Reading Files in Clojure

Clojure especially embraces interop with Java in favor of wrapping Java features into Clojure functions, so it's part of the official pain.

Can't say it was as easy as in Python but on the other hand, I can't say it hurts to know a bit Java, too. At least it's for a good cause :)

SPIT and SLURP are just one argument functions without much control. If the language was a bit more Lispy, they wouldn't even be standard functions, but something left to the programmer. For example:

1) There is no control over input file character encoding.

2) No provision for reading from non-file streams; say, you want to "slurp" input from a socket stream, N octets or up to a certain EOF delimiter. You have no control with clojure in that regard, at least with SLURP/SPIT, you will have to use Java modules with weird dot and bracket syntax.

3) What happens if the file you're attempting to read doesn't exist? what happens if there is no input in the file yet (should you block or return an error or return EOF?)

4) What happens if the file you're writing to doesn't exist? create? err? What if it exists? truncate? append? err?

Separate between files, streams, sequences, and I/O methods and you will have a clean user-extensive framework for I/O.

  (defgeneric slurp (stream
                         &optional &key
                         element-type
                         external-format
                         if-exists
                         if-does-not-exist)
    "Reads contents of element-type from stream")

  (defmethod slurp ((path pathname)
                         &optional &key
                         (element-type character)
                         external-format
                         (if-exists :supersede)
                         (if-does-not-exist :create))
    (with-open-file (file path :element-type element-type
                       :external-format external-format
                       :if-exists if-exists
                       :if-does-not-exist if-does-not-exist)
       (let ((buf (make-array (file-length file) 
                     :element-type element-type)))
          (read-sequence buf file)
          buf)))

  (slurp #p"/etc/passwd")   ;; read file in default encoding
 
  (slurp #p"/home/mahmud/sales/mideast2009Q2.txt :external-format :utf-8)

  (slurp #p"/var/log/hunchentoot/access.log"
         :if-does-not-exist :error)

  (defmethod slurp ((stream socket-stream) .. length)
    (let ((buf (make-array length .. :element-type '(unsigned-byte 8))))
      (read-sequence buf stream)
       buf))