Reading and evaluating code

Let's have a look at how code can be parsed and evaluated in Clojure. The most elementary way to convert text into an expression is by using the read function. This function accepts a java.io.PushbackReader instance as its first argument, as shown here:

user> (read (-> "(list 1 2 3)"
                .toCharArray
                java.io.CharArrayReader.
                java.io.PushbackReader.))
(list 1 2 3)

Note

These examples can be found in src/m_clj/c4/read_and_eval.clj of the book's source code.

In this example, a string containing a valid expression is first converted into an instance of java.io.PushbackReader and then passed to the read function. It seems like a lot of unnecessary work to read a string, but it is due to the fact that the read function deals with streams and readers, and not strings. If no arguments are passed to the read function, it will create a reader from the standard input and prompt the user to enter an expression...