Lazy caching map()?

[Jonathan M Davis]

On Friday, March 09, 2018 11:41:46 H. S. Teoh via Digitalmars-d wrote:
> Today I found myself needing a lazy, caching version of map() on an
> array.  More precisely, given an array `T[] src` of source data and a
> function func(T) that's pretty expensive to compute, return an object
> `result` such that:
>
> - result[i] == func(src[i]), for 0 ≤ i < src.length.
>
> - If result[j] is never actually used, func(src[j]) is never invoked
>   (lazy).
>
> - If result[j] is referenced multiple times, a cached value is returned
>   after the first time, i.e., the result of func(src[j]) is cached, and
>   func(src[j]) is never invoked more than once.
>
> I couldn't figure out how to build this using Phobos primitives, so I
> wrote my own implementation of it.  Pretty simple, really, it's just a
> wrapper struct that lazily initializes an array of Nullable!T and lazily
> populates it with func(j) when opIndex(j) is invoked, and just returns
> the cached value if opIndex(j) has been invoked before.
>
> Can this be done using current Phobos primitives?

Wasn't that what std.algorithm.iteration.cache was supposed to solve? I've
never used it, so I don't know how well it fits what you need, but IIRC,
using it with map was basically the use that it was invented for.

- Jonathan M Davis