What is the difference between "keyword" and "atom"?

Question

Hello dear Clojurians!
What is the difference between "keyword" and "atom"?
Sorry, I'm a total newbie :-)

Answer 1

Keywords are symbolic identifiers that evaluate to themselves. They are often used as keys in maps to name attributes or as enumerated values.

For example, in the map {:first-name "SpongeBob", :last-name "SquarePants"} there are two entries using the keys :first-name and :last-name.

See: https://clojure.org/reference/data_structures#Keywords

Atoms are effectively boxes that hold a value that can be atomically updated by applying a function to the existing value. For example, you could make a counter out of an atom and then increment it (safely from multiple threads if needed).

(def counter (atom 0))
(swap! counter inc) ;; increment counter
@counter ;; read counter: 1

See: https://clojure.org/reference/atoms

Comments

So... Atom is a variable which are mutable??

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Yes, atoms provide controlled mutability -- thread-safe when updated by applying a function to the previous value. See also Refs and Agents -- https://clojure.org/reference/refs and https://clojure.org/reference/agents

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Thank you!  :-)

Answer 2

Ignoring atoms completely, here's an informal perspective on keywords:

I like to think of keywords conceptually like a string/symbol hybrid with special superpowers. Like symbols, they're restricted to certain characters (notably, no spaces allowed) and can be namespaced: :myproject.api/foo. Like strings, they're really just a data primitive.

The main keyword superpower is that they can be called as functions to get values out of associative data structures like hash-maps: (:foo {:foo 42 "bar" 36}) => 42 and sets: (:foo #{:foo :bar}) => :foo. You can't use strings or numbers as functions.

Here's a helpful StackOverflow answer about why keywords exist in Clojure: https://stackoverflow.com/a/11655615

Comments

Thank you!  :-)