The document discusses applying SOLID principles to functional programming (FP). It argues that SOLID is often interpreted as being about object-oriented programming (OOP), but its goals of encapsulation, separation of concerns, and low coupling can also be achieved in FP. Extreme application of SOLID in OOP can result in many small, single-method classes and interfaces along with pervasive use of decorators, but FP approaches this through functions, function composition, and type signatures, leading to simpler code. An example message store implementation in F# demonstrates how FP facilitates applying SOLID principles through types and composition.