The Expression Problem & Lenses • Tony Morris • YOW! 2016

The Expression Problem deals with the challenge of adding new data constructors and functions to types without having to update all existing code

Lenses provide a solution by allowing abstraction over data types and their views/modifications while maintaining type safety

Using type classes with parametric polymorphism (P and F parameters) enables writing functions that work across multiple data types without mentioning concrete types

The approach trades some complexity (harder type errors, more complex signatures) for greater flexibility and code reuse

Prisms are specialized lenses for dealing with data types that may or may not contain a value (like Either or Maybe types)

The technique allows parsing different versions of data formats (like Java class files 1.5/1.7) without changing reference functions

Coming up with good identifier names remains one of the hardest programming challenges

Type classes help reduce boilerplate by allowing a single implementation to work across multiple data types

The solution isn’t perfect - there are still tradeoffs between adding constructors and modifying functions

Real-world applications include parsing complex data formats, detecting chess cheating patterns, and working with GPS/geodetic data