A functional developer specializes in functional programming, a paradigm that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. They write code that is declarative rather than imperative, emphasizing the concept of 'what to solve' over 'how to solve'. Their work involves writing pure functions, meaning the output value depends only on the arguments, with no side effects or external data changes. They use techniques like recursion, higher-order functions, and function composition. Languages they commonly use include Haskell, Erlang, and Scala. Their work supports maintainability, predictability, and easier testing in software development.