Wednesday, May 9, 2012

Simple Scala scripts : Scan a directory recursively

I'm using Scala increasingly as a scripting language at the moment. As my confidence with it is increasing, I'm finding it's becoming more and more useful for those throw-away scripting situations.  Especially when then end up being not so throw-away after all.

def findFiles(path: File, fileFilter: PartialFunction[File, Boolean] = {case _ => false}): List[File] = {
  (path :: path.listFiles.toList.filter {
  }.flatMap {

(replace {} with (), ditch newlines and it goes on one line well-enough, just doesn't fit in a Blogger template that way)
We might be duplicating the a shell find:

find | grep 'foo'
find ./ -name "foo"

And whilst the Scala is more complex, the Scala function can do operations on a File object, which gives you a lot of the rest of the power of the find command thrown in to the bargain. Plus, as it accepts a partial function, you can chain together filters. If you truly just wanted an analog for find:

def findFiles(path: File): List[File]  = 
  path :: path.listFiles.filter {
  }.toList.flatMap {

Which is less complex that the first. This is still more work than find, but, the list you get back is an actual list. If you added anything useful to your find, say an md5 for each file, it gets less happy
find ./ | awk '{print "\""$0"\""}' | xargs -n1 md5sum
Maybe there's a better way, but that's what I've always ended up doing. The Scala is starting to compete now. Bump up the complexity one more notch, and I think Scala actually starts becoming less code and less obscure.

You might also notice that the example above can be fit nicely within the Map/Reduce paradigm. Scripting that is not only relatively easy, but can also be thrown at Hadoop for extra pzazz, and NoSQL buzz-worthyness.

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