Add support for Scala.js

[milessabin]

This is mainly the addition of Scala.js build/publish infrastructure, however building for Scala.js revealed a couple of issues.

• The handling of boxed primitives for Typeable was missing some instances which was only apparent when the corresponding tests failed for js.
• The mini Eval that @TimWSpence put together for Foldable#foldRight test derivation blew up on js. When that was swapped out for a test dependency on cats-core it still failed, revealing that the use of Eval in the Foldable derivation was incorrect (a missing flatMap). This will also be an issue with the prototype Foldable implementation in Kittens for Scala 3.

[joroKr21]

Nice!

[joroKr21]

Hmm I'm getting an error on main:

[error] Referring to non-existent class java.lang.constant.Constable
[error]   called from shapeless3.typeable.Typeable$ValueTypeable.productElement(int)java.lang.Object
[error]   called from scala.runtime.ScalaRunTime$$anon$1.next()java.lang.Object
[error]   called from scala.util.hashing.MurmurHash3.unorderedHash(scala.collection.IterableOnce,int)int
[error]   called from constructor scala.util.hashing.MurmurHash3$.<init>()void
[error]   called from scala.runtime.ScalaRunTime$._hashCode(scala.Product)int
[error]   called from scala.scalajs.js.JavaScriptException.hashCode()int
[error]   called from java.lang.Object.toString()java.lang.String
[error]   exported to JavaScript with @JSExport

Is this a Mac thing?

[milessabin]

Have you got something standalone that reproduces that?

[joroKr21]

I'm just running test

[milessabin]

Interesting! I can't reproduce that on Linux.

[gzm0]

Looks like this is more about the JDK version. Starting with JDK12, String (and probably other types) extend Constable and ConstantDesc. These interfaces are not implemented in Scala.js.

So basically, another way to look at it is that Scala.js does not support building on JDK12.

[sjrd]

It should be possible to avoid the reference to Constable. I assume it comes up as an inferred LUB of String and Class. It should be possible to avoid it being inferred with some explicit type somewhere.

Perhaps Scala 3 should also consider Constable as a transparent trait to avoid that problem once and for all.

[joroKr21]

It should be possible to avoid the reference to Constable. I assume it comes up as an inferred LUB of String and Class. It should be possible to avoid it being inferred with some explicit type somewhere.

Except that productElement is generated code by the compiler: https://github.com/typelevel/shapeless-3/blob/main/modules/typeable/src/main/scala/shapeless3/typeable/typeable.scala#L71