implemented rendering of left side of abacus widget

src/Abacus/App/Widgets/Internal.hs

@@ -36,11 +36,21 @@ module Abacus.App.Widgets.Internal (
   abacusRight,
   ) where
 
+import Lens.Micro.Platform ((^.))
+
 import Abacus
 import Abacus.App.Types
 
 abacusLeft :: AppState -> [String]
-abacusLeft = undefined
+abacusLeft s =
+  "  " : map
+    (\n -> mark n : show n)
+    [0..pred (getNumRungs $ s^.abacus)]
+  ++ ["  "]
+  where
+    mark n = if n == s^.rungNum
+      then '>'
+      else ' '
 
 beads :: Abacus -> [(String, String)]
 beads = undefined