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Merged
merged 4 commits into from
Mar 13, 2023
Merged

Add code tabs to the Scala 3 Dropped Features #2723

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babff4e
Add code tabs to the Scala 3 Dropped Features
Dedelweiss Mar 8, 2023
1166c55
Corrections of the tabs
Dedelweiss Mar 10, 2023
85b3546
Change spelling Scala 3 Only to Scala 2 and 3
Dedelweiss Mar 11, 2023
d919719
Update _overviews/scala3-migration/incompat-dropped-features.md
julienrf Mar 13, 2023
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119 changes: 78 additions & 41 deletions _overviews/scala3-migration/incompat-dropped-features.md
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Original file line number Diff line number Diff line change
Expand Up @@ -27,14 +27,17 @@ But the `scala.Symbol` class still exists so that each string literal can be saf

This piece of code cannot be compiled with Scala 3:

```scala
{% tabs scala-2-literals_1 %}
{% tab 'Scala 2 Only' %}
~~~scala
val values: Map[Symbol, Int] = Map('abc -> 1)

val abc = values('abc) // Migration Warning: symbol literal 'abc is no longer supported
```
val abc = values('abc) // In Scala 3, Migration Warning: symbol literal 'abc is no longer supported
~~~
{% endtab %}
{% endtabs %}

The [Scala 3 migration compilation](tooling-migration-mode.html) rewrites the code into:

{% highlight diff %}
val values: Map[Symbol, Int] = Map(Symbol("abc") -> 1)

Expand All @@ -54,19 +57,26 @@ It is recommended to use the equivalent `while ({ <body>; <cond> }) ()` that can

The following piece of code cannot be compiled with Scala 3.

```scala
do { // Migration Warning: `do <body> while <cond>` is no longer supported
{% tabs scala-2-do_while_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
do { // In Scala 3, Migration Warning: `do <body> while <cond>` is no longer supported
i += 1
} while (f(i) == 0)
```
~~~
{% endtab %}
{% endtabs %}

The [Scala 3 migration compilation](tooling-migration-mode.html) rewrites it into.

```scala
{% tabs scala-3-do_while_2 %}
{% tab 'Scala 3 Only' %}
~~~ scala
while ({ {
i += 1
} ; f(i) == 0}) ()
```
~~~
{% endtab %}
{% endtabs %}

## Auto-application

Expand All @@ -75,16 +85,19 @@ It is deprecated in Scala 2.13 and dropped in Scala 3.

The following code is invalid in Scala 3:

```scala
{% tabs scala-2-auto_application_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
object Hello {
def message(): String = "Hello"
}

println(Hello.message) // Migration Warning: method message must be called with () argument
```
println(Hello.message) // In Scala 3, Migration Warning: method message must be called with () argument
~~~
{% endtab %}
{% endtabs %}

The [Scala 3 migration compilation](tooling-migration-mode.html) rewrites it into:

{% highlight diff %}
object Hello {
def message(): String = "Hello"
Expand All @@ -103,13 +116,16 @@ Furthermore Scala 3 does not allow eta-expansion of values to nullary functions

Thus, this piece of code is invalid in Scala 3:

```scala
{% tabs scala-2-eta_expansion_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
val x = 1
val f: () => Int = x _ // Migration Warning: The syntax `<function> _` is no longer supported;
```
val f: () => Int = x _ // In Scala 3, Migration Warning: The syntax `<function> _` is no longer supported;
~~~
{% endtab %}
{% endtabs %}

The [Scala 3 migration compilation](tooling-migration-mode.html) rewrites it into:

{% highlight diff %}
val x = 1
-val f: () => Int = x _
Expand All @@ -120,14 +136,17 @@ val x = 1

The implicit `Predef.any2stringadd` conversion is deprecated in Scala 2.13 and dropped in Scala 3.

This piece of code does not compile anymore.
This piece of code does not compile anymore in Scala 3.

```scala
val str = new AnyRef + "foo" // Error: value + is not a member of Object
```
{% tabs scala-2-any2stringadd_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
val str = new AnyRef + "foo" // In Scala 3, Error: value + is not a member of Object
~~~
{% endtab %}
{% endtabs %}

The conversion to `String` must be applied explicitly, for instance with `String.valueOf`.

{% highlight diff %}
-val str = new AnyRef + "foo"
+val str = String.valueOf(new AnyRef) + "foo"
Expand All @@ -140,9 +159,11 @@ This rewrite can be applied by the `fix.scala213.Any2StringAdd` Scalafix rule in
Early initializers are deprecated in Scala 2.13 and dropped in Scala 3.
They were rarely used, and mostly to compensate for the lack of [Trait parameters]({{ site.scala3ref }}/other-new-features/trait-parameters.html) which are now supported in Scala 3.

That is why the following piece of code does not compile anymore.
That is why the following piece of code does not compile anymore in Scala 3.

```scala
{% tabs scala-2-initializer_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
trait Bar {
val name: String
val size: Int = name.size
Expand All @@ -151,7 +172,9 @@ trait Bar {
object Foo extends {
val name = "Foo"
} with Bar
```
~~~
{% endtab %}
{% endtabs %}

The Scala 3 compiler produces two error messages:

Expand All @@ -161,7 +184,6 @@ The Scala 3 compiler produces two error messages:
| ^
| `extends` must be followed by at least one parent
{% endhighlight %}

{% highlight text %}
-- [E009] Syntax Error: src/main/scala/early-initializer.scala:8:2
8 |} with Bar
Expand All @@ -171,51 +193,66 @@ The Scala 3 compiler produces two error messages:

It suggests to use trait parameters which would give us:

```scala
{% tabs scala-3-initializer_2 %}
{% tab 'Scala 3 Only' %}
~~~ scala
trait Bar(name: String) {
val size: Int = name.size
}

object Foo extends Bar("Foo")
```
~~~
{% endtab %}
{% endtabs %}

Since trait parameters are not available in Scala 2.13, it does not cross-compile.
If you need a cross-compiling solution you can use an intermediate class that carries the early initialized `val`s and `var`s as constructor parameters.

```scala
{% tabs shared-initializer_4 %}
{% tab 'Scala 2 and 3' %}
~~~ scala
abstract class BarEarlyInit(val name: String) extends Bar

object Foo extends BarEarlyInit("Foo")
```
~~~

In the case of a class, it is also possible to use a secondary constructor with a fixed value, as shown by:

```scala
~~~ scala
class Fizz private (val name: String) extends Bar {
def this() = this("Fizz")
}
```
~~~
{% endtab %}
{% endtabs %}

## Existential Type

Existential type is a [dropped feature]({{ site.scala3ref }}/dropped-features/existential-types.html), which makes the following code invalid.

```scala
def foo: List[Class[T]] forSome { type T } // Error: Existential types are no longer supported
```

{% tabs scala-2-existential_1 %}
{% tab 'Scala 2 Only' %}
~~~ scala
def foo: List[Class[T]] forSome { type T } // In Scala 3, Error: Existential types are no longer supported
~~~
{% endtab %}
{% endtabs %}

> Existential type is an experimental feature in Scala 2.13 that must be enabled explicitly either by importing `import scala.language.existentials` or by setting the `-language:existentials` compiler flag.

The proposed solution is to introduce an enclosing type that carries the dependent type:
In Scala 3, the proposed solution is to introduce an enclosing type that carries the dependent type:

```scala
{% tabs scala-3-existential_1 %}
{% tab 'Scala 3 Only' %}
~~~ scala
trait Bar {
type T
val value: List[Class[T]]
}

def foo: Bar
```
~~~
{% endtab %}
{% endtabs %}

Note that using a wildcard argument, `_` or `?`, is often simpler but is not always possible.
For instance you could replace `List[T] forSome { type T }` by `List[?]`.