Description
With the current extension methods, (as of 2badce8), you cannot chain filterKeys and mapValues on an IterableView[_, _ <: Map]. I believe the reason for this is that both take a CanBuildFrom and return a That, unlike their corresponding 2.12 methods. The filterKeys extension is even particularly egregious, as filter methods always return the same type and do not ever take a CanBuildFrom.
The methods are currently defined as:
class MapViewExtensionMethods[K, V, C <: scala.collection.Map[K, V]](
private val self: IterableView[(K, V), C])
extends AnyVal {
def mapValues[W, That](f: V => W)(
implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, W), That]): That =
self.map[(K, W), That] { case (k, v) => (k, f(v)) }
def filterKeys[That](p: K => Boolean)(
implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, V), That]): That =
self.collect[(K, V), That] { case (k, v) if p(k) => (k, v) }
}The following definitions do not use CanBuildFrom and can chain with each other:
class MapViewExtensionMethods[K, V, C <: scala.collection.Map[K, V]](
private val self: IterableView[(K, V), C])
extends AnyVal {
def mapValues[W](f: V => W): IterableView[(K, W), C] =
// the implementation of `self.map` also casts the result
self.map({ case (k, v) => (k, f(v)) }).asInstanceOf[IterableView[(K, W), C]]
def filterKeys(p: K => Boolean): IterableView[(K, V), C] =
self.filter { case (k, _) => p(k) }
}Unfortunately, mapValues has already been in previous releases, so its signature cannot be changed without a new release. However, we can still change the implementation of filterKeys (at least as of the aforementioned commit), as it has not been in any release yet. This allows the (I believe more common) use of .view.filterKeys(...).mapValues(...), though not .view.mapValues(...).filterKeys(...).