Clojure Cookbook

Chapter 4. Local I/O

4.0. Introduction

We’ve done a lot of work in the last few chapters, but clearly, the rubber has to meet the road somewhere. How did we get all of this data into our Clojure programs, and more importantly, how do we get it out? This chapter is all about input and output to a local computer—the primary place where most applications’ data hits the road, so to speak.

There are a variety of modes and mediums for communicating with a local machine. What do we communicate with, in what way, and in what format? It’s a little like the classic board game Clue: was it plain text, in the console, with command-line arguments; or Clojure data, in a file, as configuration data? In this chapter we’ll explore files, formats, and applications of both GUI and console flavors, to name a few topics.

While it isn’t possible for us to enumerate every possible combination, it is our hope that this chapter will give you a strong idea of what is possible. Handily enough, most good solutions in Clojure compose; you should have little trouble sticking together any number of recipes in this chapter to suit your needs.

4.1. Writing to STDOUT and STDERR

by Alan Busby

Problem

You want to write to STDOUT and STDERR.

Solution

By default, the print and println functions will print content passed to them to STDOUT:

(println "This text will be printed to STDOUT.")
;; *out*
;; This text will be printed to STDOUT.

(do
  (print "a")
  (print "b"))
;; *out*
;; ab

Change the binding of *out* to *err* to print to ...