even more baking ahead of time

lib/Homework/Ch01/Hanoi.hs

@@ -25,8 +25,7 @@ move = do
   if topDiscA >= topDiscC
     then return Nothing
     else do
-      popPegA
-      pushPegC $ fromJust topDiscA
+      pushPegC =<< popPegA
       Just <$> makeMove "a" "c"
 
 {------------------------------------------------------------------------------}
@@ -106,10 +105,13 @@ makeMove from to = do
   putPegs pegs {pegsMoves = move' : pegsMoves pegs}
   return move'
 
-popPegA :: (Monad m) => PegStep m ()
+popPegA :: (Monad m) => PegStep m Disc
 popPegA = do
   peg <- getPegA
-  putPegA $ peg {pegDiscs = init $ pegDiscs peg}
+  let disc = last $ pegDiscs peg
+      rest = init $ pegDiscs peg
+  putPegA $ peg {pegDiscs = rest}
+  return disc
 
 pushPegC :: (Monad m) => Disc -> PegStep m ()
 pushPegC disc = do