Consider a Set type (or something similar)

[uint]

@dakom mentioned he'd sometimes see usage of the Map where the value is (), and the last key component is the actual value.

[dakom]

yeah, just to elaborate a bit - and this may be two separate issues:

one use-case is exactly as the title says, something like a Set.. it's not that bad to just use Map<Foo, ()>, it could almost work just like a type alias.

the other use-case is a bit trickier, where I want a list with a dynamic key. This is probably solved with Deque::new_dyn, but that's a relatively recent addition and I haven't had the pleasure of needing it hands-on yet :)

Specifically, for the "list with a dynamic key" problem, without Deque::new_dyn, I'd solve it using a composite key and a prefix, e.g.: Map<(Foo, Bar), ()>. Then the "bars for a given foo" can be iterated over via .prefix(foo).keys(...). It works perfectly fine, but it's a bit odd.

[uint]

the other use-case is a bit trickier, where I want a list with a dynamic key

Ah, right. For this one, I have a feeling we're already good. storey's version of this would be something like Map<Foo, Column<Bar>>. The Map handles dynamic keys of type Foo, and then Column is a bit like a list, only every entry is indexed with u32.

If the column isn't right, it should be fairly easy to implement some other list-like collection in storey, and then plug it into the map.

An example of a "column in a map"

[dakom]

nice! so just to expand on the use-case a bit - in case this helps:

where i've needed it, it's usually just a dynamic list, and so Column with an index sounds like it covers it completely

but sometimes it's not exactly a list by index, but rather something else... for a made-up example, consider NFTs where TokenId is not numeric but some other string that's created outside of the contracts for some reason.

so TOKENS_PER_OWNER = Map<Addr, Column(TokenId)> could work, but the index isn't something that would really be used - i.e. I'd want to be able to remove an item by (Addr, TokenId) without knowing the internal index

Also, since it's a Map I assume all the usual range/keys stuff just works- but just to make sure, it's important to be able to paginate and in both directions (both Ascending and Descending)