Scala: error: missing parameter type

Question

I'm trying to write some library functions to enhance the basic collections. Most of it has gone smoothly, but I'm having an issue with this one.

class EnhancedGenTraversableLike[A, Repr <: GenTraversable[A]](self: GenTraversableLike[A, Repr]) {
  def mapValuesStrict[T, U, R, That](f: U => R)(implicit ev: A <:< (T, U), bf: CanBuildFrom[Repr, (T, R), That]) = {
    val b = bf(self.asInstanceOf[Repr])
    b.sizeHint(self.size)
    for ((k: T, v: U) <- self) b += k -> f(v)
    b.result
  }
}
implicit def enhanceGenTraversableLike[A, Repr <: GenTraversable[A]](self: GenTraversableLike[A, Repr]) = new EnhancedGenTraversableLike[A, Repr](self)

Here's what happens when I go to use it:

scala> List((1,2),(2,3),(3,4),(2,5)).mapValuesStrict((_:Int).toString)
res0: List[(Int, java.lang.String)] = List((1,2), (2,3), (3,4), (2,5))

scala> List((1,2),(2,3),(3,4),(2,5)).mapValuesStrict(x => x.toString)
<console>:13: error: missing parameter type
              List((1,2),(2,3),(3,4),(2,5)).mapValuesStrict(x => x.toString)
                                                            ^

So Scala is unable to determine the type of x.

This answer indicates Scala doesn't use one parameter to resolve another, but that separate parameter lists can fix the problem. In my case, however, this isn't so easy since the type information is found in the implicit parameters.

Is there a way around this so that I don't have to specify the type every time I call the method?

Update: Based on Owen's advice, I ended up creating a enriched class specific to a traversable of pairs:

class EnrichedPairGenTraversableLike[T, U, Repr <: GenTraversable[(T, U)]](self: GenTraversableLike[(T, U), Repr]) {
  def mapValuesStrict[R, That](f: U => R)(implicit bf: CanBuildFrom[Repr, (T, R), That]) = {
    val b = bf(self.asInstanceOf[Repr])
    b.sizeHint(self.size)
    for ((k: T, v: U) <- self) b += k -> f(v)
    b.result
  }
}
implicit def enrichPairGenTraversableLike[T, U, Repr <: GenTraversable[(T, U)]](self: GenTraversableLike[(T, U), Repr]) = new EnrichedPairGenTraversableLike(self)

Owen Owen · Accepted Answer · 2012-02-01T03:26:43

Yes, there is. Let me give a simpler example. I hope this will also work with your more complicated use case.

say we have

trait Foo[A]

class Bar {
    def methWithImplicits[A,B](f: A => B)(implicit foo: Foo[A]) = null
}

implicit def fooInt: Foo[Int] = null

Now this has exactly the problem you describe, since

(new Bar).methWithImplicits(x => x)

gives "missing parameter type".

So what we would like to do, is to move the implicit parameter "behind" the explicitly supplied function, so that Scala sees the implicit first. Well, one way we can do this is to add an extra layer of indirection:

class Bar {
    def methWithImplicits2[A](implicit foo: Foo[A]) = new {
        def apply[B](f: A => B) = null
    }
}

(new Bar).methWithImplicits2.apply(x => x)

This works, though the syntax is not so pretty. One way you might consider prettying the syntax is to look at your current design and see if you can sneak the implicit into any of the "earlier" stages. For example, since the mapValuesStrict method is only meaningful once the implicit has been supplied, you might make the implicit a property of the object instead of passed to the method.

But if that is not convenient in your design, you could use an extra implicit conversion to sneak it back. This is what we would like to do:

implicit def addFoo[A](bar: Bar)(implicit foo: Foo[A]) = new {
    def methWithImplicits3[B](f: A => B) = null
}

But unfortunately there is what I suspect is a bug in Scala that causes it to search for an implicit value that is too polymorphic, causing it to complain:

could not find implicit value for parameter foo: test.Foo[A]

This only happens when using implicit conversions, which is why I think it is a bug. So, we can take it back even further: (and, requiring -Xexperimental for dependent method types):

trait FooWrapper {
    type AA
    val foo: Foo[AA]
}

implicit def wrapFoo[A](implicit theFoo: Foo[A]) = new FooWrapper {
    type AA = A
    val foo = theFoo
}

implicit def addFoo(bar: Bar)(implicit foo: FooWrapper) = new {
    def methWithImplicits3[B](f: foo.AA => B) = null
}

And now

(new Bar).methWithImplicits3(x => x)

works perfectly ;)

Update

In your particular case, I think your best bet is to work the implicit into enhanceGenTraversable, though, alas, the same hack is required to work around the possible bug:

// Notice `ev` is now a field of the class
class EnhancedGenTraversableLike[A, Repr <: GenTraversable[A], T, U]
    (self: GenTraversableLike[A, Repr], ev: A <:< (T, U))
{
    def mapValuesStrict[R, That](f: U => R)(implicit bf: CanBuildFrom[Repr, (T, R), That]) = {
        val b = bf(self.asInstanceOf[Repr])
        b.sizeHint(self.size)
        for ((k: T, v: U) <- self) b += k -> f(v)
        b.result
    }
}

// The Hack
trait WrappedPairBound[A] {
    type TT
    type UU
    val bound: A <:< (TT, UU)
}

implicit def wrapPairBound[A,T,U](implicit ev: A <:< (T,U)) = new WrappedPairBound[A] {
    type TT = T
    type UU = U
    val bound = ev
}

// Take the implicit here
implicit def enhanceGenTraversableLike[A, Repr <: GenTraversable[A]]
        (self: GenTraversableLike[A, Repr])(implicit ev: WrappedPairBound[A]) =
    new EnhancedGenTraversableLike[A, Repr, ev.TT, ev.UU](self, ev.bound)

Scala: error: missing parameter type

1 Answers