Solidity – How to Clear OpenZeppelin Enumerable Map

After reading docs about types thoroughly, I found that calling delete on dynamic array frees all allocated memory.

delete a assigns the initial value for the type to a. I.e. for integers it is equivalent to a = 0, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements reset.

Internally, calling delete on array assigns it to a new empty array, like I did.

It is important to note that delete a really behaves like an assignment to a, i.e. it stores a new object in a.

Transaction cost is lower for delete call, than for manual assignment to empty array (not speaking about storing this empty array in storage memory). In my example, deleting an array of 1000 addresses by calling delete cost 5060808 gas; deleting the same array by manual assignment cost 5270025 gas.

A contract can only see the current state when it executes, not previous states. This restriction allows validating nodes to work only with the current state, rather than needing to store and be able to access all the previous states.

In theory it should be possible to have a contract storing block hashes, and have the user provide a merkle proof showing that a given piece of data is in the state tree for a previous block. However, I don't know of any practical implementations of this.

If you control the contract where the data is written, one option would be to write a complete history to storage. However, this will make normal operations costly. You could do the same thing more cheaply by only writing the hash of the history, and requiring the user to resend the relevant history.