Rewriting shared nodes

[etorreborre]

Hi Tony,

Here is a graph:

case class U(name: String) {
  override def toString = s"${getClass.getSimpleName}($name: ${System.identityHashCode(this)})"
  override def equals(a: Any) = System.identityHashCode(this) == System.identityHashCode(a)
}
case class V(name: String, u: U) {
  override def toString = s"${getClass.getSimpleName}($name: ${System.identityHashCode(this)}, $u)"
  override def equals(a: Any) = System.identityHashCode(this) == System.identityHashCode(a)
}
case class T(u: U, v1: V, v2: V){
  override def toString = s"${getClass.getSimpleName}(${System.identityHashCode(this)}, $u, $v1, $v2)"
}

val v = V("v1", U("u2"))
val graph = T(U("u1"), v, v)

   T

 / |\

U1 | \

   | /

   V

   |

   U2

If I rewrite this graph with this strategy, which makes singletons in a tree:

def singletonStrategy[S](implicit tag: ClassTag[S]): Strategy = {
  var s: Option[S] = None
  strategy[Any] {
    case tag(v) =>
      s match {
        case Some(singleton) =>
          Some(singleton)
        case None =>
          s = Some(v.asInstanceOf[S])
          Some(v)
      }
    case other => Some(other)
  }
}

Then rewrite(everywhere(singletonStrategy[U]))(graph) do its job and replace every instance of `U` with the first instance found.

Unfortunately it will also duplicate the shared node `v`:

   T
 / |\
U1 | \
   |  \
   V1  V2
   |   |
   U1  U1


My question is: is there a way to rewrite a graph so that shared nodes are preserved?

I tried to look at previous messages on this mailing-list and there is supposedly a solution which I previously used for this:

https://groups.google.com/d/msg/kiama/umAJIhGuzzM/4UeuJKMVtpgJ

but I cannot recall what it was. Do you think it is even possible to do what I want?

Thanks,

Eric.

[etorreborre]

I think I found the solution: MemoRewriter!

[Tony Sloane]

Hi Eric,

Yep, that was it. Hopefully MemoRewriter will work for your current application too.

cheers,

Tony

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[etorreborre]

It worked great, I am using it for the DI library I told you about: https://github.com/zalando/grafter.

I feel just bad that it took me so long to re-discover something which you already fixed for me year ago :-).

Happy New Year 2017!

Eric.

[Tony Sloane]

And Happy New Year to you too Eric (and everyone else here). It's good to hear that your DI library is going strong.

cheers,

Tony