Understanding the difference between lexical and dynamic scope in Clojure.

[clj].lexical-dynamic-scope.md

concept

• lexical scope (static scope): dependent only on the program text.
• dynamic scope: dependent on the runtime call stack.

clojure

• (def x 1) default Var is static, using let for local Var, with-redefs to change the root binding var within its scope (visible in all threads).
• (def ^:dynamic x 1) dynamic Var, using binding to change value (thread-local, cannot be seen by any other thread).

In example bellow:

• binding only changes the value of *dynamic-var* within the scope of the binding expression
• def changes the root definition of non-dynamic-var from the point of execution on
• using let to rebind non-dynamic-var, the change is only local and won’t affect the top-level non-dynamic-var

(def non-dynamic-var "this is a non dynamic var")
(def ^:dynamic *dynamic-var* "this is a dynamic var")

(defn function-using-dynamic-var []
  (println *dynamic-var*))

(defn function-using-non-dynamic-var []
  (println non-dynamic-var))

(defn some-function []

  (function-using-dynamic-var) ;;=> this is a dynamic var
  
  ;; dynamically rebind dynamic-var
  (binding [*dynamic-var* "this is some new content for the dynamic var"]
    (function-using-dynamic-var)) ;;=> this is some new content for the dynamic var


  (function-using-non-dynamic-var) ;;=> this is a non dynamic var
  
  ;; locally rebind non-dynamic-var
  (let [non-dynamic-var "this is some new content that won't be used"]
    (function-using-non-dynamic-var)) ;;=> this is a non dynamic var
    
  ;; lexically (and globally) rebind non-dynamic-var
  (def non-dynamic-var "this is some new content that will be used")
  (function-using-non-dynamic-var) ;;=> this is some new content that will be used
)

• https://clojure.org/reference/vars
• http://www.peterfessel.com/2014/11/understanding-the-difference-between-lexical-and-dynamic-scope-in-clojure/
• https://stuartsierra.com/2013/03/29/perils-of-dynamic-scope