Handle list head and tail updates with explicit empty-list errors

fieldglass.ml

@@ -13,3 +13,15 @@ let compose (outer_get, outer_put) (inner_get, inner_put) =
 let _1 = (fun (left, _) -> left), (fun left (_, right) -> left, right)
 let _2 = (fun (_, right) -> right), (fun right (left, _) -> left, right)
 let _id = (fun value -> value), (fun value _ -> value)
+
+let split = function
+  | head :: tail -> head, tail
+  | [] -> invalid_arg "Fieldglass: empty list"
+
+let _hd =
+  (fun source -> fst (split source)),
+  (fun head source -> head :: snd (split source))
+
+let _tl =
+  (fun source -> snd (split source)),
+  (fun tail source -> fst (split source) :: tail)