Modules, Types & Structs

import Tabs from ‘@theme/Tabs’;
import TabItem from ‘@theme/TabItem’;
import {CodeTabs, Language, Github} from "@site/components/codetabs"

When writing smart contracts you will leverage common programming concepts such:

• Modules
• Data types & Collections
• Classes & Structures

---

Modules

Modules help you to organize your code and reuse third-party libraries.

The main module you will use in your contract will be the NEAR SDK, which: gives you access to the execution environment, allows you to call other contracts, transfer tokens, and much more.

:::info Using external libraries

As a general rule of thumb for Rust, anything that supports wasm32-unknown-unknown will be compatible with your smart contract.

However, we do have a size limit for a compiled contract binary which is ~4.19 MB, so it is possible that certain large libraries will not be compatible.

::

---

Native Types

When writing contracts you have access to all the language’s native types.

“`ts
number, bigint, string, [], {} …
“`

“`rust
u8, u16, u32, u64, u128, i8, i16, i32, i64, i128, Vec, HashMap …
“`

“`ts
u8, u16, u32, u64, i8, i16, i32, i64, Array, Map …
“`

:::tip

Always prefer native types in the contract’s interface. The only exception is values larger than 52 bytes (such as u64 and u128), for which string-like alternatives are preferred.

::

::warning
Always make sure to check for underflow and overflow errors. For Rust, simply add overflow-checks=true in your Cargo.

::

---

SDK Collections

Besides the native types, the NEAR SDK implements collections such as Vector and UnorderedMap
to help you store complex data in the contract’s state.

:::tip

Always prefer SDK collections over native ones in the contract’s attributes (state).

::

---

Internal Structures

You can define and instantiate complex objects through classes and structures.

🦀 Notice that the class is decorated with multiple macros:
– `BorshDeserialize` & `BorshSerialize` allow the structure to be read and
written into the contract’s state
– `Serialize` & `Deserialize` allow the structure to be used as an input type and
return type of the contract’s methods.

:::tip
If you are curious on why the (de)serialization is needed read our [serialization documentation](./serialization.md)
:::