Generator of "valid" and "invalid" data in a type class

For example, if you created

data Person = Anonymous { getId::String } | Client { getUsername::String, getName::String, getEmail::String} deriving(Show,Eq)

We can provide means to generate valid and invalid data, like:

instance Variant Person where
  valid = do
    id <- alternative
    username <- alternative
    name <- alternative
    domain <- alternative
    ext <- alternative
    (oneof . return) [Anonymous id, Client username name (username++"@"++domain++ext)]
  invalid = do
    username <- alternative
    name <- alternative
    return $ Client username name ""

See this post for more details

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