chunkBy limitation?

[Ali Çehreli via Digitalmars-d-learn]

On 10/02/2015 02:21 AM, Dmitri wrote:
 > I wondered if this is a current limitation of chunkBy implementation:
 >
 > // http://dpaste.dzfl.pl/52d6a0c5d0ab
 > void bar(int[] foo)
 > {
 >      bool less(int a, int b) // contrived
 >      {
 >          return a < b;
 >      };
 >
 >      foo.sort!less.groupBy;
 > }
 >
 > resulting in:
 >
 > /opt/compilers/dmd2/include/std/algorithm/iteration.d(1529): Error:
 > function f351.bar.SortedRange!(int[],
 > less).SortedRange.groupBy!().groupBy.ChunkByImpl!(__lambda1,
 > int[]).ChunkByImpl.Group.popFront cannot access frame of function
 > f351.bar.SortedRange!(int[], less).SortedRange.groupBy!().groupBy
 > ...
 >
 > I can see that the implementation of chunkBy is nested:
 > struct ChunkByImpl
 > {
 >          static struct Group {}
 > }
 >
 > and that the predicate (less in this case) is accessed from the nested
 > struct Group which is, probably, what's causing the compilation error.
 >
 > The question is if this is an implementation error (i.e. Group should
 > probably be moved to the top level) or a compiler issue?
 >
 > Thanks in advance.

This is a known D issue. Currently, objects have a single context 
pointer. Here is a bug discussion about it:

   https://issues.dlang.org/show_bug.cgi?id=5710

A workaround for your example is making less() static, which removes its 
context pointer:

void bar(int[] foo)
{
	import std.algorithm : sort;

	static bool less(int a, int b) // contrived
	{
		return a < b;
	}

	foo.sort!less.groupBy;
}

void main() {}

I've just realized that I don't know how to handle the case where less() 
really needs some other state (e.g. it may need to use a local variable 
in main). What can we do?

Ali