Add Elm Maybe.andThen page.

src/Pages/Software/elm/en/main.md

@@ -1,5 +1,6 @@
 ### Elm
 
 * [Formatting](./formatting)
+* [Maybe.andThen](./maybeandthen)
 * [Maybe.map](./maybemap)
 * [Don't Repeat Yourself](./dry)

src/Pages/Software/elm/en/maybeandthen.md

@@ -0,0 +1,89 @@
+Elm - Maybe AndThen
+===================
+
+Given are these functions, **hasPrecedingReleasedVersions** and **getSelectedItemIndex**, need to be optimized.
+
+
+```elm
+
+hasPrecedingReleasedVersions =
+    case getSelectedItemIndex data.versionData of
+        Just index ->
+            Array.slice 0 index data.versionData.allItems
+                |> Array.toList
+                |> List.any
+                    (\item ->
+                        case item of
+                            ReleasedVersion _ ->
+                                True
+                            _ ->
+                                False
+                    )
+        Nothing ->
+            False
+```
+
+Currently, **getSelectedItemIndex** is defined as such:
+
+```elm
+
+getSelectedItemIndex : Model id -> Maybe Int
+getSelectedItemIndex model =
+    case getSelectedItem model of
+        Just selected ->
+            Array.toIndexedList model.allItems
+                |> List.filter (\( _, item ) -> item == selected)
+                |> List.head
+                |> Maybe.map Tuple.first
+
+        Nothing ->
+            Nothing
+
+
+```
+
+But using [Maybe.andThen](https://package.elm-lang.org/packages/elm/core/latest/Maybe#andThen) will allow to get rid of the **Nothing -> Nothing** here.
+Also the **List.filter** and **List.head** combination can be shortened with a **List.find**.
+
+```elm
+
+getSelectedItemIndex : Model id -> Maybe Int
+getSelectedItemIndex model =
+    getSelectedItem model
+        |> Maybe.andThen
+            (\selected ->
+                Array.toIndexedList model.allItems
+                    |> List.find (\( _, item ) -> item == selected)                
+                    |> Maybe.map Tuple.first
+            )
+
+
+```
+
+Finally, define a custom funcion to determine the boolean:
+
+```elm
+
+itemIsReleased : VersionData id -> Bool
+itemIsReleased versionData =
+    case versionData of
+        WorkingVersion _ ->
+            False
+        ReleasedVersion
+            True
+
+```
+
+The original function can now be changed to:
+
+```elm
+
+hasPrecedingReleasedVersions =
+    case getSelectedItemIndex data.versionData of
+        Just index ->
+            Array.slice 0 index data.versionData.allItems
+                |> Array.toList
+                |> List.any itemIsReleased
+        Nothing ->
+            False
+```

src/Pages/Software/elm/nl/main.md

@@ -1,5 +1,6 @@
 ### Elm
 
 * [Formatting](./formatting)
+* [Maybe.andThen](./maybeandthen)
 * [Maybe.map](./maybemap)
 * [Don't Repeat Yourself](./dry)

src/Pages/Software/elm/nl/maybeandthen.md

@@ -0,0 +1,89 @@
+Elm - Maybe AndThen
+===================
+
+Gegeven zijn deze functies: **hasPrecedingReleasedVersions** en **getSelectedItemIndex**. Deze moeten aangepast worden.
+
+
+```elm
+
+hasPrecedingReleasedVersions =
+    case getSelectedItemIndex data.versionData of
+        Just index ->
+            Array.slice 0 index data.versionData.allItems
+                |> Array.toList
+                |> List.any
+                    (\item ->
+                        case item of
+                            ReleasedVersion _ ->
+                                True
+                            _ ->
+                                False
+                    )
+        Nothing ->
+            False
+```
+
+Momenteel is **getSelectedItemIndex** zo gedefineerd:
+
+```elm
+
+getSelectedItemIndex : Model id -> Maybe Int
+getSelectedItemIndex model =
+    case getSelectedItem model of
+        Just selected ->
+            Array.toIndexedList model.allItems
+                |> List.filter (\( _, item ) -> item == selected)
+                |> List.head
+                |> Maybe.map Tuple.first
+
+        Nothing ->
+            Nothing
+
+
+```
+
+Door een [Maybe.andThen](https://package.elm-lang.org/packages/elm/core/latest/Maybe#andThen) te gebruiken, kunnen we de **Nothing -> Nothing** weglaten.
+Ook de **List.filter** en **List.head** combinatie kan verkort werden met een **List.find**.
+
+```elm
+
+getSelectedItemIndex : Model id -> Maybe Int
+getSelectedItemIndex model =
+    getSelectedItem model
+        |> Maybe.andThen
+            (\selected ->
+                Array.toIndexedList model.allItems
+                    |> List.find (\( _, item ) -> item == selected)                
+                    |> Maybe.map Tuple.first
+            )
+
+
+```
+
+Ten slotte, defineer een nieuwe functie om de boolean te achterhalen:
+
+```elm
+
+itemIsReleased : VersionData id -> Bool
+itemIsReleased versionData =
+    case versionData of
+        WorkingVersion _ ->
+            False
+        ReleasedVersion
+            True
+
+```
+
+De originele functie kan nu veranderd worden naar:
+
+```elm
+
+hasPrecedingReleasedVersions =
+    case getSelectedItemIndex data.versionData of
+        Just index ->
+            Array.slice 0 index data.versionData.allItems
+                |> Array.toList
+                |> List.any itemIsReleased
+        Nothing ->
+            False
+```

src/Routers.js

@@ -8,7 +8,7 @@ export const elmRoutes = languages.map(lang => {
     const main = <Route key={`${lang}-elm-main`} exact path={`/${lang}/software/elm/`} element={<ElmPage mdPath={`Software/elm/${lang}/main.md`} />} />
 
     // Pages within Elm.
-    const md = ['formatting', 'maybemap', 'dry'];
+    const md = ['dry', 'formatting', 'maybeandthen', 'maybemap'];
     const entries = md.map(entry => {
         const path = `/${lang}/software/elm/${entry}`;
         const mdPath = `Software/elm/${lang}/${entry}.md`;