEntries, SpanMode;
>     import std.stdio : writeln;
>     foreach(file; dirEntries(args[1], SpanMode.depth))
>         writeln(file.name);
> }
>
> Modify this program such that it will print "<file.name> access denied" instead of crashing with an exception whenever it hits a permissions problem. Remember that you might not even have permission to read the directory given in args[1]. Remember that access permissions can change at any time.
>
> It can be done, but it is seriously ugly.

https://github.com/D-Programming-Language/phobos/pull/931

I had to move to some urgent stuff instead of improving the PR and later I forgot about it. The discussion points out what not to do when solving this issue. :)

On Friday, 18 September 2015 at 11:54:32 UTC, Robert burner Schadek wrote:
> http://dlang.org/phobos/std_exception.html#.handle
>
> foreach(file; dirEntries(args[1], SpanMode.depth)
>         .handle!(Exception, RangePrimitive.front, (e, r) => DirEntry())) {
>     writeln(file.name);
> }
>
> change Exception to the Exception Type to handle and select the throwing range primitive (RangePrimitive). Then just supply a delegate that does the actual handling.
> This will not break any range chain!

Cool, I wish I had this idea back in 2012.

On Friday, 18 September 2015 at 11:54:32 UTC, Robert burner Schadek wrote:
> On Friday, 18 September 2015 at 11:35:45 UTC, John Colvin wrote:
>> Posting here instead of learn because I think it uncovers a design flaw
>>
>> void main(string[] args)
>> {
>>     import std.file : dirEntries, SpanMode;
>>     import std.stdio : writeln;
>>     foreach(file; dirEntries(args[1], SpanMode.depth))
>>         writeln(file.name);
>> }
>>
>> Modify this program such that it will print "<file.name> access denied" instead of crashing with an exception whenever it hits a permissions problem. Remember that you might not even have permission to read the directory given in args[1]. Remember that access permissions can change at any time.
>>
>> It can be done, but it is seriously ugly.
>
> http://dlang.org/phobos/std_exception.html#.handle
>
> foreach(file; dirEntries(args[1], SpanMode.depth)
>         .handle!(Exception, RangePrimitive.front, (e, r) => DirEntry())) {
>     writeln(file.name);
> }
>
> change Exception to the Exception Type to handle and select the throwing range primitive (RangePrimitive). Then just supply a delegate that does the actual handling.
> This will not break any range chain!

That's neat, didn't know about std.exception.handle

Unfortunately, I think there are two problems with your solution:

1) DirIteratorImpl will throw on popFront, not front. I had to look at source to find out. Is this a failure of documentation or is it actually an implementation detail?

2) it doesn't cover the case where args[1] itself is unreadable, because dirEntries will throw when it's created.

On Friday, 18 September 2015 at 12:17:25 UTC, John Colvin wrote:
> That's neat, didn't know about std.exception.handle

It is at least a year old. I created it because I had a range that threw, and there was nothing to keep a range going or reenter it.

>
> Unfortunately, I think there are two problems with your solution:
>
> 1) DirIteratorImpl will throw on popFront, not front. I had to look at source to find out. Is this a failure of documentation or is it actually an implementation detail?

That one is trivial RangePrimitive.popFront

foreach(file; dirEntries(args[1], SpanMode.depth)
         .handle!(Exception, RangePrimitive.popFront, (e, r) =>  DirEntry())) {
     writeln(file.name);
 }

>
> 2) it doesn't cover the case where args[1] itself is unreadable, because dirEntries will throw when it's created.

well, creating a DirEntry is not a range operation. handle can't help you there.

On Friday, 18 September 2015 at 12:27:37 UTC, Robert burner Schadek wrote:
> On Friday, 18 September 2015 at 12:17:25 UTC, John Colvin wrote:
>> That's neat, didn't know about std.exception.handle
>
> It is at least a year old. I created it because I had a range that threw, and there was nothing to keep a range going or reenter it.
>
>>
>> Unfortunately, I think there are two problems with your solution:
>>
>> 1) DirIteratorImpl will throw on popFront, not front. I had to look at source to find out. Is this a failure of documentation or is it actually an implementation detail?
>
> That one is trivial RangePrimitive.popFront
>
> foreach(file; dirEntries(args[1], SpanMode.depth)
>          .handle!(Exception, RangePrimitive.popFront, (e, r) =>
>  DirEntry())) {
>      writeln(file.name);
>  }

Yes, but implicit in this being an OK solution for people is that no-one ever reorganises the internals of DirIteratorImpl. I guess One could use handle to deal with *all* the range primitives as a defensive countermeasure.

>> 2) it doesn't cover the case where args[1] itself is unreadable, because dirEntries will throw when it's created.
>
> well, creating a DirEntry is not a range operation. handle can't help you there.

Yup :(

On Friday, 18 September 2015 at 12:42:26 UTC, John Colvin wrote:
> Yes, but implicit in this being an OK solution for people is that no-one ever reorganises the internals of DirIteratorImpl. I guess One could use handle to deal with *all* the range primitives as a defensive countermeasure.

yes, "handle" is something you do after the building has started burning

On Friday, 18 September 2015 at 12:42:26 UTC, John Colvin wrote:
>> well, creating a DirEntry is not a range operation. handle can't help you there.
>
> Yup :(

I think you could use std.exception.ifThrown in this case.

foreach(file; dirEntries(args[1], SpanMode.depth)
                  .ifThrown(DirEntry.init)
                  .handle!(Exception, RangePrimitive.popFront, (e, r) =>  DirEntry()))
{
     writeln(file.name);
}

Although I'm not exactly sure what to return for the "error" value... I'm not sure if DirEntry.init will work or not; you'll probably have to mess around with it.

On 18-Sep-2015 15:03, Adrian Matoga wrote:
> On Friday, 18 September 2015 at 11:35:45 UTC, John Colvin wrote:
>> Posting here instead of learn because I think it uncovers a design flaw
>>
>> void main(string[] args)
>> {
>>     import std.file : dirEntries, SpanMode;
>>     import std.stdio : writeln;
>>     foreach(file; dirEntries(args[1], SpanMode.depth))
>>         writeln(file.name);
>> }
>>
>> Modify this program such that it will print "<file.name> access
>> denied" instead of crashing with an exception whenever it hits a
>> permissions problem. Remember that you might not even have permission
>> to read the directory given in args[1]. Remember that access
>> permissions can change at any time.
>>
>> It can be done, but it is seriously ugly.
>
> https://github.com/D-Programming-Language/phobos/pull/931
>
> I had to move to some urgent stuff instead of improving the PR and later
> I forgot about it. The discussion points out what not to do when solving
> this issue. :)

FYI

https://github.com/D-Programming-Language/phobos/pull/2768

-- 
Dmitry Olshansky