pure vs std.range.retro

[bearophile]

John Carter:

> Thanks for all your help understanding the algorithm chaining 
> problem...
>
> Now I have another gotcha.
>
> This code compiles and runs OK if I remove the keyword "pure" 
> or if I remove the ".retro"

Some functions can't be pure. Some functions can be pure but in 
Phobos are not yet pure, often because they call other basic 
functions that are not yet pure. And probably some functions are 
not yet pure because of some compiler bug/limits. To know what is 
your case you have to take a look at the Phobos code.

Bye,
bearophile