Software design patterns were forged at a time when object oriented programming (OOP) reigned. This led to "design patterns" becoming somewhat synonymous with "OOP design patterns". But design patterns are solutions to problems and problems are relative to the context in which they occur. A design problem in OOP is not necessarily one in functional programming (FP), and vice versa.

From a Haskell perspective, many (but not all) of the well known "Gang of Four" patterns [Design patterns, Gamma et al.] become so easy to solve that it is not worth going to the trouble of treating them as patterns.

However, we still want to identify patterns of problems and solutions, so that when we have a dejavu moment of having experienced a problem before, we are somewhat prepared. Design patterns remain relevant for Haskell, after all, "dejavu is language neutral" (Erich Gamma).

In order to have a meaningful conversation about Haskell design patterns, we'll begin our exploration by looking at the three primary kinds of "glue" in Haskell: first-class functions, the Haskell type system, and lazy evaluation. This chapter revisits the Haskell you already know through the lens of design patterns, and looks at: