Re: [Haskell-cafe] user type declarations in Haskell

2006/6/22, 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

a1 and a2 have type HType (not QADouble, nor Double). GHC doesn't know how to use (+) on HType because (+) is meaningful only for Num instances : try to type ':t (+)' in ghci : it will give you the type of (+). try to type also ':t a1'. hope it helps, vo minh thu