jsuereth/scala-arm

This project aims to be the Scala Incubator project for Automatic-Resource-Management in the scala library

jpcooper

@jpcooper

@jsuereth Have you ever needed to work with a few hundred resource simultaneously? I've written a function that takes T[ManagedResource[U]] to ManagedResource[T[U]] for T <: Traversable. However, this method relies on foldLeft and flatMap. This leads to StackOverflowError if the input is too long. Do you know of any better way to do this?

I was considering writing case class ManagedResourceSequence[a](resource: Seq[a]). Do you see any problems with this?

jpcooper

@jpcooper

Well, one problem is that ManagedResource doesn't expose the resource

jpcooper

@jpcooper

I think what we're actually looking for is making ManagedResource Applicative without flatMap

Josh Suereth

@jsuereth_twitter

yeah

Using flatMap is "ok" I think, if we trampoline

I think having an applicative + traverse would be really nice. What do you need to make that happen?

I'm thinking of non/cats extension

jpcooper

@jpcooper

The problem right now as that ManagedResource doesn't have an open method like AbstractManagedResource does

Josh Suereth

@jsuereth_twitter

right, so you need to assume things about it.....

is "zip" enough on managed resource?

jpcooper

@jpcooper

Yes, but how would it be implemented?

Josh Suereth

@jsuereth_twitter

it already is?

jpcooper

@jpcooper

without flatMap?

I don't see zip in ManagedResource

I guess I'm looking for and without flatMap

jpcooper

@jpcooper

@jsuereth Doesn't that still involve a big stack, or am I missing something?

This is what I'm doing:

def sequenceManagedResources[R, T[X] <: TraversableOnce[X]](managedResources: T[ManagedResource[R]])(implicit canBuildFrom: CanBuildFrom[T[ManagedResource[R]], R, T[R]]): ManagedResource[T[R]] = {
    val initialBuilderResource = unitManaged(canBuildFrom(managedResources))

    val finalManagedBuilder = managedResources.foldLeft(initialBuilderResource) { case (builderResource, managedR) ⇒
      builderResource.flatMap { builder ⇒
        managedR.map(r ⇒ builder += r)
      }
    }

    finalManagedBuilder.map(_.result())
  }

def unitManaged[T](value: => T): ManagedResource[T] = {
    managed(()).map(_ ⇒ value)
  }

Josh Suereth

@jsuereth

hey, sorry

so here's my lame impl: https://github.com/jsuereth/scala-arm/blob/master/src/main/scala/resource/package.scala#L57

and zip: https://github.com/jsuereth/scala-arm/blob/master/src/main/scala/resource/package.scala#L44 which would have the issue of stack overflow....

they may both have that issue....

for popping the stack, we'd need to use an alternative mechanism for this. I have an idea for how to do it, need to experiment

jpcooper

@jpcooper

@jsuereth I suppose we would need to allow one ManagedResource to control another ManagedResource's resource

jpcooper

@jpcooper

@jsuereth I'm going to give it a go with continuations

jpcooper

@jpcooper

Sorry for the mess. I realise the implementation of and in my version of scala-arm uses .reflect. Why was this removed in the latest version?

Josh Suereth

@jsuereth_twitter

because CPS transformed were removed from being supported in scala :(

I like continuations

SO, what we should do here

is basically optimise for the "AbstractMangedResource" case

jpcooper

@jpcooper

@jsuereth Do you mean they'll be removed in 2.12?

Josh Suereth

@jsuereth_twitter

yeah

support was dropped in 2.11

jpcooper

@jpcooper

Shame

Josh Suereth

@jsuereth_twitter

agreed

jpcooper

@jpcooper

How do you mean that support has been dropped though? Isn't just a separate module which can be depended on?

Josh Suereth

@jsuereth_twitter

which no one is maintaining

i.e. as language features change, I wouldn't rely on that module

jpcooper

@jpcooper

I understand

Josh Suereth

@jsuereth_twitter

hmmm

actually, looks like someone picked i tup

er picked it up

we removed it from ARM before that happened

Yang, Bo

@Atry

Hi @jsuereth_twitter , could you review my PR #54 , please?

@jsuereth @jsuereth_twitter , which account are you using?

Yang, Bo

@Atry

Hi @jsuereth_twitter @jsuereth , would you mind changing @Arty to @Atry in https://github.com/jsuereth/scala-arm/releases/tag/v2.0, please

jpcooper

@jpcooper

@jsuereth_twitter You may remember that we talked a while ago about how to sequence lots of ManagedResources without getting a stack overflow on calling .acquireAndGet. We discussed the possibility of continuations. I've been wondering whether it could be done more easily. Have you thought about the problem since at all?

Oh I see that that conversation is just a few lines up

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