Re: [Haskell-beginners] defining own arbitrary :: Gen Int

On 7 Mar 2009, at 13:54, ben wrote:

Hello,

I would like to define my own arbitrary which should only give naturals, i. e. which should behave like

instance Arbitrary Integer where arbitrary = elements ([0..]).

This definition gives a "multiple definition" error. Is there a way to hide the definition of arbitrary::Gen Int of QuickCheck, such that I can use my own definition ?

The easiest way is to newtype it, and use fromIntegral to convert: newtype Natural = N Integer instance Arbitrary Integer where arbitrary = N <$> elements [0..] instance Integral Natural where fromIntegral (N x) = fromIntegral x Bob