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When the manifest is unavailable

def bar[T: Manifest](a: Array[T]) = Array.ofDim[T](3) class Foo bar(Array(new Foo)) //Array[Foo] = Array(null, null, null)

Manifests seem to exist implicitly for arbitrary types, as shown above.

Since we have context-related, this means that there will be some types for which there is no implicit manifest - what is it?

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2 answers

The manifest should be “transferred” from the place where the specific type of the latter appears in the source code, up to the type parameters in the place where it is necessary.

But everything has a manifest.

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I'm not sure your conclusion is correct. I have not seen types for which there is no manifest, but I have seen situations where the inferencer type does not seem to be able to provide it.

In particular, when embedding such output situations:

 scala> def bar[T: Manifest](a: Array[T]) = Array.ofDim[T](3) bar: [T](a: Array[T])(implicit evidence$1: Manifest[T])Array[T] scala> def bar2[T](a: Array[T]) = bar(a) <console>:8: error: No Manifest available for T. def bar2[T](a: Array[T]) = bar(a) ^

It seems that if the manifest did not “pass”, it is not available at a lower level, so we can say

 scala> def bar2[T: Manifest](a: Array[T]) = bar(a) bar2: [T](a: Array[T])(implicit evidence$1: Manifest[T])Array[T]

or

 scala> def bar2[T](a: Array[T])(implicit m: Manifest[T]) = bar(a) bar2: [T](a: Array[T])(implicit m: Manifest[T])Array[T]

However, why I do not know such behavior.

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Source: https://habr.com/ru/post/901862/

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