Hi. I am a student, who get interesting in Haskell. So, nowdays, I study Hakell with "Introduction to Functional Programming using Haskell - second edition" And today, I face an error from bellow code. ---------------------------- module OperationTest where data Nat = Zero | Succ Nat deriving (Show) (+) :: Nat -> Nat -> Nat m + Zero = m m + Succ n = Succ (m + n) ---------------------------- But above source make an error like bellow Reading file "g:\Haskell\OperationTest.hs": Dependency analysis ERROR "g:\Haskell\OperationTest.hs":8 - Ambiguous variable occurrence "+" *** Could refer to: OperationTest.+ Prelude.+ above source is from the text book - "Introduction to Functional Programming using Haskell - second edition" and I use Hugs to interpret it. I understand what interpreter say, but I don't know How I can solve this problem. Please, Help me!!! - 5 - P.S. Tank you very much to read my broken english letter.