I'm trying to use Scala's manifests to instantiate a type, and I'm running into problems when that type is parameterized on types with a view bound. I've distilled the problem down to the following code:
class foo[X <% Ordered[X]]() {}
def boo[T](implicit m : Manifest[T]) = { m.erasure.newInstance().asInstanceOf[T] }
boo[foo[String]]
java.lang.InstantiationException: foo
at java.lang.Class.newInstance0(Class.java:357)
at java.lang.Class.newInstance(Class.java:325)
. . .
So you can see we have a simple class, foo, which is parameterized on X; which is view bounded by Ordered[X]. The boo function simply attempts to instantiate a new instance of a foo[String] using manifests. However, when this function is called, things go horribly awry and I get the stack trace that begins as I have shown. When the type parameter of foo is not view bounded, the instantiation works without issue. I assume this has something to do with the fact that the view bound is just syntactic sugar for the existence of an implicit conversion of X => Ordered[X], and that somehow the manifest being dependent on another manifest is causing a problem. However, I have no idea what's really happening or, more importantly, how to fix it. Is this even possible in Scala, and if not, how do people achieve something similar?
newInstance works only if T has a parameterless constructor. foo has not. The view bound <% (just as a the context bound :) is a shortcut for an implicit parameter in the constructor.
class foo[X <% Ordered[X]] is the same as class foo(implicit freshName: X => Ordered[X]). Lacking a parameterless constructor for foo, newInstance fails.