scala - "Parameter type in structural refinement may not refer to an abstract type defined outside that refinement"

Question

When I compile:

object Test extends App {
  implicit def pimp[V](xs: Seq[V]) = new {
    def dummy(x: V) = x
  }
}                                                                                                                                                                                                              

I get:

$ fsc -d aoeu go.scala
go.scala:3: error: Parameter type in structural refinement may not refer to an abstract type defined outside that refinement
    def dummy(x: V) = x
        ^
one error found

Why?

(Scala: "Parameter type in structural refinement may not refer to an abstract type defined outside that refinement" doesn't really answer this.)

Answer 1

It's disallowed by the spec. See 3.2.7 Compound Types.

Within a method declaration in a structural re?nement, the type of any value parameter may only refer to type parameters or abstract types that are contained inside the re?nement. That is, it must refer either to a type parameter of the method itself, or to a type de?nition within the re?nement. This restriction does not apply to the function’s result type.

Before Bug 1906 was fixed, the compiler would have compiled this and you'd have gotten a method not found at runtime. This was fixed in revision 19442 and this is why you get this wonderful message.

The question is then, why is this not allowed?

Here is very detailed explanation from Gilles Dubochet from the scala mailing list back in 2007. It roughly boils down to the fact that structural types use reflection and the compiler does not know how to look up the method to call if it uses a type defined outside the refinement (the compiler does not know ahead of time how to fill the second parameter of getMethod in p.getClass.getMethod("pimp", Array(?))

But go look at the post, it will answer your question and some more.

Edit:

Hello list.

I try to define structural types with abstract datatype in function parameter. ... Any reason?

I have heard about two questions concerning the structural typing extension of Scala 2.6 lately, and I would like to answer them here.

1. Why did we change Scala's native values (“int”, etc.) boxing scheme to Java's (“java.lang.Integer”).
2. Why is the restriction on parameters for structurally defined methods (“Parameter type in structural refinement may not refer to abstract type defined outside that same refinement”) required.

Before I can answer these two questions, I need to speak about the implementation of structural types.

The JVM's type system is very basic (and corresponds to Java 1.4). That means that many types that can be represented in Scala cannot be represented in the VM. Path dependant types (“x.y.A”), singleton types (“a.type”), compound types (“A with B”) or abstract types are all types that cannot be represented in the JVM's type system.

To be able to compile to JVM bytecode, the Scala compilers changes the Scala types of the program to their “erasure” (see section 3.6 of the reference). Erased types can be represented in the VM's type system and define a type discipline on the program that is equivalent to that of the program typed with Scala types (saving some casts), although less precise. As a side note, the fact that types are erased in the VM explains why operations on the dynamic representation of types (pattern matching on types) are very restricted with respect to Scala's type system.

Until now all type constructs in Scala could be erased in some way. This isn't true for structural types. The simple structural type “{ def x: Int }” can't be erased to “Object” as the VM would not allow accessing the “x” field. Using an interface “interface X { int x{}; }” as the erased type won't work either because any instance bound by a value of this type would have to implement that interface which cannot be done in presence of separate compilation. Indeed (bear with me) any class that contains a member of the same name than a member defined in a structural type anywhere would have to implement the corresponding interface. Unfortunately this class may be defined even before the structural type is known to exist.

Instead, any reference to a structurally defined member is implemented as a reflective call, completely bypassing the VM's type system. For example def f(p: { def x(q: Int): Int }) = p.x(4) will be rewritten to something like:

  def f(p: Object) = p.getClass.getMethod("x", Array(Int)).invoke(p, Array(4))

And now the answers.

1. “invoke” will use boxed (“java.lang.Integer”) values whenever the invoked method uses native values (“int”). That means that the above call must really look like “...invoke(p, Array(new java.lang.Integer(4))).intValue”.

Integer values in a Scala program are already often boxed (to allow the “Any” type) and it would be wasteful to unbox them from Scala's own boxing scheme to rebox them immediately as java.lang.Integer.

Worst still, when a reflective call has the “Any” return type, what should be done when a java.lang.Integer is returned? The called method may either be returning an “int” (in which case it should be unboxed and reboxed as a Scala box) or it may be returning a java.lang.Integer that should be left untouched.

Instead we decided to change Scala's own boxing scheme to Java's. The two previous problems then simply disappear. Some performance-related optimisations we had with Scala's boxing scheme (pre-calculate the boxed form of the most common numbers) were easy to use with Java boxing too. In the end, using Java boxing was even a bit faster than our own scheme.

2. “getMethod”'s second parameter is an array with the types of the parameters of the (structurally defined) method to lookup — for selecting which method to get when the name is overloaded. This is the one place where exact, static types are needed in the process of translating a structural member call. Usually, exploitable static types for a method's parameter are provided with the structural type definition. In the example above, the parameter type of “x” is known to be “Int”, which allows looking it up.

Parameter types defined as abstract types where the abstract type is defined inside the scope of the structural refinement are no problem either: def f(p: { def x[T](t: T): Int }) = p.xInt In this example we know that any instance passed to “f” as “p” will define “x[T](t: T)” which is necessarily erased to “x(t: Object)”. The lookup is then correctly done on the erased type: def f(p: Object) = p.getClass.getMethod("x", Array(Object)).invoke(p, Array(new java.lang.Integer(4)))

But if an abstract type from outside the structural refinement's scope is used to define a parameter of a structural method, everything breaks: def f[T](p: { def x(t: T): Int }, t: T) = p.x(t) When “f” is called, “T” can be instantiated to any type, for example: f[Int]({ def x(t: Int) = t }, 4) f[Any]({ def x(t: Any) = 5 }, 4) The lookup for the first case would have to be “getMethod("x", Array(int))” and for the second “getMethod("x", Array(Object))”, and there is no way to know which one to generate in the body of “f”: “p.x(t)”.

To allow defining a unique “getMethod” call inside “f”'s body for any instantiation of “T” would require any object passed to “f” as the “p” parameter to have the type of “t” erased to “Any”. This would be a transformation where the type of a class' members depend on how instances of this class are used in the program. And this is something we definitely don't want to do (and can't be done with separate compilation).

Alternatively, if Scala supported run-time types one could use them to solve this problem. Maybe one day ...

But for now, using abstract types for structural method's parameter types is simply forbidden.

Sincerely, Gilles.