class A has one type parameter, but type B has one

Question

Recently I stumbled across a strange (to me) compiler error message. Consider the following code:

trait Foo {
  type Res <: Foo
  type Bar[X <: Res]
}

class MyFoo extends Foo {
  override type Res = MyFoo
  override type Bar[X <: Res] = List[X]
}

type FOO[F <: Foo, R <: Foo, B[_ <: R]] = F { type Res = R; 
                                              type Bar[X <: R] = B[X] }

def process[F <: Foo, R <: Foo, B[_ <: R]](f: FOO[F, R, B]) {}

Now, if I want to call the process method I have to explicitly write the type parameters:

process[MyFoo, MyFoo, List](new MyFoo) // fine

If I write:

process(new MyFoo)

or

process((new MyFoo): FOO[MyFoo, MyFoo, List])

I get the following error message:

inferred kinds of the type arguments (MyFoo,MyFoo,List[X]) do not conform to the expected kinds of the type parameters (type F,type R,type B). List[X]'s type parameters do not match type B's expected parameters: class List has one type parameter, but type B has one

Why isn´t the compiler able to infer the types (although I explicitly stated them at call parameter)? And what does that class List has one type parameter, but type B has one mean? Something has one, but the other has also one, and that´s why they don´t fit together???

Answer 1

If we look to the Scala compiler, the sources could help us understanding what the problem is. I have never contributed to the Scala compiler, but I found the sources very readable and I have already investigated on that.

The class responsible for type inference is scala.tools.nsctypechecker.Infer which you can find simply by looking in the Scala compiler sources for a part of your error. You'll find out the following fragment:

  /** error if arguments not within bounds. */
    def checkBounds(pos: Position, pre: Type, owner: Symbol,
                    tparams: List[Symbol], targs: List[Type], prefix: String) = {
      //@M validate variances & bounds of targs wrt variances & bounds of tparams
      //@M TODO: better place to check this?
      //@M TODO: errors for getters & setters are reported separately
      val kindErrors = checkKindBounds(tparams, targs, pre, owner)

      if(!kindErrors.isEmpty) {
        error(pos,
          prefix + "kinds of the type arguments " + targs.mkString("(", ",", ")") +
          " do not conform to the expected kinds of the type parameters "+ tparams.mkString("(", ",", ")") + tparams.head.locationString+ "." +
          kindErrors.toList.mkString("\n", ", ", ""))
      } 

So now the point is understanding why checkKindBounds(tparams, targs, pre, owner) returns those errors. If you go down the method call chain, you will see that the checkKindBounds call another method

val errors = checkKindBounds0(tparams, targs, pre, owner, true)

You'll see the problem is connected to checking bounds of higher-kinded type, at line 5784, inside checkKindBoundsHK :

 if (!sameLength(hkargs, hkparams)) {
        if (arg == AnyClass || arg == NothingClass) (Nil, Nil, Nil) // Any and Nothing are kind-overloaded
        else {error = true; (List((arg, param)), Nil, Nil) } // shortcut: always set error, whether explainTypesOrNot
      }

The test is not passed, it appears that in my debugger:

hkargs$1 = {scala.collection.immutable.Nil$@2541}"List()"
arg$1 = {scala.tools.nsc.symtab.Symbols$ClassSymbol@2689}"class List"
param$1 = {scala.tools.nsc.symtab.Symbols$TypeSymbol@2557}"type B"
paramowner$1 = {scala.tools.nsc.symtab.Symbols$MethodSymbol@2692}"method process"
underHKParams$1 = {scala.collection.immutable.$colon$colon@2688}"List(type R)"
withHKArgs$1 = {scala.collection.immutable.Nil$@2541}"List()"
exceptionResult12 = null
hkparams$1 = {scala.collection.immutable.$colon$colon@2688}"List(type R)"

So it appears like there is one higher kinded param, type R, but there is no provided value for that.

If you actually go back to the to checkKindBounds, you see that after the snippet:

 val (arityMismatches, varianceMismatches, stricterBounds) = (
        // NOTE: *not* targ.typeSymbol, which normalizes
        checkKindBoundsHK(tparamsHO, targ.typeSymbolDirect, tparam, tparam.owner, tparam.typeParams, tparamsHO)
      )

the arityMismatches contains a tuple List, B. And now you can also see that the error message is wrong:

inferred kinds of the type arguments (MyFoo,MyFoo,List[X]) do not conform to the expected kinds of the type parameters (type F,type R,type B). List[X]'s type parameters do not match type B's expected parameters: class List has one type parameter, but type B has ZERO

In fact if you put a breakpoint at line 5859 on the following call

checkKindBoundsHK(tparamsHO, targ.typeSymbolDirect, tparam, tparam.owner, tparam.typeParams, tparamsHO)

you can see that

tparam = {scala.tools.nsc.symtab.Symbols$TypeSymbol@2472}"type B"
targ = {scala.tools.nsc.symtab.Types$UniqueTypeRef@2473}"List[X]"

Conclusion:

For some reason, when dealing with complex higher-kinded types such as yours, Scala compiler inference is limited. I don't know where it does come from, maybe you want to send a bug to the compiler team