for comprehensions seem broken

[Profpatsch]

I noticed this strange behaviour:

    val foo = for {
      p1 <- managed(UIPeer(ID("1")))
    } yield "foo"
    foo.either.right.get

That returns "foo", as expected.

    val foo = for {
      p1 <- managed(UIPeer(ID("1")))
      p2 <- managed(UIPeer(ID("2")))
    } yield "foo"
    foo.either.right.get

[error]  value either is not a member of resource.ManagedResource[String]
[error]     foo.either.right.get
[error]         ^

Compare:

    val foo = for {
      b1 <- Some("foo")
      b2 <- Some("baz")
    } yield "bar"
    foo.get

works just fine.

Maybe the map and/or flatMap implementation is broken (i.e. not following the monad laws)?

[jsuereth]

I finally figured this one out. IT's the difference between "ExtractableManagedResource" and a vanilla "ManagedResource". I think the library is assuming the value from teh for-expression is still "raw", i.e. you are trying to leak a resource out of the for-expression.

ManagedResource is not a Monad in the sense of "the category of all types", a.k.a. Hask. It is only a monad in a limited subset of types + functions, hence the laws only apply within this subset. However, I do think this is a bug and I know how to fix it.

[jsuereth]

To be clear, this line needs to change: https://github.com/jsuereth/scala-arm/blob/master/src/main/scala/resource/ManagedResource.scala#L56 and should return "ExtractableManagedResource"

[EronWright]

@jsuereth would you consider making this change now?

[pseiferth]

@jsuereth would you consider making this change now? :)

[cchantep]

@jsuereth If I understand that would mean a breaking change as updating the return type of flatMap? Could only be done if bumping to a new majorversions?

[jsuereth]

Yes, this would mean bumping major version.