Merge pull request #42 from KurtRMueller/master

Create splitWhen function that attempts to split a list at the element where the given predicate is true

src/List/Extra.elm

@@ -40,6 +40,7 @@ module List.Extra
         , scanr1
         , unfoldr
         , splitAt
+        , splitWhen
         , takeWhileRight
         , dropWhileRight
         , span
@@ -879,6 +880,17 @@ splitAt n xs =
     ( take n xs, drop n xs )
 
 
+{-| Attempts to split the list at the first element where the given predicate is true. If the predicate is not true for any elements in the list, return nothing. Otherwise, return the split list.
+
+    splitWhen (\n -> n == 3) [1,2,3,4,5] == Just ([1,2],[3,4,5])
+    splitWhen (\n -> n == 6) [1,2,3,4,5] == Nothing
+-}
+splitWhen : (a -> Bool) -> List a -> Maybe ( List a, List a )
+splitWhen predicate list =
+    findIndex predicate list
+    |> Maybe.map (\i -> splitAt i list)
+
+
 {-| Take elements from the right, while predicate still holds.
 
     takeWhileRight ((<)5) (range 1 10) == [6,7,8,9,10]

tests/Tests.elm

@@ -217,6 +217,14 @@ all =
                 \() ->
                     Expect.equal (splitAt (-1) [ 1, 2, 3 ]) ( [], [ 1, 2, 3 ] )
             ]
+        , describe "splitWhen" <|
+            [ test "returns split list when predicate is true" <| 
+                \() ->
+                    Expect.equal (splitWhen (\n -> n == 3) [ 1, 2, 3, 4, 5]) (Just ([1, 2], [3, 4, 5]))
+            , test "returns nothing when predicate is false" <| 
+                \() ->
+                    Expect.equal (splitWhen (\n -> n == 6) [ 1, 2, 3, 4, 5]) Nothing
+            ]
         , describe "takeWhileRight" <|
             [ test "" <|
                 \() ->