I have some use cases where I am trying to iterate over a collection in a pure function. The collection doesn't change, but the foreach loop body updates a variable elsewhere in the function body. (I'm reimplementing reduce as pure nothrow, and the variable in question is the accumulator).

Since opApply is implemented as a delegate, though, this prevents my function from being pure. The delegate isn't pure because it writes through its context pointer - but the context pointer shouldn't capture anything outside of the calling function's scope  (I have no idea how to verify this).

In other words, if I replaced the foreach with a for-loop, my function would be pure.

I see 3 possible takes on this, please tell me if I'm hot or cold:

1) foreach is the idiomatic D way to iterate, but since I am insisting on purity, I have to accept that I might have to do things in a non-idiomatic way and go for the for-loop.

2) ranges are the other idiomatic D way to iterate, and if I want purity then I should be working with ranges anyway (as they have a more mathematical flavor to them than foreach or for)

3) opApply ought to be considered pure if the supplied delegate doesn't modify anything outside of the calling function's scope (bug/enhancement)