Kotlin / Bob / DigDeeper: Add a pattern matching approach using when-expression (#629)

* Add approach to introduction

* Add snippet.txt and content.md

* Add entry in config.json

* Fix typo in config.json

* Reduce snippet length to 8 lines

* Use proposed one-liner in function isYelling

Author: micha-b

exercises/practice/bob/.approaches/config.json

@@ -22,6 +22,15 @@
       "authors": [
         "bobahop"
       ]
+    },
+    {
+      "uuid": "7aebc1df-c23a-42c9-90c4-f022e40d67e9",
+      "slug": "when-expression",
+      "title":"when expression",
+      "blurb": "Use a when expression to return the answer.",
+      "authors": [
+        "micha-b"
+      ]
     }
   ]
 }

exercises/practice/bob/.approaches/introduction.md

@@ -56,6 +56,23 @@ object Bob {
 
 For more information, check the [Answer `List` approach][approach-answer-list].
 
+## Approach: `when` expression
+
+```kotlin
+object Bob {
+    fun hey(statement: String): String =
+        when {
+            statement.isQuestion() && statement.isYelling() -> "Calm down, I know what I'm doing!"
+            statement.isQuestion() -> "Sure."
+            statement.isYelling() -> "Whoa, chill out!"
+            statement.isSilence() -> "Fine. Be that way!"
+            else -> "Whatever."
+        }
+}
+```
+
+For more information, check the [`when` expression approach][approach-when]
+
 ## Which approach to use?
 
 The choice between `if` expressions and answers `List` can be made by perceived readability.
@@ -65,3 +82,4 @@ The choice between `if` expressions and answers `List` can be made by perceived
 [dry]: https://en.wikipedia.org/wiki/Don%27t_repeat_yourself
 [approach-if]: https://exercism.org/tracks/kotlin/exercises/bob/approaches/if-expressions
 [approach-answer-list]: https://exercism.org/tracks/kotlin/exercises/bob/approaches/answer-list
+[approach-when]: https://exercism.org/tracks/kotlin/exercises/bob/approaches/when-expression

exercises/practice/bob/.approaches/when-expression/content.md

@@ -0,0 +1,54 @@
+# `when` expressions
+
+```kotlin
+object Bob {
+    fun hey(statement: String): String =
+        when {
+            statement.isQuestion() && statement.isYelling() -> "Calm down, I know what I'm doing!"
+            statement.isQuestion() -> "Sure."
+            statement.isYelling() -> "Whoa, chill out!"
+            statement.isSilence() -> "Fine. Be that way!"
+            else -> "Whatever."
+        }
+
+    private fun String.isSilence(): Boolean = this.isBlank()
+    private fun String.isQuestion(): Boolean = this.trim().endsWith('?')
+    private fun String.isYelling(): Boolean = any(Char::isLetter) && toUpperCase() == this
+}
+```
+
+In this approach you have a `when` expression containing different so-called branches that can be matched using the corresponding patterns. 
+As soon as one of these patterns on the left side of the arrow (`->`) is matched ...
+
+1. the value on the right side
+2. the block on the right side is executed and the value retuned from the block 
+
+... is returned from the `when` expression.
+
+Only one branch is matched in one execution of the `when` expression. The branches are matched from top to bottom and the first branch in which the condition evaluates to `true` is selected. 
+If none of the given conditions matches the `else` branch is selected and returned.
+
+~~~~exercism/caution
+Depending on what patterns are on the left side of your branches the order of branches is important to the correct execution of the `when` expression since the first matching branch is selected.
+~~~~
+
+An [object declaration][object] is used to define `Bob` as essentially a [singleton][singleton] object instantiation of the class.
+This is sufficient, since there is no object state that needs to change with each call of the `hey` method.
+
+Inside this object there are some `private` [extension methods as members][extension-members] of the `object`. This adds these methods to the `String` data type for `private` usage of these methods in this `object`. This allows calling the methods directly on the `String` instead of passing the `String` to the method.
+(More about extension methods in general [here][extension-general])
+
+These extension methods check for:
+
+1. `isSilence()`: a blank string
+2. `isQuestion()`: a string with the last non-whitespace-character being a question mark
+3. `isYelling()`: a string with all letters in uppercase
+
+When combining these methods as in the `when` expression above you can map all the cases required by the exercise.
+
+
+[when]: https://kotlinlang.org/docs/control-flow.html#when-expression
+[object]: https://kotlinlang.org/docs/object-declarations.html#object-declarations-overview
+[singleton]: https://en.wikipedia.org/wiki/Singleton_pattern
+[extension-members]: https://kotlinlang.org/docs/extensions.html#declaring-extensions-as-members
+[extension-general]: https://kotlinlang.org/docs/extensions.html

exercises/practice/bob/.approaches/when-expression/snippet.txt

@@ -0,0 +1,8 @@
+fun hey(statement: String): String =
+    when {
+        statement.isQuestion() && statement.isYelling() -> "Calm down, I know what I'm doing!"
+        statement.isQuestion() -> "Sure."
+        statement.isYelling() -> "Whoa, chill out!"
+        statement.isSilence() -> "Fine. Be that way!"
+        else -> "Whatever."
+    }