In Clojure 1.3, How to read and write a file

Question

I\'d like to know the \"recommended\" way of reading and writing a file in clojure 1.3 .

1. How to read the whole file
2. How to read a file line by line

Answer 1

If the file fits into memory you can read and write it with slurp and spit:

(def s (slurp "filename.txt"))

(s now contains the content of a file as a string)

(spit "newfile.txt" s)

This creates newfile.txt if it doesnt exit and writes the file content. If you want to append to the file you can do

(spit "filename.txt" s :append true)

To read or write a file linewise you would use Java's reader and writer. They are wrapped in the namespace clojure.java.io:

(ns file.test
  (:require [clojure.java.io :as io]))

(let [wrtr (io/writer "test.txt")]
  (.write wrtr "hello, world!\n")
  (.close wrtr))

(let [wrtr (io/writer "test.txt" :append true)]
  (.write wrtr "hello again!")
  (.close wrtr))

(let [rdr (io/reader "test.txt")]
  (println (.readLine rdr))
  (println (.readLine rdr)))
; "hello, world!"
; "hello again!"

Note that the difference between slurp/spit and the reader/writer examples is that the file remains open (in the let statements) in the latter and the reading and writing is buffered, thus more efficient when repeatedly reading from / writing to a file.

Here is more information: slurp spit clojure.java.io Java's BufferedReader Java's Writer