Theory: if function

Clojure has an if function that evaluates an expresssion. If that expression is true, then the first value is returned, if false then the second argument is returned.

In psudo-code, the if function in Clojure works as follows

If (this expression is true ?)
    then return this value
    else return this value

In our code, if the web address, :uri, taken from the request map is equal to / then the first response map with the hello message is returned. If the :uri value is not equal to / then the second resource map with an error message is returned.

(if (= "/" (:uri request))
    {:status 200
     :body "<h1>Hello, Clojure World</h1>
            <p>Welcome to your first Clojure app.</p>"
     :headers {}}
    {:status 404
     :body "<h1>This is not the page you are looking for</h1>
            <p>Sorry, the page you requested was not found!></p>"
     :headers {}}))

In the case where an if expression is defined with only one value and the expression is false, then the value nil is returned.

Advanced if functions with do

Each of the two possible values the if function can return can come from evaluating a function. For example

(if (true)
  (str "I am the truth")
  (str "I am the path to darkness")

However, each value can only be defined by one expression. If you need to have multiple steps you can use a function called do

(if (true)
  (do (some-function)
      (another-function))
  (else-function))

The do function calls each function evaluation in turn, returning the result of the last function called.

The if function is a very simplistic way to define routes in our web application. Soon we will look at Compojure, a library for elegantly managing routing for our Clojure application.