Definition (def)#
(def name meta? value)
This special form binds a value to a global symbol. A definition cannot be redefined at a later point.
(def my-name "phel")
(def sum-of-three (+ 1 2 3))
To each definition metadata can be attached. Metadata is either a Keyword, a String or a Map.
(def my-private-definition :private 12)
(def my-name "Stores the name of this language" "Phel")
(def my-other-name {:private true :doc "This is my doc"} "My value")
Local bindings (let)#
(let [bindings*] expr*)
Creates a new lexical context with assignments defined in bindings. Afterwards the list of expressions is evaluated and the value of the last expression is returned. If no expression is given nil is returned.
(let [x 1
y 2]
(+ x y)) # Evaluates to 3
(let [x 1
y (+ x 2)]) # Evaluates to nil
All assignments defined in bindings are immutable and cannot be changed.
Variables#
(var value)
Variables provide a way to manage mutable state. Each variable contains a single value. To create a variable use the var function.
(def foo (var 10)) # Define a variable with value 10
The deref function can be used to extract the value from the variable. The set! function is use to set a new value to the variable.
(def foo (var 10))
(deref foo) # Evaluates to 10
(set! foo 20) # Set foo to 20
(deref foo) # Evaluates to 20
To update a variable with a function the swap! function can be used.
(def foo (var 10))
(swap! foo + 2) # Evaluates to 12
(deref foo) # Evaluates to 12