When to use let vs. if-let in Clojure

Question

When does using if-let rather than let make code look better and does it have any performance impact?

Answer 1

I guess if-let should be used when you'd like to reference an if condition's value in the "then" part of the code:

i.e. instead of

(let [result :foo]   (if result     (do-something-with result)     (do-something-else))) 

you write:

(if-let [result :foo]   (do-something-with result)   (do-something-else)) 

which is a little neater, and saves you indenting a further level. As far as efficiency goes, you can see that the macro expansion doesn't add much overhead:

(clojure.core/let [temp__4804__auto__ :foo]   (if temp__4804__auto__     (clojure.core/let [result temp__4804__auto__]       (do-something-with result))     (do-something-else))) 

This also illustrates that the binding can't be referred to in the "else" part of the code.

Answer 2

A good use case for if-let is to remove the need to use anaphora. For example, the Arc programming language provides a macro called aif that allows you to bind a special variable named it within the body of an if form when a given expression evaluates to logical true. We can create the same thing in Clojure:

(defmacro aif [expr & body]   `(let [~'it ~expr] (if ~'it (do ~@body))))  (aif 42 (println it)) ; 42 

This is fine and good, except that anaphora do not nest, but if-let does:

(aif 42 (aif 38 [it it])) ;=> [38 38]  (aif 42 [it (aif 38 it)]) ;=> [42 38]  (if-let [x 42] (if-let [y 38] [x y])) ;=> [42 38]