CODEWITHSUNDEEP
scalatypesimplicitimplicit-typing
user2051561
user2051561

Reputation: 837

Using a case class apply: No member of type class A in scope for T

I am having trouble compiling a function due to implicit conversions. I have the following base case classes:

sealed abstract class Expr0[T](implicit ev: VectorSpace0[T]) extends ID {...
case class Neg0[T](e: Expr0[T])(implicit ev: VectorSpace0[T]) extends Expr0[T] { ...

Then in an object I have the following function

  def simplify[T](e: Expr0[T])(implicit ev: VectorSpace0[T]): Expr0[T] = {
    def s(expr: Expr0[T])(implicit ev: VectorSpace0[T]): Result[Boolean, T] = expr match {

      case Neg0(e)                            => 
        val re = s(e)
        val ne = Neg0.apply(re.e)
        if (re.r) new TR(ne) else FR(ne)

The code above compiles correctly with no problem. Now I want to create a function that will execute that sequence of statements in the case. So I created the following helper function: 

  def C[T](f: Expr0[T] => Expr0[T], re: Result[Boolean, T])(implicit ev: VectorSpace0[T]) = {
    val ne = f(re.e); if (re.r) new TR(ne) else FR(ne)
  }

And now I try to use so:

  def simplify[T](e: Expr0[T])(implicit ev: VectorSpace0[T]): Expr0[T] = {
    def s(expr: Expr0[T])(implicit ev: VectorSpace0[T]): Result[Boolean, T] = expr match {

      case Neg0(e)                            => 
        C(Neg0.apply, s(e))

and I get the error:

 /src/main/scala/ann/unit/Expr0.scala:412: No member of type class ann.uinit.VectorSpace in scope for T
        C(Neg0.apply, s(e))
               ^

I have been poking and prodding this for several hours but no luck. I think the problem here is with the definition of C[T] (3rd code snippet). Maybe I have to add something to the definition of the first parameter f, which is a function, so that the implicit T is correctly determined.

Can anyone advise me on how to correct or further diagnose this problem?

TIA

Upvotes: 1

Views: 93

Answers (1)

0__
0__

Reputation: 67300

It looks like a bit of a mess there with the nested implicits of the same type and name. The message might be misleading, perhaps the eta expansion in Neg0.apply is the culprit, perhaps a missing type inference for T in C[T].

I suggest you make that call explicit first and see if the error narrows down:

C[T](Neg0[T](_), s(e))

Upvotes: 1

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