Why does choose not work here

[berni]

On Thursday, 1 August 2019 at 21:26:10 UTC, Matt wrote:
> Anyone have any other thoughts?

I tried to simplify your example a little bit:

import std.stdio;
import std.range;
import std.algorithm;

auto myFilter(R1, R2)(R1 a, R2 b)
{
     return a.filter!(c => c==b.front);
}

struct A
{
     int[] starts;

     auto intervalRange() @property
     {
         return starts;
     }
}

auto uniqIntervalsA(A primary, A* previous) @property
{
     return choose(previous is null,
                   primary.intervalRange,
                   primary.intervalRange.filter!(a => 
a==previous.intervalRange.front));
}

auto uniqIntervalsB(A primary, A* previous) @property
{
     return choose(previous is null,
                   primary.intervalRange,
                   
primary.intervalRange.myFilter(previous.intervalRange));
}

unittest
{
     auto a1 = A([1]);

     writeln(uniqIntervalsA(a1,&a1));
     writeln(uniqIntervalsA(a1,null));
     writeln(uniqIntervalsB(a1,&a1));
     writeln(uniqIntervalsB(a1,null));
}

The strange thing is, that even if you replace "return 
a.filter!(c => c==b.front);" by "return a.filter!(c => true);" 
the problem remains (but now you can use lazy). As I'm not an 
expert myself, I'm not sure how to analyze this. But I think, 
both parameters to "choose" are evaluated (that is they are not 
lazy), but "myFilter" uses it's parameter immediately to give "b" 
a value, while "filter" just hands in an anonymous function 
without using it ever.

Hope, this helps to get further down the way to a solution...