Add functional programming conventions

Author: tillydray

functional-programming/CONVENTIONS.md

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+When writing code, you MUST follow these principles:
+- Code should be easy to read and understand.
+- Keep the code as simple as possible. Avoid unnecessary complexity.
+- Use meaningful names for variables, functions, etc. Names should reveal
+  intent.
+- Functions should be small and do one thing well. They should not exceed a few
+  lines.
+- Function names should describe the action being performed.
+- Prefer fewer arguments in functions. Ideally, aim for no more than two or
+  three.
+- Only use comments when necessary, as they can become outdated. Instead, strive
+  to make the code self-explanatory.
+- When comments are used, they should add useful information that is not readily
+  apparent from the code itself.
+- Properly handle errors and exceptions to ensure the software's robustness.
+- Use exceptions rather than error codes for handling errors.
+- Consider security implications of the code. Implement security best practices
+  to protect against vulnerabilities and attacks.
+- Adhere to these 4 principles of Functional Programming:
+  1. Pure Functions
+  2. Immutability
+  3. Function Composition
+  4. Declarative Code
+- Do not use object oriented programming.

functional-programming/README.md

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+# Functional Programming Conventions
+
+This conventions file provides guidelines for writing good functional code. The definition of "good" here is largely influenced by [Clean Code by Robert C. Martin](https://www.goodreads.com/book/show/3735293-clean-code) ([web archive in case of url rot](https://web.archive.org/web/20250127215242/https://www.goodreads.com/book/show/3735293-clean-code))
+
+## Purpose
+
+These conventions ensure that code is
+- simple
+- easy to understand
+- maintainable
+
+## Use Cases
+
+This convention is intended for use when writing code using functional paradigm
+
+## Usage
+
+[See How to Use These Conventions](https://github.com/Aider-AI/conventions#how-to-use-these-conventions)