Vladimir Portnykh wrote:
I am trying to define the following types
data MyStringType a = String deriving (Eq, Ord, Show) data QADouble a = Double deriving (Eq, Ord, Show)
These are not what you think they are. MyStringType has a phantom type parameter and only one value, which is the constant String (but not of type String). What you actually meant can only be guessed, and I'm not even trying.
So HType can represent strings or doubles. later I want to do something like the following: let a1 =QADouble 1 let a2 =QADouble 2 let a3 = a1 + a2
data HType = QADouble Double | QAString Double
First, it is not working because Haskell complains about a3. it does not know how to calculate it.
What did you expect? What's the sum of a Double and a String if not an error? You have to define (+), which is already defined. Use some other name and give a definition: QADouble x `plus` QADouble y = ... QADouble x `plus` QAString y = ... QAString x `plus` QAString y = ... QAString x `plus` QADouble y = ... Udo. -- Lieber vom Fels zertrümmert als bei einer Frau verkümmert. -- aus einem Gipfelbuch