Collections

Clojure’s core data structures are immutable persistent collections. That phrase is worth learning early:

• Immutable: operations do not modify a collection in place.
• Persistent: updates return a new collection while sharing structure with the old one, so value-oriented updates are practical in real systems.

Java mental model: instead of mutating an ArrayList, HashMap, or HashSet, you usually produce a new value and pass it forward. That makes tests, concurrency, and review easier.

The Big Four Plus Seqs

Practical Defaults

• Use vectors for most ordered data.
• Use maps for records, configuration, and structured domain values.
• Use sets for membership checks and deduplication.
• Use lists mostly when working with code-as-data or head-first sequence algorithms.
• Use sequence functions such as map, filter, and reduce across all of them.

Gotchas Worth Memorizing

    flowchart TD
	    A["Need a collection"] --> B{"Ordered by position?"}
	    B -->|Yes| C["Vector"]
	    B -->|No| D{"Key-value structure?"}
	    D -->|Yes| E["Map"]
	    D -->|No| F{"Membership or uniqueness?"}
	    F -->|Yes| G["Set"]
	    F -->|No| H{"Code form or head-first sequence?"}
	    H -->|Yes| I["List"]
	    H -->|No| J["Use seq functions over existing data"]

The goal of this section is to make you fluent at reading collection literals, predicting what common operations return, and choosing the default collection that keeps your code simple.

In this section

• Lists in Clojure
  Understand Clojure lists as immutable head-first sequences, code forms, and occasional data structures, while avoiding Java List assumptions about indexed access.
• Vectors in Clojure
  Use Clojure vectors as the default ordered collection for indexed, immutable data, with practical guidance on conj, assoc, subvec, sequence functions, and Java ArrayList comparisons.
• Maps in Clojure
  Use Clojure maps as immutable records and lookup tables with keyword keys, assoc, update, dissoc, merge, nested updates, and Java HashMap comparisons.
• Sets in Clojure
  Use Clojure sets for uniqueness, membership checks, set algebra, deduplication, and Java Set comparisons while preserving immutable value semantics.