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 pseudo-code, the if function in Clojure works as follows

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

In the project code an if function checks the web address by returning the value associated with :uri in the request map.

If the :url value 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 {}}))

Hint::Single path if function

In the case where an if expression is defined with only one value and the expression is false, then the value nil is returned. when function is the idiomatic choice over a single path if function.

Multiple expression if function with do

Each of the two possible values the if function returns can come from only evaluating one expression. For example

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

If you need multiple expressions they can be wrapped in the do function

(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.

Hint::Compojure for managing routes

The if function is a very simplistic way to define routes in our web application. Compojure is a library for elegantly managing routing for our Clojure server-side applications.