simple-arm

simple-arm is based on the Managed typeclass, which opens and closes resources. There are three ways to use it: for-comprehensions, the using function, and ResourceScopes:

import com.rojoma.simplearm.util._

val linesCopied = for {
  in <- managed(new java.io.FileReader(inName))
  out <- managed(new java.io.FileWriter(outName))
} {
  copyLines(in, out)
}

val linesCopied = using(new java.io.FileReader(inName), new java.io.FileWriter(outName)) { (in, out) =>
  copyLines(in, out)
}

// return an iterator which is managed by the given ResourceScope.
// Closing the iterator through the scope will also ensure the
// underlying Source is closed.
def fileAsIterator(in: File, rs: ResourceScope): Iterator[String] = {
  val source = rs.open(Source.fromFile(in))
  rs.openUnmanaged(source.getLines(), transitiveClose = List(source))
}

The first two are almost completely equivalent; the for-comprehension requires using managed but the using function separates the resource from its name. In a for-comprehension, earlier resources are available at the time later ones are initialized, of course.

Managed is a monad; invoking flatMap or map on it will produce a new Managed. To actually cause the effects to occur, run or foreach must be used. This is a change from simple-arm 1, where Managed supported the for-comprehension syntactic sugar without actually being a monad. foreach and run are synonyms; in particular, foreach will return the result of the function it is passed:

val x = for(r <- managed(...)) yield 5 // x is Managed[Int]
val y = for(r <- managed(...)) 5 // y is Int

Getting it

SBT:

libraryDependencies += "com.rojoma" %% "simple-arm-v2" % "2.3.2"

While for Maven, the pom snippets are:

<dependencies>
  <dependency>
    <groupId>com.rojoma</groupId>
    <artifactId>simple-arm-v2_${scala.version}</artifactId>
    <version>2.3.2</version>
  </dependency>
</dependencies>

Details

Resource-management is defined as follows:

1. Create the resouce by evaluating the by-name parameter passed to managed or using.
2. Open the resource by calling the openPreTry method on the typeclass instance with it. By default, this is a no-op. If the open returns normally, the resource is considered to be under management and will be closed.
3. Open the resource by calling the openPostTry method on the typeclass instance. By default, this is a no-op.
4. Do whatever is required with this resource.
5. If the "whatever" returns normally ("normally" includes via ControlThrowable), invoke close on the typeclass instance with it. Otherwise, invoke closeAbormally with both the resource object and the exception. By default, this simply defers to close. If closeAbnormally throws a non-ControlThrowable exception, it is added to the original exception's suppressed list.

Resources in using are acquired in left-to-right order and released in the opposite order. ResourceScope will, unless resources are explicitly closed early, also close resources in the opposite order from which they were added. Note that transferring resources between ResourceScopes may not preserve the exact order. Instead, it uses some order consistent with the DAG produced by resources' transitiveClose parameters.