On Mon, Feb 06, 2012 at 10:31:50AM +0100, Adrien Haxaire wrote:
Hello,
In the library I am writing, I declare a type Vector for Data.Vector Double. I then create a Num instance for it as it doesn't have one yet. GHC tells me that this instance is an orphan. After reading several answers to issues like mine, I want to get rid of these orphans. The advice I saw mostly is to use the newtype keyword.
Is there a way to do it nicely, instead of copying most of the API of Data.Vector ?
Now, the only thing I can see is do like in this example:
import Data.Vector as V
newtype VectorD = VectorD (V.Vector Double)
map :: (Double -> Double) -> VectorD -> VectorD map f (VectorD v) = VectorD $ V.map f v
Is there a better way ?
Yes: don't bother. Having an orphan instance is really not that big of a deal. And it's certainly not worth making a newtype just to avoid the warning. If you want to turn off the warning you can add {-# OPTIONS_GHC -fno-warn-orphans #-} to the top of your file. -Brent