Reading Files in Clojure(lethain.com)
lethain.com
Reading Files in Clojure
http://lethain.com/entry/2009/nov/15/reading-file-in-clojure/
2 comments
[deleted]
Too much duct-tape, imo.
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.
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))
I don't entirely agree. In CL, people pass around slurp-file snippets like:
http://groups.google.com/group/comp.lang.lisp/msg/89501db253...
Further, Clojure's slurp has an optional encoding argument. (I'm glancing at its sourcecode, in the same emacs buffer that I'm typing this post in.)
If I understand your argument, slurp's too unlispy to include in the language. But at least in the CL world, the user is constantly adding to Lisp as she codes away. And there are many things in the CL spec which often considered unlispy. (Take loop and format. Loop is even criticized for being not well-defined, as well as being incomplete in ways you wouldn't expect, as you start pushing its limits. Though I should mention that I love both loop and format.)
What you wrote was beautiful and more complete (I'm not being sarcastic; it looks like the High CL style, which I consider very aesthetic and good, though that's not the only aesthetic style I appreciate), but I think you're comparing a Clojure feature with something which exists mostly as a CL snippet passed around informally. (And is maybe now in some opensource library.)
Further, Clojure's slurp has an optional encoding argument. (I'm glancing at its sourcecode, in the same emacs buffer that I'm typing this post in.)
If I understand your argument, slurp's too unlispy to include in the language. But at least in the CL world, the user is constantly adding to Lisp as she codes away. And there are many things in the CL spec which often considered unlispy. (Take loop and format. Loop is even criticized for being not well-defined, as well as being incomplete in ways you wouldn't expect, as you start pushing its limits. Though I should mention that I love both loop and format.)
What you wrote was beautiful and more complete (I'm not being sarcastic; it looks like the High CL style, which I consider very aesthetic and good, though that's not the only aesthetic style I appreciate), but I think you're comparing a Clojure feature with something which exists mostly as a CL snippet passed around informally. (And is maybe now in some opensource library.)
You can redirect I/O in Clojure, so you possibly can spit/slurp things other than files. I was sure you could use with-input-stream and with-output-stream but I don't see it now.. oh well. Anyway, this (taken from Wikipedia) works with standard I/O and I can only assume ti works with file I/O too (if not, then I agree this is a flaw in the Clojure library design):
(def bit-bucket-writer
(proxy [java.io.Writer] []
(write [buf] nil)
(close [] nil)
(flush [] nil)))
(defmacro noprint
"Evaluates the given expressions with all printing to *out* silenced."
[& forms]
`(binding [*out* bit-bucket-writer]
~@forms))
(noprint
(println "Hello, nobody!"))
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 :)