gistfile1.scala

// method bodies in Foo are just dummies

scala> class Foo[T] { def parse(s: String): T = null.asInstanceOf[T] ; def format(t: T): String = ""}
defined class Foo

scala> class C; class D
defined class C
defined class D

scala> val foos = List[Foo[_]](new Foo[C], new Foo[D])
foos: List[Foo[_]] = List(Foo@6f36c2f0, Foo@f58853c)

// try #1: doesn't work, existential type fail

scala> val foo = foos(0)
foo: Foo[_] = Foo@6f36c2f0

scala> foo.format(foo.parse(""))
<console>:13: error: type mismatch;
 found   : (some other)_1(in value foo)
 required: _1(in value foo)
              foo.format(foo.parse(""))
                                  ^
// try #2: works (but why?)

scala> foos(0) match { case foo => foo.format(foo.parse("")) }
res1: String = ""

// try #3: also works (path-dependent type win?)

scala> abstract class Foo { type T; def parse(s: String): T = null.asInstanceOf[T] ; def format(t: T): String = ""}
defined class Foo

scala> val foos = List[Foo](new Foo { type T = C }, new Foo { type T = D })
foos: List[Foo] = List($anon$1@15d49048, $anon$2@7098b907)

scala> val foo = foos(0)
foo: Foo = $anon$1@15d49048

scala> foo.format(foo.parse(""))
res4: String = ""