@Jose_A_Alonso Exercitium: Mayor capicúa producto de dos números de n cifras

mayorCapicuaP.hs

-- www.glc.us.es/~jalonso/exercitium/mayor-capicua-producto-de-dos-numeros-de-n-cifras

mayorCapicuaP n = alpha 0 (t-1)
  where 
    t = 10^n
    b = div t 10
    alpha a x | x > b     = let a' = case beta x of
                                       Just z  -> max z a
                                       Nothing -> a
                            in alpha a' (x-1)
              | otherwise = a

      where 
        beta y | y > b    = let z = x*y
                                s = show z
                            in case s == reverse s of
                                 True  -> Just z
                                 False -> beta (y-1)
               | otherwise = Nothing