def and Immutable Values
Prerequisites: Reading Clojure code
def gives a value a name at the top level of a namespace (a
module; the default one in scripts is user):
(def city "Lagos")
(def temperature 24)
(println "It is" temperature "degrees in" city)
(println (+ temperature 3))
(println temperature)
It is 24 degrees in Lagos
27
24
(+ temperature 3) didn't change temperature — it returned a new
value. In Clojure, values never change. Numbers, strings, and also
vectors, maps, and sets are immutable.
Immutable collections
(def fruits ["apple" "banana"])
(def more-fruits (conj fruits "cherry"))
(println fruits)
(println more-fruits)
[apple banana]
[apple banana cherry]
conj ("conjoin") returned a new vector with an extra item; fruits
is untouched. This sounds wasteful, but Clojure's persistent data
structures share almost all their memory between versions, so a
"copy" costs very little.
Why it's worth it:
- Any function can receive your data without being able to break it.
- Values can be shared between threads with no locks.
- Old versions remain available, which makes undo, history, and debugging easy.
Redefining
(def greeting "Hello")
(def greeting "Hi")
(println greeting)
Hi
Evaluating def again replaces what the name refers to. That's
meant for the REPL workflow (re-evaluating a definition while you
develop), not for program logic — never use def like an assignment
inside a function. For local names, use let (a few lessons ahead); for
changing state, use an atom (Level 2).
Naming
- Names are kebab-case:
max-retries,user-name. - Almost any character is allowed:
valid?(returns a boolean),swap!(changes state),->json(converts),*debug*(a dynamic var). - Names are case-sensitive.
Practice
Define product and price, then print Coffee costs 3.5.
Show solution
(def product "Coffee")
(def price 3.5)
(println product "costs" price)
Coffee costs 3.5