Fixing type inference for higher kinded type

Question

Ok, so I have this wonderfully simple setup:

trait Sys[S <: Sys[S]] trait Elem[S <: Sys[S]] trait AttrElem[S <: Sys[S]] { type E <: Elem[S] def attributes: Any def element: E }

And a factory:

object Factory { def apply[S <: Sys[S], E1 <: Elem[S]]( elem: E1): AttrElem[S] { type E = E1 } = new Impl(elem) private class Impl[S <: Sys[S], E1 <: Elem[S]](val element: E1) extends AttrElem[S] { type E = E1 def attributes = 1234 } }

Now in practice the f*** Scala type inference breaks down:

def test[S <: Sys[S]](elem: Elem[S]): Unit = { Factory(elem) } <console>:62: error: inferred type arguments [Nothing,Elem[S]] do not conform to method apply's type parameter bounds [S <: Sys[S],E1 <: Elem[S]] Factory(elem) ^

So my next attempt is existential types:

object Factory { def apply[S <: Sys[S], E1[~] <: Elem[~] forSome { type ~ <: Sys[~] }]( elem: E1[S]): AttrElem[S] { type E = E1[S] } = new Impl(elem) private class Impl[S <: Sys[S], E1[~] <: Elem[~] forSome { type ~ <: Sys[~] }]( val element: E1[S]) extends AttrElem[S] { type E = E1[S] def attributes = 1234 } }

This gives me this following lovely message:

<console>:62: error: inferred kinds of the type arguments (S,E1[S]) do not conform to the expected kinds of the type parameters (type S,type E1) in class Impl. E1[S]'s type parameters do not match type E1's expected parameters: type E1 has one type parameter, but type E1 (in class Impl) has one elem: E1[S]): AttrElem[S] { type E = E1[S] } = new Impl(elem) ^

"type E1 has one type parameter, but type E1 has one" -- huh?

Question: How can I define the factory's apply method to infer the types?

Can you describe what are you trying to achieve in "human words". Too hard for me to infer the meaning of Sys/Elem/AttrElem. — Eugene Zhulenev
– Eugene Zhulenev, Commented Apr 24, 2014 at 17:40
I want to be able to write Factory(elem) when elem is fully known, a sub-type of Elem[S]. Without having to type Factory[S, SomeSubtypeOfElem[S]](elem). In other words, I want to use the advertised feature of type-inference — 0__
– 0__, Commented Apr 24, 2014 at 17:42

0__ · Accepted Answer · 2014-04-24 17:45:12Z

The following "redundancy" seems to satisfy the compiler:

def apply[S <: Sys[S], E1 <: Elem[S]](elem: E1 with Elem[S]): AttrElem[S] { type E = E1 } = ...

That is, adding with Elem[S]. It looks like a needless deficiency of the Scala compiler to not infer S from E1 <: Elem[S] where Elem is invariant in S.

Or am I missing a crucial bit?

Here is another instance of the same problem: gist.github.com/ScrapCodes/10713689

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