Merge pull request #3034 from scalacenter/scala3book/fun-eta-expansion

improve fun-eta-expanion.md wording

Author: sjrd

_overviews/scala3-book/fun-eta-expansion.md

@@ -1,22 +1,22 @@
 ---
-title: Eta Expansion
+title: Eta-Expansion
 type: section
-description: This page discusses Eta Expansion, the Scala technology that automatically and transparently converts methods into functions.
+description: This page discusses Eta-Expansion, the Scala technology that automatically and transparently converts methods into functions.
 languages: [ru, zh-cn]
 num: 31
 previous-page: fun-function-variables
 next-page: fun-hofs
 ---
 
 
-When you look at the Scaladoc for the `map` method on Scala collections classes, you see that it’s defined to accept a _function_:
+When you look at the Scaladoc for the `map` method on Scala collections classes, you see that it’s defined to accept a _function_ value:
 
 {% tabs fun_1 %}
 {% tab 'Scala 2 and 3' for=fun_1 %}
 
 ```scala
-def map[B](f: (A) => B): List[B]
-           -----------
+def map[B](f: A => B): List[B]
+//            ^^^^^^ function type from `A` to `B`
 ```
 
 {% endtab %}
@@ -26,7 +26,7 @@ Indeed, the Scaladoc clearly states, “`f` is the _function_ to apply to each e
 But despite that, somehow you can pass a _method_ into `map`, and it still works:
 
 {% tabs fun_2 %}
-{% tab 'Scala 2 and 3' for=fun_2 %}
+{% tab 'Scala 2 and 3' %}
 
 ```scala
 def times10(i: Int) = i * 10   // a method
@@ -36,80 +36,96 @@ List(1, 2, 3).map(times10)     // List(10,20,30)
 {% endtab %}
 {% endtabs %}
 
-Have you ever wondered how this works---how you can pass a _method_ into `map`, which expects a _function_?
-
-The technology behind this is known as _Eta Expansion_.
+Why does this work? The process behind this is known as _eta-expansion_.
 It converts an expression of _method type_ to an equivalent expression of _function type_, and it does so seamlessly and quietly.
 
 ## The differences between methods and functions
 
-{% comment %}
-NOTE: I got the following “method” definition from this page (https://dotty.epfl.ch/docs/reference/changed-features/eta-expansion-spec.html), but I’m not sure it’s 100% accurate now that methods can exist outside of classes/traits/objects.
-I’ve made a few changes to that description that I hope are more accurate and up to date.
-{% endcomment %}
-
-Historically, _methods_ have been a part of the definition of a class, although in Scala 3 you can now have methods outside of classes, such as [Toplevel definitions][toplevel] and [extension methods][extension].
+The key difference between methods and functions is that _a function is an object_, i.e. it is an instance of a class, and in turn has its own methods (e.g. try `f.apply` on a function `f`).
 
-Unlike methods, _functions_ are complete objects themselves, making them first-class entities.
+_Methods_ are not values that can be passed around, i.e. they can only be called via method application (e.g. `foo(arg1, arg2, ...)`). Methods can be _converted_ to a value by creating a function value that will call the method when supplied with the required arguments. This is known as eta-expansion.
 
-Their syntax is also different.
-This example shows how to define a method and a function that perform the same task, determining if the given integer is even:
+More concretely: with automatic eta-expansion, the compiler automatically converts any _method reference_, without supplied arguments, to an equivalent _anonymous function_ that will call the method. For example, the reference to `times10` in the code above gets rewritten to `x => times10(x)`, as seen here:
 
-{% tabs fun_3 %}
-{% tab 'Scala 2 and 3' for=fun_3 %}
+{% tabs fun_2_expanded %}
+{% tab 'Scala 2 and 3' %}
 
 ```scala
-def isEvenMethod(i: Int) = i % 2 == 0         // a method
-val isEvenFunction = (i: Int) => i % 2 == 0   // a function
+def times10(i: Int) = i * 10
+List(1, 2, 3).map(x => times10(x)) // eta expansion of `.map(times10)`
 ```
 
 {% endtab %}
 {% endtabs %}
 
-The function truly is an object, so you can use it just like any other variable, such as putting it in a list:
+> For the curious, the term eta-expansion has its origins in the [Lambda Calculus](https://en.wikipedia.org/wiki/Lambda_calculus).
+
+## When does eta-expansion happen?
 
-{% tabs fun_4 %}
-{% tab 'Scala 2 and 3' for=fun_4 %}
+Automatic eta-expansion is a desugaring that is context-dependent (i.e. the expansion conditionally activates, depending on the surrounding code of the method reference.)
+
+{% tabs fun_5 class=tabs-scala-version %}
+{% tab 'Scala 2' %}
 
+In Scala 2 eta-expansion only occurs automatically when the expected type is a function type.
+For example, the following will fail:
 ```scala
-val functions = List(isEvenFunction)
+def isLessThan(x: Int, y: Int): Boolean = x < y
+
+val methods = List(isLessThan)
+//                 ^^^^^^^^^^
+// error: missing argument list for method isLessThan
+// Unapplied methods are only converted to functions when a function type is expected.
+// You can make this conversion explicit by writing `isLessThan _` or `isLessThan(_,_)` instead of `isLessThan`.
 ```
 
+See [below](#manual-eta-expansion) for how to solve this issue with manual eta-expansion.
 {% endtab %}
-{% endtabs %}
 
-{% tabs fun_5 class=tabs-scala-version %}
-{% tab 'Scala 2' for=fun_5 %}
+{% tab 'Scala 3' %}
+
+New to Scala 3, method references can be used everywhere as a value, they will be automatically converted to a function object with a matching type. e.g.
 
 ```scala
-// this example shows the Scala 2 error message
-val methods = List(isEvenMethod)
-                   ^
-error: missing argument list for method isEvenMethod
-Unapplied methods are only converted to functions when a function type is expected.
-You can make this conversion explicit by writing `isEvenMethod _` or `isEvenMethod(_)` instead of `isEvenMethod`.
-```
+def isLessThan(x: Int, y: Int): Boolean = x < y
 
-Conversely, a method technically isn’t an object, so in Scala 2 you couldn’t put a method in a `List`, at least not directly, as shown in this example:
+val methods = List(isLessThan)       // works
+```
 
 {% endtab %}
+{% endtabs %}
+
+## Manual eta-expansion
 
-{% tab 'Scala 3' for=fun_5 %}
+You can always manually eta-expand a method to a function value, here are some examples how:
+
+{% tabs fun_6 class=tabs-scala-version %}
+{% tab 'Scala 2' %}
 
 ```scala
-val functions = List(isEvenFunction)   // works
-val methods = List(isEvenMethod)       // works
+val methodsA = List(isLessThan _)               // way 1: expand all parameters
+val methodsB = List(isLessThan(_, _))           // way 2: wildcard application
+val methodsC = List((x, y) => isLessThan(x, y)) // way 3: anonymous function
 ```
 
-The important part for Scala 3 is that the Eta Expansion technology is improved, so now when you attempt to use a method as a variable, it just works---you don’t have to handle the manual conversion yourself.
+{% endtab %}
+
+{% tab 'Scala 3' %}
+
+```scala
+val methodsA = List(isLessThan(_, _))           // way 1: wildcard application
+val methodsB = List((x, y) => isLessThan(x, y)) // way 2: anonymous function
+```
 
 {% endtab %}
 {% endtabs %}
 
+## Summary
+
 For the purpose of this introductory book, the important things to know are:
 
-- Eta Expansion is the Scala technology that lets you use methods just like functions
-- The technology has been improved in Scala 3 to be almost completely seamless
+- eta-expansion is a helpful desugaring that lets you use methods just like functions,
+- the automatic eta-expansion been improved in Scala 3 to be almost completely seamless.
 
 For more details on how this works, see the [Eta Expansion page][eta_expansion] in the Reference documentation.