Software development is a craft, combining elements of both theory and practice. This book aims to cover both.

Writing a technical book that seeks to address developers of every programming language equally is not easy, resulting in overly abstract discussion and adoption strategies are left for the reader to puzzle out. Instead, this book is intended for Clojure developers.

That is not to say that Clojure is the best and certainly not the only language available for writing modular applications. Clojure is a young language, and still rapidly evolving. That said, the treatment of shared mutable state in Clojure makes it favorable to the construction of software applications from interchangeable components, for example, the lack of opportunity for two separate components to affect each other via a shared address space improves the chances of component re-use.

In our treatment of computer programs as graphs we are particular concerned with the avoidance of side-effects.

Side-effects, in computer systems, are unexpected behaviours that are caused by the breakdown of the abstraction we are using to instruct the computer. They punch 'holes' through logical façades we erect to protect us from considering the low-level implementation details of a computer.

Side-effects imply the existence of 'hidden' edges in our graph, ones that have no respect for the boundaries we draw between our clusters and can thwart our attempts to reduce coupling.

Clojure represents a significant breakthrough in the design of computer languages in that it provides functional data structures, offering programmers greater scope to avoid side-effects in their functions. While most functional programming languages encourage the development of 'pure' functions operating on values, Clojure significantly deepens the choice of data structure that can be passed to and from functions as values. In Clojure, rich data structures including collections, sets and maps (association sets) behave as if they were values - operations on them in no way affect the original value.

While Clojure is not unique in offering 'functional data structures', ensuring their ubiquity by the exclusion of alternative 'mutable' choices is a key constraint not found in most other functional languages, with the exception of Haskell (or one from its family of descendants). Yet Haskell is chiefly concerned with the orthogonal theory of types. For the purposes of this discussion on modularity, therefore, we prefer to focus on the connectedness of edges rather than their correctness.

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