> ## Documentation Index
> Fetch the complete documentation index at: https://base-a060aa97-eb-mini-app-ia-overhaul.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Imports

> Learn to import code into your contract.

In this lesson, we'll learn how to import code written by others into your contracts. We'll also explore the [OpenZeppelin] library of smart contracts.

***

## Objectives

By the end of this lesson you should be able to:

* Import and use code from another file
* Utilize OpenZeppelin contracts within Remix

***

## OpenZeppelin

[OpenZeppelin] has a robust [library] of well-[documented] smart contracts. These include a number of standard-compliant token implementations and a suite of utilities. All the contracts are audited and are therefore safer to use than random code you might find on the internet (you should still do your own audits before releasing to production).

### Docs

The [docs] start with installation instructions, which we'll return to when we switch over to local development. You do **not** need to install anything to use these contracts in Remix.

Find the documentation for the `EnumerableSet` under *Utils*. This library will allow you to create [sets] of `bytes32`, `address`, and `uint256`. Since they're enumerated, you can iterate through them. Neat!

### Implementing the OpenZeppelin EnumerableSet

Create a new file to work in and add the `pragma` and license identifier.

In Remix, you can import libraries directly from GitHub!

```solidity
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/structs/EnumerableSet.sol";
```

You should see `EnumerableSet.sol` pop into your workspace files, nested deeply in a bunch of folders.

### Trying It Out

Add a contract called `SetExploration`. Review the extensive comments within the contract itself.

To use the `EnumerableSet`, you need to use the [`using`] keyword. This directive attaches all of the library methods to the type. Doing so allows you to call the method on the variable with dot notation, and the variable itself will be supplied as the first argument.

Follow the pattern in the example in the comments, but name the variable `visitors`:

```
using EnumerableSet for EnumerableSet.AddressSet;

EnumerableSet.AddressSet private visitors;
```

Add a function called `registerVisitor` that makes use of the library's `add` function to add the sender of the message to the `visitors` set.

<Tip>
  There's also an `_add` function, which is private.
</Tip>

<Accordion title="Reveal code">
  ```solidity
  function registerVisitor() public {
      visitors.add(msg.sender);
  }
  ```
</Accordion>

Add another function to return the `numberOfVisitors`. Thanks to `using`, this can cleanly call the `length` function:

<Accordion title="Reveal code">
  ```solidity
  function numberOfVisitors() public view returns (uint) {
      return visitors.length();
  }
  ```
</Accordion>

***

## Conclusion

In this lesson, you imported a library from [OpenZeppelin] and implemented some of its functions. You also learned how to use the `using` keyword.

***

[OpenZeppelin]: https://www.openzeppelin.com/

[library]: https://github.com/OpenZeppelin/openzeppelin-contracts

[documented]: https://docs.openzeppelin.com/contracts/4.x/

[docs]: https://docs.openzeppelin.com/contracts/4.x/

[sets]: https://en.wikipedia.org/wiki/Set_\(abstract_data_type\)

[`using`]: https://docs.soliditylang.org/en/v0.8.17/contracts.html#using-for