[Haskell-cafe] user type declarations in Haskell

[Vladimir Portnykh]

I am trying to define the following types

data MyStringType a = String deriving (Eq, Ord, Show)
data QADouble a = Double deriving (Eq, Ord, Show)
data HType a = QADouble a| DDTraceType a deriving (Eq, Ord, Show)

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

First, it is not working because Haskell complains about a3. it does not 
know how to calculate it.
Is it a way to give him a hint? QADouble is Double...am I doing something 
absolutely wrong and silly?

many thanks, vladimir

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