by @yokolet

Level: easy

Assignment let

  • Advice to coaches

    The let binding is frequently used in function definitions. We have functions with destructuring and functions with let/destructuring sections. It may be good to come back to this lab after or during the function lab.

Like def, let creates a binding. In other languages, it is called a local variable assignment. In Clojure, it has the different name: lexical binding. Similar to local variables in other languages, Clojure’s lexically bound variables are available to use in a limited code block (scope). However, unlike other languages, the values never change (immutable). For convenience, this document uses the word variable but its value won’t vary once the value is assigned.

The syntax is (let [bindings*] exprs*).

Let’s try this in a REPL.

user> (let [her-number 234] (println her-number))
234
nil
user> her-number
CompilerException java.lang.RuntimeException: Unable to resolve symbol: her-number in this context
, compiling:(/private/var/folders/4b/c9gsjvv12tq9n4mph065qs480000gn/T/form-init6323661111322215411
.clj:1:743)
user> ; because her-number is available only within let parenthesis (body).

We can write as many bindings (key-value pairs) as we want within the square brackets. Those bindings are evaluated one by one, so we can use the evaluated result within a single square bracket. Try these examples below:

user> (let [x 1
            y 1
            z (+ x y)
            z (* 2 z) ] (println z))
4
nil
user> (let [+ str    ; locally, + works as string concatenation
            w (+ "a" "b" "c")]
        (println (+ "I got " w)))
I got abc
nil

As seen in the examples above, the value part of bindings can be a function. We can use println function in let bindings, too. This is often used for debugging so that you can check the value.

user> (let [x 1
            y 1
            z (+ x y)
            _ (println (str "z was " z))
            z (* 2 z)]
        z)
z was 2
4

References