book

Practical Clojure

by Luke VanderHart, Stuart Sierra

June 2010

Beginner to intermediate

226 pages

5h 33m

English

Apress

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Copyright
About the Authors
About the Technical Reviewer
Acknowledgments
1. The Clojure Way
1.1. Clojure's Philosophy and Special Features1.1.1. A Next-Generation Language1.1.2. Dynamic and Powerful (Yes, It's a Lisp)1.1.3. The Java Platform1.2. Functional Programming1.2.1. Purely Functional Programming1.2.2. Clojure's Compromise1.2.3. Immutability1.2.4. What about Object-Oriented Programming?1.2.4.1. Structure of a Clojure Program1.2.5. State Management1.2.6. State and Identity1.2.7. Software Transactional Memory1.3. Summary
2. The Clojure Environment
2.1. "Hello World" in Clojure2.2. Clojure Forms2.2.1. Literals2.2.2. Symbols2.2.3. Composite Forms2.2.4. Special Forms2.3. Writing and Running Source Files2.4. Vars, Namespaces, and the Environment2.5. Symbols and Symbol Resolution2.5.1. Symbol Names2.5.2. Symbol Resolution and Scope2.6. Namespaces2.6.1. Declaring Namespaces2.6.2. Referencing Namespaces2.6.3. Structuring Source Files2.7. Summary
3. Controlling Program Flow
3.1. Functions3.1.1. First-Class Functions3.1.2. Defining Functions with fn3.1.3. Defining Functions with defn3.1.4. Functions of Multiple Arities3.1.5. Functions with Variable Arguments3.1.6. Shorthand Function Declaration3.2. Conditional Expressions3.3. Local Bindings3.4. Looping and Recursion3.4.1. Tail Recursion3.4.1.1. Clojure'srecur3.4.1.2. Using loop3.5. Deliberate Side Effects3.5.1.3.5.1.1. Using do3.5.1.2. Side Effects in Function Definitions3.6. Functional Programming Techniques3.6.1. First-Class Functions3.6.1.1. Consuming First-Class Functions3.6.1.2. Producing First-Class Functions3.6.2. Closures3.6.3. Currying and Composing Functions3.6.3.1. Using partial to Curry Functions3.6.3.2. Using comp to Compose Functions3.6.4. Putting It All Together
4. Data in Clojure
4.1. How to Represent and Manipulate Data4.1.1. Nil4.2. Primitive Types4.2.1. Numbers4.2.1.1. Common Numeric Functions4.2.1.1.1. Addition (+)4.2.1.1.2. Subtraction (–)4.2.1.1.3. Multiplication (*)4.2.1.1.4. Division (/)4.2.1.1.5. inc4.2.1.1.6. dec4.2.1.1.7. quot4.2.1.1.8. rem4.2.1.1.9. min4.2.1.1.10. max4.2.1.1.11. Equals Function (==)4.2.1.1.12. Greater-Than Function (<)4.2.1.1.13. Greater-Than-or-Equals Function (<=)4.2.1.1.14. Less-Than (>)4.2.1.1.15. The Less-Than-or-Equals (>=)4.2.1.1.16. zero?4.2.1.1.17. pos?4.2.1.1.18. neg?4.2.1.1.19. number?4.2.2. Strings4.2.2.1. Common String Functions4.2.2.1.1. str4.2.2.1.2. subs4.2.2.1.3. string?4.2.2.1.4. print & println4.2.2.2. Regular Expression Functions4.2.2.2.1. re-pattern4.2.2.2.2. re-matches4.2.2.2.3. re-matcher4.2.2.2.4. re-find4.2.2.2.5. re-groups4.2.2.2.6. re-seq4.2.3. Boolean4.2.3.1. Common Boolean Functions4.2.3.1.1. not4.2.3.1.2. and4.2.3.1.3. or4.2.4. Characters4.2.4.1. char4.2.5. Keywords4.2.5.1. keyword4.2.5.2. keyword?4.3. Collections4.3.1. Lists4.3.1.1. list4.3.1.2. peek4.3.1.3. pop4.3.1.4. list?4.3.2. Vectors4.3.2.1. vector4.3.2.2. vec4.3.2.3. get4.3.2.4. peek4.3.2.5. vector?4.3.2.6. conj4.3.2.7. assoc4.3.2.8. pop4.3.2.9. subvec4.3.3. Maps4.3.3.1. Struct Maps4.3.3.2. Maps As Objects4.3.3.2.1. assoc4.3.3.2.2. dissoc4.3.3.2.3. conj4.3.3.2.4. merge4.3.3.2.5. merge-with4.3.3.2.6. get4.3.3.2.7. contains?4.3.3.2.8. map?4.3.3.2.9. keys4.3.3.2.10. vals4.3.4. Sets4.3.4.1. Common Set Functions4.3.4.1.1. clojure.set/union4.3.4.1.2. clojure.set/intersection4.3.4.1.3. clojure.set/difference4.4. Summary
5. Sequences
5.1. What Are Sequences?5.1.1. Sequenceable Types5.2. Anatomy of a Sequence5.3. Constructing Sequences5.4. Lazy Sequences5.4.1. An Example of Laziness5.4.2. Constructing Lazy Sequences5.4.2.1. Constructing Lazy Sequences Directly5.4.2.2. Constructing Lazy Sequences Using Sequence Generator Functions5.4.3. Lazy Sequences and Memory Management5.5. The Sequence API5.5.1. Sequence Creation5.5.1.1. seq5.5.1.2. vals5.5.1.3. keys5.5.1.4. rseq5.5.1.5. lazy-seq5.5.1.6. repeatedly5.5.1.7. iterate5.5.1.8. repeat5.5.1.9. range5.5.1.10. distinct5.5.1.11. filter5.5.1.12. remove5.5.1.13. cons5.5.1.14. concat5.5.1.15. lazy-cat5.5.1.16. mapcat5.5.1.17. cycle5.5.1.18. interleave5.5.1.19. interpose5.5.1.20. rest5.5.1.21. next5.5.1.22. drop5.5.1.23. drop-while5.5.1.24. take5.5.1.25. take-nth5.5.1.26. take-while5.5.1.27. drop-last5.5.1.28. reverse5.5.1.29. sort5.5.1.30. sort-by5.5.1.31. split-at5.5.1.32. split-with5.5.1.33. partition5.5.1.34. map5.6. Summary
6. State Management
6.1. State in an Immutable World6.1.1. The Old Way6.1.2. State and Identity6.1.3. State and Identity in Clojure6.1.3.1. Coordinated vs. Independent State6.1.3.2. Synchronous vs. Asynchronous Updates6.2. Refs and Transactions6.2.1. Creating and Accessing refs6.2.2. Updating refs6.2.2.1. Transactions6.2.2.2. Tools for Updating refs6.2.2.3. Examples6.3. Atoms6.3.1. Using Atoms6.3.2. When to Use Atoms6.4. Asynchronous Agents6.4.1. Creating and Updating Agents6.4.1.1. Update Semantics6.4.2. Errors and Agents6.4.2.1. Dealing with Agents in a Failed State6.4.3. Waiting for Agents6.4.4. Shutting Down Agents6.4.5. When to Use Agents6.5. Vars and Thread-Local State6.5.1. When to Use Thread-Local Vars6.6. Keeping Track of Identities6.6.1. Validators6.6.2. Watches6.7. Summary

7. Namespaces and Libraries
7.1. Organizing Clojure Code7.2. Namespace Basics7.2.1. Switching Namespaces with in-ns7.2.2. Referring to Other Namespaces7.3. Loading Other Namespaces7.3.1. Loading from a File or Stream7.3.2. Loading from the Classpath7.3.2.1. Namespace Names vs. File Names7.3.2.2. Loading Resources from the Classpath7.3.2.3. Loading Namespaces from the Classpath7.3.3. Loading and Referring Namespaces in One Step7.3.4. Importing Java Classes7.4. Bringing It All Together: Namespace Declarations7.5. Symbols and Namespaces7.5.1. Namespace Metadata7.5.2. Forward Declarations7.5.3. Namespace-Qualified Symbols and Keywords7.5.4. Constructing Symbols and Keywords7.5.5. Public and Private Vars7.6. Advanced Namespace Operations7.6.1. Querying Namespaces7.6.2. Manipulating Namespaces7.7. Namespaces As References7.8. Summary
8. Metadata
8.1. Describing Your Code, in Code8.2. Reading and Writing Metadata8.3. Metadata-Preserving Operations8.4. Read-Time Metadata8.5. Metadata on Vars8.5.1. Type Tags8.5.2. Private Vars8.6. Metadata on Reference Types8.7. Summary
9. Multimethods and Hierarchies
9.1. Runtime Polymorphism Without Classes9.2. Multimethods9.2.1. Multiple Dispatch9.2.2. Default Dispatch Values9.3. Hierarchies9.3.1. Querying Hierarchies9.4. Hierarchies with Multimethods9.4.1. Hierarchies with Java Classes9.4.2. More Hierarchy Queries9.4.3. Resolving Conflicts9.4.4. Type Tags9.5. User-Defined Hierarchies9.6. Summary
10. Java Interoperability
10.1. Calling Java from Clojure10.1.1. Java Interop Special Forms10.1.2. Java Interop Preferred Forms10.1.3. Clojure Types and Java Interfaces10.1.4. Java Arrays10.1.4.1. Creating Arrays10.1.4.2. Manipulating Arrays10.1.4.3. Iterating Over Arrays10.2. Calling Clojure from Java10.2.1. Loading and Evaluating Clojure Code10.2.2. Using Clojure Functions and Vars10.3. Creating Java Classes10.3.1. Proxying Java Classes10.3.2. Generating Java Classes10.3.2.1. Ahead-of-Time Compilation10.3.2.2. Basic gen-class Options10.3.2.3. Defining Methods for the Generated Class10.3.2.4. Adding State to the Generated Class10.3.2.5. Adding Methods to the Generated Class10.3.2.6. Adding Constructors and Factories10.3.2.7. Exposing Superclass Members10.3.2.8. Generating Command-Line Programs10.3.2.9. Loading the Implementation10.3.2.10. Namespace Declarations with gen-class10.3.2.11. Simple Command-Line Program10.4. Summary
11. Parallel Programming
11.1. Parallelism in Clojure11.2. Agents11.2.1. Agent Thread Pools11.2.2. Agent Example11.2.3. Concurrent Agent Performance11.3. Concurrency Functions11.3.1. Overhead and Performance11.4. Futures and Promises11.4.1. Futures11.4.1.1. Controlling Futures11.4.2. Promises11.5. Java-based Threading11.5.1. Creating a Thread11.6. Summary
12. Macros and Metaprogramming
12.1. What Is Metaprogramming?12.1.1. Code vs. Data12.1.2. Homoiconicity12.2. Macros12.2.1. Working with Macros12.2.1.1. Debugging Macros12.2.2. Code Templating12.2.2.1. Splicing Unquotes12.2.3. Generating Symbols12.2.4. When to Use Macros12.2.5. Using Macros12.2.5.1. Implementing a Control Structure12.2.5.2. Implementing a Macro with Variadic Arguments12.2.5.3. Implementing a Macro Using Recursion12.2.6. Using Macros to Create DSLs12.3. Summary
13. Datatypes and Protocols
13.1. Protocols13.1.1. Protocols As Interfaces13.2. Datatypes13.3. Implementing Protocols and Interfaces13.3.1. In-Line Methods13.3.2. Extending Java Interfaces13.3.3. Datatypes As Classes13.4. Extending Protocols to Pre-Existing Types13.4.1. Extending Java Classes and Interfaces13.5. Reifying Anonymous Datatypes13.6. Working with Datatypes and Protocols13.6.1. A Complete Example13.7. Advanced Datatypes13.8. Summary
14. Performance
14.1. Profiling on the JVM14.1.1. General Tips for Java Performance14.1.2. Simple Profiling with Time14.1.3. Using Java Profiling Tools14.2. Memoization14.3. Reflection and Type Hints14.4. Working with Primitives14.4.1. Loop Primitives14.4.2. Unchecked Integer Arithmetic14.4.3. Primitive Arrays14.5. Transients14.6. Var Lookups14.7. Inlining14.7.1. Macros and definline14.8. Summary

Content preview from Practical Clojure

Chapter 9. Multimethods and Hierarchies

Runtime Polymorphism Without Classes

Clojure is not an object-oriented language in the traditional sense of classes and methods, although it is built on Java's object-oriented foundation.

Most mainstream object-oriented languages, such as Java and C++, use classes to define a tree-like hierarchy of types and to provide implementations of the methods supported by those types.

Clojure separates type hierarchies from method implementations, which greatly simplifies thorny issues such as multiple inheritance. In addition, it permits you to define multiple, independent hierarchies over the same types. This makes it possible to define IS-A relationships that more closely model the real world.

Multimethods

Clojure multimethods ...

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