April 19, 2012
Perhaps I'm just misunderstanding something about closures but the following code seems to behave oddly:

    import std.stdio, std.range, std.algorithm, std.string;

    void main()
    {
        auto lst = ["a", "b"];
        auto rng = range_gen(lst);
        writeln(rng.take(5));
    }
    auto range_gen(string[] lst)
    {
        auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0], a))();
        return chain(lst, a); // access violation
        //return a; // works
    }

Returning with the chain() gives an access violation after the writeln has processed the first two elements and gets to the elements generated by map(sequence()) (the output is '["a", "b", ').  If I just return the map(sequence()) it works correctly.  If I don't use lst[0] in the map and instead use a literal it works without issue.  It also works without issue if I use a global in place of lst[0].

I can work around this. Moving the chain() outside to the calling function seems to work fine but I was making using of ResultOf to type a member variable which is why I had the function in the first place (I can manually type it but it's pretty ugly).

Regards,
Brad Anderson
April 19, 2012
"Brad Anderson" , dans le message (digitalmars.D.learn:34902), a écrit :
> Perhaps I'm just misunderstanding something about closures but the following code seems to behave oddly:
> 
>      import std.stdio, std.range, std.algorithm, std.string;
> 
>      void main()
>      {
>          auto lst = ["a", "b"];
>          auto rng = range_gen(lst);
>          writeln(rng.take(5));
>      }
>      auto range_gen(string[] lst)
>      {
>          auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0],
> a))();
>          return chain(lst, a); // access violation
>          //return a; // works
>      }

My guess is that chain takes lst by reference, just like the delegates for map, wo both are working on the same slice instance. The chain first pops elements from lst, and then calls the mapped sequence. At that time, lst is empty.

You can just copy lst before you give it to chain (or to map's delegate) to solve this bug:

auto range_gen(string[] lst)
{
     auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0], a))();
     string[] lst2 = lst;
     return chain(lst2, a); // access violation
}

April 19, 2012
On Thu, Apr 19, 2012 at 7:25 AM, Christophe <travert@phare.normalesup.org>wrote:

> "Brad Anderson" , dans le message (digitalmars.D.learn:34902), a écrit :
> > Perhaps I'm just misunderstanding something about closures but the following code seems to behave oddly:
> >
> >      import std.stdio, std.range, std.algorithm, std.string;
> >
> >      void main()
> >      {
> >          auto lst = ["a", "b"];
> >          auto rng = range_gen(lst);
> >          writeln(rng.take(5));
> >      }
> >      auto range_gen(string[] lst)
> >      {
> >          auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0],
> > a))();
> >          return chain(lst, a); // access violation
> >          //return a; // works
> >      }
>
> My guess is that chain takes lst by reference, just like the delegates for map, wo both are working on the same slice instance. The chain first pops elements from lst, and then calls the mapped sequence. At that time, lst is empty.
>
> You can just copy lst before you give it to chain (or to map's delegate)
> to solve this bug:
>
> auto range_gen(string[] lst)
> {
>     auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0], a))();
>      string[] lst2 = lst;
>     return chain(lst2, a); // access violation
> }
>
>
Ah, that would make sense.  I'll test and make sure when I get home. Range consumption tricks me more often than I wish.  I'll eventually learn to look out for it more actively.

Regards,
Brad Anderson


April 22, 2012
On Thursday, 19 April 2012 at 16:19:05 UTC, Brad Anderson wrote:
> On Thu, Apr 19, 2012 at 7:25 AM, Christophe <travert@phare.normalesup.org>wrote:
>
>> "Brad Anderson" , dans le message (digitalmars.D.learn:34902), a écrit :
>> > Perhaps I'm just misunderstanding something about closures but
>> > the following code seems to behave oddly:
>> >
>> >      import std.stdio, std.range, std.algorithm, std.string;
>> >
>> >      void main()
>> >      {
>> >          auto lst = ["a", "b"];
>> >          auto rng = range_gen(lst);
>> >          writeln(rng.take(5));
>> >      }
>> >      auto range_gen(string[] lst)
>> >      {
>> >          auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0],
>> > a))();
>> >          return chain(lst, a); // access violation
>> >          //return a; // works
>> >      }
>>
>> My guess is that chain takes lst by reference, just like the delegates
>> for map, wo both are working on the same slice instance. The chain first
>> pops elements from lst, and then calls the mapped sequence. At that
>> time, lst is empty.
>>
>> You can just copy lst before you give it to chain (or to map's delegate)
>> to solve this bug:
>>
>> auto range_gen(string[] lst)
>> {
>>     auto a = sequence!"n+1"().map!(a=>format("%s%d", lst[0], a))();
>>      string[] lst2 = lst;
>>     return chain(lst2, a); // access violation
>> }
>>
>>
> Ah, that would make sense.  I'll test and make sure when I get home. Range
> consumption tricks me more often than I wish.  I'll eventually learn to
> look out for it more actively.
>
> Regards,
> Brad Anderson

Ok, so this wasn't the problem.  I have no idea what the problem is.  Using string[] lst = list; doesn't help nor does lst.save.  I can get some really weird behavior messing around with this.  Access violations that appear or disappear when symbolic debug info is enabled.  Here's an example of a weird one:

    import std.stdio, std.range, std.algorithm, std.string;

    void main()
    {
        writeln(gen(["a", "b"]).take(5));
    }

    auto gen(string[] lst)
    {
        auto prefix = lst[0];
        auto a = iota(10).map!(a=>format("%s%d", prefix, a))();
        //auto a = sequence!"n+1"().map!(a=>format("%s%d", prefix, a))();
        return chain(lst.save, a);
    }


Compiling that using DMD 2.059 without -g results in a UTF exception (memory corruption?) after outputting ["a", "b",. Compiling with -g produces no exception but the output is incorrect (["a", "b", "0", "1", "2"] should be ["a", "b", "a0", "a1", "a2"]).  Now, if you replace the iota line with with the commented out sequence line it works the other way. Without -g gives no exception (but correct output this time), with -g results in a UTF exception. It's a bit unsettling that adding debug info can change the behavior of the program.

I suppose this makes this a bug but I'm not sure what I'd call it.  Is it a bug with chain() or with closures?  Is the symbolic debug information differences a different bug?

Regards,
Brad Anderson

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