On 2014-04-28 20:26, Lorenzo Tabacchini wrote:
Imagine the following data type:
data Person = Child { childAge :: Int, school :: School } | Adult { adultAge :: Int, job :: Job }
Both the alternatives share an "age" field, but in order to access it we are obliged to match all the constructors:
personAge :: Person -> Int personAge (Child {childAge = age}) = age personAge (Adult {adultAge = age}) = age
Is there a way to define a common field in a data type (somehow like inheritance in the OOP world)?
Since you already have dedicated types for the school and the job, why not have a descriptive name for the age as well so that you can drop the accessor functions altogether? E.g. type Age = Int data Person = Child Age School | Adult Age Job If needed, you could of course still define age :: Person -> Age age (Child x _) = x age (Adult x _) = x but you may also find that you don't even need such a function in the first place, pattern matching may do the job just fine: mayRideRollerCoaster :: Person -> Bool mayRideRollerCoaster (Child age _) = age > 12 mayRideRollerCoaster _ = True -- Frerich Raabe - raabe@froglogic.com www.froglogic.com - Multi-Platform GUI Testing