Lenses are composable getters and setters that facilitate the manipulation of immutable data structures in functional programming.

To elaborate, a lens is a first-class value that encapsulates two primary operations: viewing (or getting) a specific subpart of a data structure and updating (or setting) that subpart. Lenses are particularly valuable in functional programming languages, especially those that prioritize immutability, such as Haskell and Scala.

The concept of lenses originates from category theory in mathematics. However, in practical terms, you can envision a lens as a tool that allows you to “zoom in” on a specific part of a data structure. For instance, if you have a complex nested data structure, such as a JSON object, you can utilize a lens to focus on a particular nested field. This abstraction enables you to interact with that field as if it were a top-level entity, thereby simplifying your code.

The true strength of lenses lies in their composability. You can combine multiple lenses to delve deeper into a data structure, and the resulting lens remains a first-class value that can be easily passed around within your program. This composability makes lenses an exceptionally flexible tool for managing complex data.

In functional programming, lenses uphold the principles of immutability and pure functions. In contrast to imperative languages, where you might update a field of an object by directly assigning a new value, functional languages typically treat data structures as immutable. This means once a data structure is created, it cannot be altered. Instead, you must create a new copy of the data structure that reflects the updated value. This approach can be cumbersome, especially for deeply nested structures. Lenses offer a clean and efficient mechanism to perform these updates without violating functional programming principles.

In summary, lenses are a powerful tool for working with immutable data structures in functional programming. They enable you to focus on specific parts of a data structure and update those parts in a manner that adheres to the core principles of functional programming.