Implicit conversion from array to list
Solution 1
implicit def arrayToList[A](a: Array[A]) = a.toList
Seems to work as expected. My guess is that there's already a genericArrayOps
in Predef
that has the signature for implicit conversion from Array[T] -> ArrayOps[T]
, ArrayOps[T]
has a method .toList(): List[T]
. Your method has the signature Array[T] -> List[T]
, which also makes the method .toList[T]
available. The body is asking for an implicit conversion with that signature. The compiler doesn't know that using arrayToList
will make that method go into an infinite loop, hence the ambiguity error. However, type-inferencing the return type seems to be able to work around this problem. Implicits resolution don't jive very well with type-inference it seems.
Also worth noting is that since there is already an implicit conversion that will get you what you want by default, there's no need to have an implicit conversion from Array
to List
.
Solution 2
There's no need for a Manifest
or ClassManifest
when converting from arrays, as Array
is the one "collection" type that gets special treatment on the JVM and doesn't undergo type erasure.
This means that you can go with the obvious/trivial approach, no trickery required:
implicit def arrayToList[A](arr: Array[A]) = arr.toList
One question though... Given that .toList
is already such a trivial operation, what do you gain by making it implicit?
Miguel Fernandez
Updated on February 14, 2020Comments
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Miguel Fernandez about 4 years
How do I write an implicit conversion from
Array[_]
toList[_]
type? I tried the following but it doesn't seem to work.scala> implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList <console>:5: error: type mismatch; found : Array[A] required: ?{val toList: ?} Note that implicit conversions are not applicable because they are ambiguous: both method arrayToList in object $iw of type [A](a: Array[A])(implicit evidence$1: ClassManifest[A])List[A] and method genericArrayOps in object Predef of type [T](xs: Array[T])scala.collection.mutable.ArrayOps[T] are possible conversion functions from Array[A] to ?{val toList: ?} implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList ^
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Kevin Wright about 13 yearsThe naming convention
XxxOps
always suggests that you're seeing pimped extension methods for the typeXxx
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Raman almost 10 yearsThis was useful to me. I have an array that might be null (returned from a JDBC call, so
Option
is not an option), and using your explanation/workaround I was able to create an implicit to handle it:implicit def nullableArrayToList[T](array: Array[T]) Option(array).fold(List.empty[T]) { _.toList }