A Hacklang developer is responsible for writing and debugging code using Hack, a programming language developed by Facebook that operates on a HipHop Virtual Machine (HHVM). They build efficient, testable, and reusable PHP modules, often for web development. They solve complex performance problems and architectural challenges, integrating data storage solutions. They also ensure the technical feasibility of UI/UX designs and optimize applications for maximum speed and scalability. They collaborate with other team members and stakeholders, understanding the needs to develop solutions, and they also test, maintain and recommend software improvements to ensure strong functionality and optimization.