[Haskell-beginners] Type constructors sharing a common field

Lorenzo Tabacchini lortabac at gmx.com
Mon Apr 28 18:26:21 UTC 2014

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)?
It would be practical if we could define "age :: Int" as a common field 
and do something like:

personAge (_ {age = age}) = age

An alternative solution could be extracting the varying fields in a 
different type:

data WorkingPerson = Child School | Adult Job

data Person = Person { age :: Int, workingPerson :: WorkingPerson }

Or to make Person a type class (which would probably require existential 
types in order to be useful).

But the first one looks simpler and more natural.

Does this feature exist (maybe in other forms) and, if not, is there a 
Would it make sense to have a GHC extension (or even to make a Template 
Haskell hack) for this feature?

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