Structural Types and Primitives

Question

I was playing with Scala structural types when I discovered that it looked like a bug to me. Here is my code:

type toD = { def toDouble: Double }
def foo(t: toD) = t.toDouble
foo(5)

And I got this error:

java.lang.NoSuchMethodException
at scala.runtime.BoxesRunTime.toDouble(Unknown Source)
at .foo(<console>:9)
at .<init>(<console>:11)
at .<clinit>(<console>)
at .<init>(<console>:11)
at .<clinit>(<console>)
at $print(<console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920)
at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
at scala.tools.nsc.io.package$$anon$2.run(package.scala:25)
at java.lang.Thread.run(Unknown Source)

Firstly, I do not know why this does not work. Secondly, it is strange that the code compiles just fine and throws an exception at runtime, saying that the method does not actually exist.

Does anyone have an explanation?

Answer 1

I just played around a bit with this and it really seems like a mistake. However, it works when you just set the return type to Any:

type toD = { def toDouble: Any }

I think this may have something to do with auto boxing and how primitives are handled.

edit:

I just found a workaround:

type toD[A] = { def toDouble: A }
def foo[A](x: toD[A])(implicit y: A =:= Double) = x.toDouble

, toDouble (A) Double