Immutable

What does immutable mean?

In programming, immutable refers to an object, data structure, or variable that cannot be modified after it's created. The term immutable is used both as a concept and as a keyword in various programming languages.

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The immutable keyword

Many programming languages and smart contract languages use the immutable keyword to declare variables or properties that cannot be changed after initialization. Programming languages that do not have the explicit immutable keyword, achieve the same effect using different keywords or specific designs or declarations. Here's how immutable is used in different contexts:

In general-purpose programming languages:

• Java: Java has no immutable keyword and achieves immutability through final variables and careful class design.
• C#: C# uses readonly for runtime constants, const for compile-time constants, and init for init-only properties. So it has immutable  objects in concept, but not by name.

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In smart contract languages:

• Solidity: Uses the immutable keyword for contract-level variables that can only be assigned once during contract creation.
• Vyper: Achieves similar functionality to the immutable keyword through careful variable declaration and initialization.

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Examples of immutable in smart contracts

Let's look at a practical Solidity example:

contract ExampleContract {
    address public immutable owner;
    uint256 public constant MINIMUM_STAKE = 100 ether;

    constructor() {
        owner = msg.sender;
    }
}

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In this example:

• The owner variable is declared as immutable. It can only be set once in the constructor and cannot be changed afterward.
• The MINIMUM_STAKE is declared as a constant, which is implicitly immutable. Its value is fixed at compile-time and cannot be changed.

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Benefits of using immutable variables

Using immutable variables in programming and smart contracts provides several advantages:

• Security: Prevents unauthorized modifications to critical values.
• Reliability: Ensures consistent behavior throughout the program or contract's lifetime.
• Gas efficiency (in smart contracts): Immutable variables can be embedded directly into the contract bytecode, often resulting in lower gas costs compared to regular state variables.
• Thread safety: In multi-threaded environments, immutable objects are inherently thread-safe, reducing the risk of race conditions.

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Examples of immutability as a concept

Immutability as a concept extends beyond programming languages. Here are some real-world examples:

• Git commits: Once a commit is made in Git, it becomes immutable. The commit's hash, timestamp, and content cannot be changed without creating a new commit.
• Blockchains: Each block in a blockchain is immutable once it's added to the chain. This property ensures the integrity and security of the entire blockchain.
• Functional programming: Languages like Haskell emphasize immutable data structures, making it easier to reason about program behavior.
• Database transactions: In ACID-compliant databases, committed transactions are immutable, ensuring data consistency and reliability.