add a couple of tests

test/test.sml

@@ -0,0 +1,48 @@
+open Railroad
+open Railroad.Test
+open InputMsg
+
+val emptyVec = Vector.fromList []
+
+val cursorTests = describe "cursor operations"
+  [ test "'w' moves cursor to start of next word in contiguous string" (fn _ =>
+      let
+        (* arrange *)
+        val buffer = LineGap.fromString "hello world"
+        val app = AppType.init (buffer, 0, 0)
+
+        (* act *)
+        val ({cursorIdx, ...}, _) = AppUpdate.update (app, CHAR_EVENT #"w")
+
+        (* assert *)
+        val chr = String.sub ("hello world", cursorIdx)
+      in
+        Expect.isTrue (chr = #"w")
+      end)
+
+  , test "'w' moves cursor to start of next word in split string" (fn _ =>
+      let
+        (* arrange *)
+        val buffer =
+          { idx = 0
+          , line = 0
+          , leftStrings = []
+          , leftLines = []
+          , rightStrings = ["hello ", "world"]
+          , rightLines = [emptyVec, emptyVec]
+          }
+        val app = AppType.init (buffer, 0, 0)
+
+        (* act *)
+        val ({cursorIdx, ...}, _) = AppUpdate.update (app, CHAR_EVENT #"w")
+
+        (* assert *)
+        val chr = String.sub ("hello world", cursorIdx)
+      in
+        Expect.isTrue (chr = #"w")
+      end)
+  ]
+
+val tests = concat [cursorTests]
+
+val _ = run tests