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Practical Clojure by Luke VanderHart, Stuart Sierra

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Chapter 8. Metadata

Describing Your Code, in Code

Programmers often talk about metadata, or data about data. The definition of metadata varies in different contexts. Clojure provides mechanisms to attach metadata to objects, but it has a very specific definition: metadata is a map of data attached to an object that does not affect the value of the object.

Two objects with the same value and different metadata are considered equal (and have the same hash code). However, metadata has the same immutable semantics as Clojure's other data structures; modifying an object's metadata yields a new object, with the same value (and the same hash code) as the original object.

When updating a value, some operations preserve metadata and some do not, which this ...

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