[Haskell-beginners] Type constructors sharing a common field

[Frerich Raabe]

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 at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing