I'm new in Clojure and i read that it is a functional language. It says that Clojure doesn't have variables, still when i find (def n 5), what's the difference between it and a variable?
I can change the value of the var after, so is it really that different from a variable? I don't understand the difference.
Assuming that by variable you mean a refer to a mutable storage location, I guess the main difference(depending against which language you compare) is that if you dynamically rebind the var in Clojure is on a per-thread basis.
But the long answer is that you don't usually use a var in Clojure unless you really need a reference to a mutable storage location.
Clojure favors immutability and programming using values instead of references.
You can watch Rich Hickey's talk about values.
A summary would be, when you're programming in Clojure what you have are values , not references to locations that may change (maybe even changed by another thread).
So.
(let [a 1
_ (println a) => prints 1
a 2
_ (println a) => prints 2
])
Even if you get the illusion of "changing a" in that code, you're not changing the "old" a you just have a new value. (if someone would have looked at the first definition it would still be seeing the value 1).
Actually you can see that sequence of assignments as a composed function calls where a is being replaced in scope, but not the same "variable" at all.
((fn [a]
(println a) => prints 1
((fn [a]
(println a) => prints 2
) 2) 1)
None the less, if you need to have a mutable storage with potentially many threads accessing that storage, Clojure gives you vars, atoms, refs, etc.