A "mock" is a term in software development that refers to a technique where a simulated object or module is created to mimic the behavior of a real component. Mocks are commonly used in testing environments, particularly in unit tests.

Here are some key points about mocks:

1. Simulating Dependencies: In a typical software application, modules or objects may depend on each other. However, when you want to test a component in isolation without being influenced by other dependent components, you can use mock objects to simulate the behavior of these other components.

2. Simple Implementation: Mocks are often simple placeholders or stubs used to mimic specific functions or methods. They are specifically designed for testing purposes and often contain predefined behaviors to simulate certain scenarios.

3. Control Over Testing Environment: By using mocks, developers can have better control over the testing environment and simulate specific conditions or edge cases more easily. This increases the predictability and reproducibility of tests.

4. Reducing External Dependencies: Using mocks can help avoid or reduce external dependencies, such as databases or APIs, increasing test speed and making tests more independent.

Mocks are an important tool in a software developer's toolkit, especially when it comes to writing tests that are robust, maintainable, and independent of each other.

In software development, a directive typically refers to a form of instruction or a specific tag used to provide instructions to the compiler, interpreter, or other build systems. These instructions control how the code should be processed or treated. Directives can vary across different programming languages and serve different purposes.

Some examples of directives in software development include:

1. Preprocessor directives in C/C++: Used to provide instructions to the compiler on how to handle the code before compilation, such as #include to include header files or #define to define macros.

2. Comment directives: These could be special instructions within the code recognized by specific tools or IDEs to perform certain actions. For instance, comment directives in some development environments might be used to generate automatic documentation.

3. Statements for the compiler or interpreter: Some languages have specific statements that communicate instructions to the compiler or interpreter on how to process the code. For example, pragma directives in C/C++ that provide specific compiler instructions.

4. Coding style guidelines: In certain cases, directives might be used to establish particular coding styles or formatting rules for the code, which are then interpreted by tools or analysis programs.

In essence, directives in software development serve to control the development process, establish specific behaviors, or provide special instructions to the compiler/interpreter on how to treat the code.

Immutability refers to the state of being unchangeable or unalterable. In software development, it often refers to immutable data structures or objects. When something is deemed "immutable," it means that once it's created, it cannot be modified.

Immutable data is emphasized in programming languages such as functional programming to ensure that once data is created, it cannot be inadvertently changed. Instead of modifying existing data, immutable structures create new data by making copies of existing data with the desired modifications. This often facilitates writing safer and more error-resistant code, as there's less room for unexpected side effects or unintended alterations.