In this talk, we will explore how we built a library to define declarative UIs using an in-house functional language. The discussion will center around three key components: a declarative view, an update function, and a model. We will address various challenges such as background tasks, efficient updates, and composition, and analyze the performance implications of different design decisions. The presentation is grounded in a production library that has evolved over more than ten years of experience. This library has been instrumental in the creation of over 100 internal applications.