问题
User Régis Jean-Gilles gracefully answered my previous question where I was struggling with CanBuildFrom and enrichment functions (aka "pimp my library" or "enrich my library"):
Creating an implicit function that wraps map() in Scala with the right type: Not for the faint at heart
But this time I've got an even more complicated issue.
I have a function to implement variations on intersectWith, for intersecting collections by their keys. I've managed to make them work like proper collection functions:
implicit class IntersectUnionWithPimp[K, A, Repr](a: GenTraversableLike[(K, A), Repr]) {
/**
* Intersect two collections by their keys. This is identical to
* `intersectWith` except that the combiner function is passed the
* key as well as the two items to combine.
*
* @param b Other collection to intersect with.
* @param combine Function to combine values from the two collections.
*/
def intersectWithKey[B, R, That](b: GenTraversable[(K, B)])(
combine: (K, A, B) => R)(
implicit bf: CanBuildFrom[Repr, (K, R), That]): That = {
...
}
/**
* Intersect two collections by their keys. Keep the ordering of
* objects in the first collection. Use a combiner function to
* combine items in common. If either item is a multi-map, then
* for a key seen `n` times in the first collection and `m`
* times in the second collection, it will occur `n * m` times
* in the resulting collection, including all the possible
* combinations of pairs of identical keys in the two collections.
*
* @param b Other collection to intersect with.
* @param combine Function to combine values from the two collections.
*/
def intersectWith[B, R, That](b: GenTraversable[(K, B)])(
combine: (A, B) => R)(
implicit bf: CanBuildFrom[Repr, (K, R), That]): That =
a.intersectWithKey(b){ case (_, x, y) => combine(x, y) }(bf)
}
Now, I currently also have non-CanBuildFrom versions of intersectBy and friends, and these I can't get working as CanBuildFrom versions.
implicit class IntersectUnionByPimp[A](a: Traversable[A]) {
/**
* Intersect two collections by their keys, with separate key-selection
* functions for the two collections. This is identical to
* `intersectBy` except that each collection has its own key-selection
* function. This allows the types of the two collections to be
* distinct, with no common parent.
*
* @param b Other collection to intersect with.
* @param key1fn Function to select the comparison key for the first
* collection.
* @param key1fn Function to select the comparison key for the first
* collection.
* @param combine Function to combine values from the two collections.
*/
def intersectBy2[K, B, R](b: Traversable[B])(key1fn: A => K
)(key2fn: B => K)(combine: (A, B) => R): Traversable[R] = {
val keyed_a = a.map { x => (key1fn(x), x) }
val keyed_b = b.map { x => (key2fn(x), x) }
keyed_a.intersectWith(keyed_b)(combine).map(_._2)
}
/**
* Intersect two collections by their keys. Keep the ordering of
* objects in the first collection. Use a combiner function to
* combine items in common. If either item is a multi-map, then
* for a key seen `n` times in the first collection and `m`
* times in the second collection, it will occur `n * m` times
* in the resulting collection, including all the possible
* combinations of pairs of identical keys in the two collections.
*
* @param b Other collection to intersect with.
* @param keyfn Function to select the comparison key.
* @param combine Function to combine values from the two collections.
*/
def intersectBy[K, B >: A](b: Traversable[B])(keyfn: B => K)(
combine: (A, B) => B): Traversable[B] = {
val keyed_a = a.map { x => (keyfn(x), x) }
val keyed_b = b.map { x => (keyfn(x), x) }
keyed_a.intersectWith(keyed_b)(combine).map(_._2)
}
}
The best version so far I can come up with is this:
implicit class IntersectUnionByPimp[A, Repr](a: GenTraversableLike[A, Repr]) {
def intersectBy2[K, B, R, That](b: Traversable[B])(key1fn: A => K)(
key2fn: B => K)(combine: (A, B) => R)(
implicit bf: CanBuildFrom[Repr, R, That]): That = {
// FIXME! How to make this work while calling `map`?
// val keyed_a = a.map { x => (key1fn(x), x) }
val keyed_a = mutable.Buffer[(K, A)]()
a.foreach { x => keyed_a += ((key1fn(x), x)) }
val keyed_b = b.map { x => (key2fn(x), x) }
keyed_a.intersectWith(keyed_b)(combine).map(_._2)
}
def intersectBy[K, B >: A, That](b: Traversable[B])(keyfn: B => K)(
combine: (A, B) => B)(
implicit bf: CanBuildFrom[Repr, B, That]): That = {
// FIXME! How to make this work while calling `map`?
// val keyed_a = a.map { x => (keyfn(x), x) }
val keyed_a = mutable.Buffer[(K, A)]()
a.foreach { x => keyed_a += ((keyfn(x), x)) }
val keyed_b = b.map { x => (keyfn(x), x) }
keyed_a.intersectWith(keyed_b)(combine).map(_._2)
}
First, I don't see why I need to rewrite the call to map that produces keyed_a with a mutable Buffer; seems like there must be a better way. But I still get the same sort of error on the bottom line:
[error] /Users/benwing/devel/textgrounder/src/main/scala/opennlp/textgrounder/util/collection.scala:1018: type mismatch;
[error] found : scala.collection.mutable.Buffer[R]
[error] required: That
[error] Note: implicit method bufferToIndexedSeq is not applicable here because it comes after the application point and it lacks an explicit result type
[error] keyed_a.intersectWith(keyed_b)(combine).map(_._2)
[error] ^
So my questions are:
- How to call
mapon a GenTraversableLike? - How to make the call to
intersectWithwork correctly? I know I have to somehow pass in aCanBuildFrombased on the one I received, and I know aboutmapResulton Builders, but I'm not sure what to do here, or if this is even possible.
An example of intersectBy, which intersects lists of floating-point numbers treating two numbers the same if their integral part is the same, and computing the absolute difference:
scala> List(4.5,2.3,4.2).intersectBy(List(4.6,4.8))(_.toInt){ case (a,b) => (a - b).abs }
res2: Traversable[Double] = List(0.09999999999999964, 0.2999999999999998, 0.39999999999999947, 0.5999999999999996)
(except that the returned type should be List[Double])
Thanks for any help.
回答1:
OK, turns out I need to create a builder to return the items, instead of trying to return them directly. The following works:
implicit class IntersectUnionByPimp[A, Repr](a: GenTraversableLike[A, Repr]) {
/**
* Intersect two collections by their keys, with separate key-selection
* functions for the two collections. This is identical to
* `intersectBy` except that each collection has its own key-selection
* function. This allows the types of the two collections to be
* distinct, with no common parent.
*
* @param b Other collection to intersect with.
* @param key1fn Function to select the comparison key for the first
* collection.
* @param key2fn Function to select the comparison key for the first
* collection.
* @param combine Function to combine values from the two collections.
*/
def intersectBy2[K, B, R, That](b: GenTraversable[B])(key1fn: A => K)(
key2fn: B => K)(combine: (A, B) => R)(
implicit bf: CanBuildFrom[Repr, R, That]): That = {
// It appears we can't call map() on `a`.
val keyed_a = mutable.Buffer[(K, A)]()
a.foreach { x => keyed_a += ((key1fn(x), x)) }
val keyed_b = b.map { x => (key2fn(x), x) }
// Nor can we return the value of map() here. Need to use a builder
// instead.
val bu = bf()
for ((_, r) <- keyed_a.intersectWith(keyed_b)(combine))
bu += r
bu.result
}
/**
* Intersect two collections by their keys. Keep the ordering of
* objects in the first collection. Use a combiner function to
* combine items in common. If either item is a multi-map, then
* for a key seen `n` times in the first collection and `m`
* times in the second collection, it will occur `n * m` times
* in the resulting collection, including all the possible
* combinations of pairs of identical keys in the two collections.
*
* @param b Other collection to intersect with.
* @param keyfn Function to select the comparison key.
* @param combine Function to combine values from the two collections.
*/
def intersectBy[K, B >: A, That](b: GenTraversable[B])(keyfn: B => K)(
combine: (A, B) => B)(
implicit bf: CanBuildFrom[Repr, B, That]): That = {
val keyed_a = mutable.Buffer[(K, A)]()
a.foreach { x => keyed_a += ((keyfn(x), x)) }
val keyed_b = b.map { x => (keyfn(x), x) }
val bu = bf()
for ((_, r) <- keyed_a.intersectWith(keyed_b)(combine))
bu += r
bu.result
}
}
I'm not completely sure why just calling map on a GenTraversableLike doesn't seem to work, but so be it.
来源：https://stackoverflow.com/questions/20734629/another-scala-canbuildfrom-issue-a-collection-enrichment-operator-that-wraps-an