Re: [Haskell-cafe] partial inheritance

On 19/07/2011, at 11:09 AM, Patrick Browne wrote:

data Emperor = Emperor data Robin = Robin

class Bird a where fly :: a -> a -> a walk :: a -> a -> a

instance Bird Robin where fly x y = y walk x y = x

instance Bird Emperor where -- No fly method walk x y = y

Note that "no 'fly' method is visibly presented" is not at all the same thing as "no 'fly' method EXISTS". From the report: "The default class method for v sub i is used if no binding for it is given in a particular instance declaration (see Section 4.3.2)." so the absence of a method in 'instance Bird Emperor' doesn't mean that it doesn't _have_ one, just that it inherits the default class method.