Conversion Between Option and Collection

_overviews/collections-2.13/conversion-between-option-and-the-collections.md

@@ -0,0 +1,53 @@
+---
+layout: multipage-overview
+title: Conversion Between Option and the Collections
+partof: collections-213
+overview-name: Collections
+
+num: 18
+previous-page: conversions-between-java-and-scala-collections
+
+permalink: /overviews/collections-2.13/:title.html
+---
+`Option` can be seen as a collection that has zero or exactly one element, and it provides a degree of interoperability with the collection types found in the package `scala.collection`. In particular, it implements the interface `IterableOnce`, which models the simplest form of collections: something that can be iterated over, at least once. However, `Option` does not implement the more comprehensive interface of `Iterable`. Indeed, we cannot provide a sensible implementation for the operation [`fromSpecific`](https://github.com/scala/scala/blob/6c68c2825e893bb71d6dc78465ac8c6f415cbd93/src/library/scala/collection/Iterable.scala#L173), which is supposed to create an `Option` from a collection of possibly more than one element. Starting from [Scala 2.13](https://github.com/scala/scala/pull/8038), `Option` was made an `IterableOnce` but not an `Iterable`.
+
+Hence `Option` can be used everywhere an `IterableOnce` is expected, for example, when calling `flatMap` on a collection (or inside a for-comprehension)
+
+```scala mdoc
+for {
+  a <- Set(1)
+  b <- Option(41)
+} yield (a + b)
+// : Set[Int] = Set(42)
+``` 
+
+since the operation `flatMap` on the type `Set[Int]` takes a function returning an `IterableOnce`:
+
+```
+def flatMap[B](f: Int => IterableOnce[B]): Set[B]
+```
+
+Although `Option` does not extend `Iterable`, there exists an [implicit conversion](https://github.com/scala/scala/blob/6c68c2825e893bb71d6dc78465ac8c6f415cbd93/src/library/scala/Option.scala#L19) between `Option` and `Iterable`
+
+
+```
+implicit def option2Iterable[A](xo: Option[A]): Iterable[A]
+```
+
+so although `Option[A]` is not a full collection it can be _viewed_ as one. For example,
+
+```scala mdoc
+Some(42).drop(1)
+// : Iterable[Int] = List()
+```
+
+expands to
+
+```scala mdoc
+Option.option2Iterable(Some(42)).drop(1)
+// : Iterable[Int] = List()
+```
+
+because `drop` is not defined on `Option`. A downside of the above implicit conversion is that instead of getting back an `Option[A]` we are left with an `Iterable[A]`. For this reason, `Option`’s documentation carries the following note:
+
+> Many of the methods in `Option` are duplicative with those in the `Iterable` hierarchy, but they are duplicated for a reason: the implicit conversion tends to leave one with an `Iterable` in situations where one could have retained an `Option`.

_overviews/collections-2.13/conversions-between-java-and-scala-collections.md

@@ -6,6 +6,7 @@ overview-name: Collections
 
 num: 17
 previous-page: creating-collections-from-scratch
+next-page: conversion-between-option-and-the-collections
 
 languages: [ru]
 permalink: /overviews/collections-2.13/:title.html