Traversals (#29)

README.md

@@ -234,11 +234,51 @@ A Optional is a weaker Lens and a weaker Prism.
  modifyAddressRegion address = Optional.modify addressRegionOptional modifyRegion address
 ```
 
+## Traversal
+
+A Traversal allows you to modify many elements at once.
+
+```elm
+ type alias Traversal a b =
+ (b -> b) -> a -> a
+```
+
+(`Traversal a b` is just an alias for a function that applies
+a transformation over `b` elements of a larger `a` structure.)
+
+###### Example
+
+```elm
+ firstNameLens : Lens Friend String
+ firstNameLens =
+ Lens .firstName (\b a -> { a | firstName = b })
+
+ bestFriendsTraversal : Traversal (List Friend) Friend
+ bestFriendsTraversal =
+ Traversal.some
+ Traversal.list
+ (\friend -> friend.value == Best)
+
+ friendsLens : Lens Account (List Friend)
+ friendsLens =
+ Lens .friends (\b a -> { a | friends = b })
+
+ firstNamesOfBestFriends : Traversal Account String
+ firstNamesOfBestFriends =
+ friendsLens
+ |> Compose.lensWithTraversal bestFriendsTraversal
+ |> Compose.traversalWithLens firstNameLens
+
+ upcaseBestFriendsFirstNames : Account -> Account
+ upcaseBestFriendsFirstNames account =
+ Traversal.modify firstNamesOfBestFriends String.toUpper
+```
+
 ## Common
 Common lenses/prisms/optionals that most projects will use.
 
 #### Step into a `Maybe` value.
-```elm 
+```elm
  maybe.set 5 Nothing
  > Just 5
 ```

elm.json

@@ -3,12 +3,13 @@
  "name": "arturopala/elm-monocle",
  "summary": "Library providing functional tools to manipulate complex records",
  "license": "MIT",
- "version": "2.1.0",
+ "version": "2.2.0",
  "exposed-modules": [
  "Monocle.Iso",
  "Monocle.Prism",
  "Monocle.Lens",
  "Monocle.Optional",
+ "Monocle.Traversal",
  "Monocle.Common",
  "Monocle.Compose"
  ],
@@ -19,4 +20,4 @@
  "test-dependencies": {
  "elm-explorations/test": "1.1.0 <= v < 1.1.1"
  }
-}
+}