Add List.splitFirst and List.splitLast

crates/compiler/builtins/roc/List.roc

@@ -51,6 +51,8 @@ interface List
         sublist,
         intersperse,
         split,
+        splitFirst,
+        splitLast,
         all,
         dropIf,
         sortAsc,
@@ -915,6 +917,41 @@ split = \elements, userSplitIndex ->
 
     { before, others }
 
+## Returns the list before the first occurrence of a delimiter, as well as the
+## rest of the list after that occurrence. If the delimiter is not found, returns `Err`.
+##
+##     List.splitFirst [Foo, Z, Bar, Z, Baz] Z == Ok { before: [Foo], after: [Bar, Baz] }
+splitFirst : List elem, elem -> Result { before : List elem, after : List elem } [NotFound]*
+splitFirst = \elements, delimiter ->
+    length = List.len elements
+
+    when List.findFirstIndex elements (\elem -> elem == delimiter) is
+        Ok index ->
+            before = List.sublist elements { start: 0, len: index }
+            len = List.len elements - index
+
+            Ok { before, len }
+
+        Err NotFound -> Err NotFound
+
+
+## Returns the list before the last occurrence of a delimiter, as well as the
+## rest of the list after that occurrence. If the delimiter is not found, returns `Err`.
+##
+##     List.splitLast [Foo, Z, Bar, Z, Baz] Z == Ok { before: [Foo, Bar], after: [Baz] }
+splitLast : List elem, elem -> Result { before : List elem, after : List elem } [NotFound]*
+splitLast = \elements, delimiter ->
+    length = List.len elements
+
+    when List.findLastIndex elements (\elem -> elem == delimiter) is
+        Ok index ->
+            before = List.sublist elements { start: 0, len: index }
+            len = List.len elements - index
+
+            Ok { before, len }
+
+        Err NotFound -> Err NotFound
+
 ## Like [List.map], except the transformation function returns a [Result].
 ## If that function ever returns `Err`, [mapTry] immediately returns that `Err`.
 ## If it returns `Ok` for every element, [mapTry] returns `Ok` with the transformed list.