What Automatic Resource Management alternatives exist for Scala?
Chris Hansen's blog entry 'ARM Blocks in Scala: Revisited' from 3/26/09 talks about about slide 21 of Martin Odersky's FOSDEM presentation. This next block is taken straight from slide 21 (with permission):
def using[T <: { def close() }]
(resource: T)
(block: T => Unit)
{
try {
block(resource)
} finally {
if (resource != null) resource.close()
}
}
--end quote--
Then we can call like this:
using(new BufferedReader(new FileReader("file"))) { r =>
var count = 0
while (r.readLine != null) count += 1
println(count)
}
What are the drawbacks of this approach? That pattern would seem to address 95% of where I would need automatic resource management...
Edit: added code snippet
Edit2: extending the design pattern - taking inspiration from python with
statement and addressing:
- statements to run before the block
- re-throwing exception depending on the managed resource
- handling two resources with one single using statement
- resource-specific handling by providing an implicit conversion and a
Managed
class
This is with Scala 2.8.
trait Managed[T] {
def onEnter(): T
def onExit(t:Throwable = null): Unit
def attempt(block: => Unit): Unit = {
try { block } finally {}
}
}
def using[T <: Any](managed: Managed[T])(block: T => Unit) {
val resource = managed.onEnter()
var exception = false
try { block(resource) } catch {
case t:Throwable => exception = true; managed.onExit(t)
} finally {
if (!exception) managed.onExit()
}
}
def using[T <: Any, U <: Any]
(managed1: Managed[T], managed2: Managed[U])
(block: T => U => Unit) {
using[T](managed1) { r =>
using[U](managed2) { s => block(r)(s) }
}
}
class ManagedOS(out:OutputStream) extends Managed[OutputStream] {
def onEnter(): OutputStream = out
def onExit(t:Throwable = null): Unit = {
attempt(out.close())
if (t != null) throw t
}
}
class ManagedIS(in:InputStream) extends Managed[InputStream] {
def onEnter(): InputStream = in
def onExit(t:Throwable = null): Unit = {
attempt(in.close())
if (t != null) throw t
}
}
implicit def os2managed(out:OutputStream): Managed[OutputStream] = {
return new ManagedOS(out)
}
implicit def is2managed(in:InputStream): Managed[InputStream] = {
return new ManagedIS(in)
}
def main(args:Array[String]): Unit = {
using(new FileInputStream("foo.txt"), new FileOutputStream("bar.txt")) {
in => out =>
Iterator continually { in.read() } takeWhile( _ != -1) foreach {
out.write(_)
}
}
}
Here's James Iry solution using continuations:
// standard using block definition
def using[X <: {def close()}, A](resource : X)(f : X => A) = {
try {
f(resource)
} finally {
resource.close()
}
}
// A DC version of 'using'
def resource[X <: {def close()}, B](res : X) = shift(using[X, B](res))
// some sugar for reset
def withResources[A, C](x : => A @cps[A, C]) = reset{x}
Here are the solutions with and without continuations for comparison:
def copyFileCPS = using(new BufferedReader(new FileReader("test.txt"))) {
reader => {
using(new BufferedWriter(new FileWriter("test_copy.txt"))) {
writer => {
var line = reader.readLine
var count = 0
while (line != null) {
count += 1
writer.write(line)
writer.newLine
line = reader.readLine
}
count
}
}
}
}
def copyFileDC = withResources {
val reader = resource[BufferedReader,Int](new BufferedReader(new FileReader("test.txt")))
val writer = resource[BufferedWriter,Int](new BufferedWriter(new FileWriter("test_copy.txt")))
var line = reader.readLine
var count = 0
while(line != null) {
count += 1
writer write line
writer.newLine
line = reader.readLine
}
count
}
And here's Tiark Rompf's suggestion of improvement:
trait ContextType[B]
def forceContextType[B]: ContextType[B] = null
// A DC version of 'using'
def resource[X <: {def close()}, B: ContextType](res : X): X @cps[B,B] = shift(using[X, B](res))
// some sugar for reset
def withResources[A](x : => A @cps[A, A]) = reset{x}
// and now use our new lib
def copyFileDC = withResources {
implicit val _ = forceContextType[Int]
val reader = resource(new BufferedReader(new FileReader("test.txt")))
val writer = resource(new BufferedWriter(new FileWriter("test_copy.txt")))
var line = reader.readLine
var count = 0
while(line != null) {
count += 1
writer write line
writer.newLine
line = reader.readLine
}
count
}
Daniel,
I've just recently deployed the scala-arm library for automatic resource management. You can find the documentation here: https://github.com/jsuereth/scala-arm/wiki
This library supports three styles of usage (currently):
1) Imperative/for-expression:
import resource._
for(input <- managed(new FileInputStream("test.txt")) {
// Code that uses the input as a FileInputStream
}
2) Monadic-style
import resource._
import java.io._
val lines = for { input <- managed(new FileInputStream("test.txt"))
val bufferedReader = new BufferedReader(new InputStreamReader(input))
line <- makeBufferedReaderLineIterator(bufferedReader)
} yield line.trim()
lines foreach println
3) Delimited Continuations-style
Here's an "echo" tcp server:
import java.io._
import util.continuations._
import resource._
def each_line_from(r : BufferedReader) : String @suspendable =
shift { k =>
var line = r.readLine
while(line != null) {
k(line)
line = r.readLine
}
}
reset {
val server = managed(new ServerSocket(8007)) !
while(true) {
// This reset is not needed, however the below denotes a "flow" of execution that can be deferred.
// One can envision an asynchronous execuction model that would support the exact same semantics as below.
reset {
val connection = managed(server.accept) !
val output = managed(connection.getOutputStream) !
val input = managed(connection.getInputStream) !
val writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(output)))
val reader = new BufferedReader(new InputStreamReader(input))
writer.println(each_line_from(reader))
writer.flush()
}
}
}
The code makes uses of a Resource type-trait, so it's able to adapt to most resource types. It has a fallback to use structural typing against classes with either a close or dispose method. Please check out the documentation and let me know if you think of any handy features to add.