How to use nested map in Move

[0xaptosj]

Asked by skite_in from discord

How do I create nested maps in Move?

[0xaptosj]

You can have nested table (which is map in Aptos) in your struct like this

    struct Sellers has key, store {
        sellers: Table<address, Table<address, bool>>
    }

[gregnazario]

There are three types of "Map-like" data structures in Aptos:

Table SmartTable and SimpleMap

There are tradeoffs for each on gas costs, and ease of use.

It depends on your use case.

I did a segment on this on twitter https://x.com/Greg_Nazario/status/1760809623194677725?s=20

But the tl;dr:

1. SimpleMap is based in a vector, it doesn't scale > 10k, so if you want a large set you should stay away from it. Every lookup may iterate through the whole map.
2. SimpleMap can't be nested.
3. Table is useful, but it's not iterable. So, if you need to iterate over multiple values, and you don't know what they are ahead of time, I would not use it.
4. SmartTable is iterable, and generally more efficient than Table

For your use case (I'm assuming some), is that you probably want to be able to iterate over the inner map.

Nested Tables

I would instead of Table<address, Table<address, bool>> probably use:

 SmartTable<address, SmartTable<address, bool>>

Compound Keys

The other way to approach this is to make a compound key and use a single SmartTable:

struct MyKey has key, store {
  outer: address,
  inner: address,
}

SmartTable<MyKey, bool>

SimpleMap in SmartTable

If the inner map is really big, this could be costly from a gas perspective, as it needs to iterate over all values in the map.

 SmartTable<address, SimpleMap<address, bool>>

[StrawberryChocolateFudge]

I don't want to open a new discussion, so I ask here:
@gregnazario
I have a struct

   struct Hello has store, drop{
    world: SmartTable<address,bool>
   }
   

Then I run aptos move prove I get:

error: type `smart_table::SmartTable<address, bool>` is missing required ability `drop`

62 │     world: SmartTable<address,bool>
   
   

What to do? How do I add the required ability drop?

If I remove the drop ability from the struct it does work but then I get errors all over my codebase where the struct is used because it needs to have that ability

[0xaptosj]

hi move struct requires inner struct to have all abilities from outer struct, since SmartTable is from aptos framework, you cannot change it. I suggest create a 2 structs, one with drop ability and the rest of fields, the other without drop ability and only contains a smart table