implemented rungL

src/Abacus.hs

@@ -24,6 +24,8 @@ License along with this program.  If not, see
 
 -}
 
+{-# LANGUAGE RankNTypes #-}
+
 module Abacus (
   Abacus,
   newAbacus,
@@ -31,9 +33,12 @@ module Abacus (
   getNumRungs,
   getRung,
   setRung,
+  rungL,
   ) where
 
 import Data.List (find)
+import Data.Maybe (fromJust)
+import Lens.Micro.Platform (Lens', lens)
 
 import Abacus.Internal
 
@@ -91,4 +96,13 @@ setRung r v a = a
       | v > getNumBeads a = getNumBeads a
       | otherwise         = v
 
+-- | Lens for a specific rung of an "Abacus"
+rungL
+  :: Int
+  -- ^ The rung number
+  -> Lens' Abacus Int
+rungL n = lens
+  (fromJust . getRung n)
+  (flip $ setRung n)
+
 --jl