implemented newAbacus

src/Abacus.hs

@@ -28,13 +28,16 @@ module Abacus (Abacus, newAbacus) where
 
 import Abacus.Internal
 
--- | Constructs the initial state of a new "Abacus"
+-- | Constructs the initial state of a new "Abacus"; will return
+-- "Nothing" if the input paramebers are invalid.
 newAbacus
   :: Int
-  -- ^ The number of beads on each rung
+  -- ^ The number of beads on each rung (must be at least one)
   -> Int
-  -- ^ The number of rungs
+  -- ^ The number of rungs (must be at least one)
   -> Maybe Abacus
-newAbacus = undefined
+newAbacus beads rungs = if beads < 1 || rungs < 1
+  then Nothing
+  else Just $ Abacus beads $ replicate rungs 0
 
 --jl