obviously it's trivial to do with a little aa cache.  and I know I can memoize a function, and turn the memoized version into an infinite range.  but suppose I have a lazy function that returns a finite range, and its expensive to calculate.

can I use Phobos to produce a memoized range?  So it will calculate on access to an element if it needs to, but will return the cached value if it has been calculated before.

On Friday, 11 September 2015 at 13:09:33 UTC, Laeeth Isharc wrote:
> obviously it's trivial to do with a little aa cache.  and I know I can memoize a function, and turn the memoized version into an infinite range.  but suppose I have a lazy function that returns a finite range, and its expensive to calculate.
>
> can I use Phobos to produce a memoized range?  So it will calculate on access to an element if it needs to, but will return the cached value if it has been calculated before.

Is your range random access?

If not, `cache` should do the trick.

[1] http://dlang.org/phobos/std_algorithm_iteration.html#cache

On Friday, 11 September 2015 at 13:09:33 UTC, Laeeth Isharc wrote:
> obviously it's trivial to do with a little aa cache.  and I know I can memoize a function, and turn the memoized version into an infinite range.  but suppose I have a lazy function that returns a finite range, and its expensive to calculate.
>
> can I use Phobos to produce a memoized range?  So it will calculate on access to an element if it needs to, but will return the cached value if it has been calculated before.

perhaps http://dlang.org/phobos/std_algorithm_iteration.html#.cache would do what you need. An AA based cache (with a normal range interface if you want) is probably the sensible way to get true random-access here, unless you have reasonably dense and monotonic accesses in which case an appender-based linear cache could work, either using Nullable!T or a separate list of bools (or bits) marking whether a result is cached yet.

On Friday, 11 September 2015 at 13:31:06 UTC, John Colvin wrote:
> On Friday, 11 September 2015 at 13:09:33 UTC, Laeeth Isharc wrote:
>> obviously it's trivial to do with a little aa cache.  and I know I can memoize a function, and turn the memoized version into an infinite range.  but suppose I have a lazy function that returns a finite range, and its expensive to calculate.
>>
>> can I use Phobos to produce a memoized range?  So it will calculate on access to an element if it needs to, but will return the cached value if it has been calculated before.
>
> perhaps http://dlang.org/phobos/std_algorithm_iteration.html#.cache would do what you need. An AA based cache (with a normal range interface if you want) is probably the sensible way to get true random-access here, unless you have reasonably dense and monotonic accesses in which case an appender-based linear cache could work, either using Nullable!T or a separate list of bools (or bits) marking whether a result is cached yet.


Thanks, John and Jacob.


Laeeth.