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<font face="monospace"><font face="sans-serif">Imagine the following
data type:</font><br>
<br>
data Person = Child { childAge :: Int, school :: School }<br>
| Adult { adultAge :: Int, job :: Job }<br>
<br>
<font face="sans-serif">Both the alternatives share an "age"
field, but in order to access it we are obliged to match all the
constructors:</font><br>
<br>
personAge :: Person -> Int<br>
personAge (Child {childAge = age}) = age<br>
personAge (Adult {adultAge = age}) = age<br>
<br>
<font face="sans-serif">Is there a way to define a common field in
a data type (somehow like inheritance in the OOP world)?<br>
It would be practical if we could define "age :: Int" as a
common field and do something like:</font><br>
<br>
personAge (_ {age = age}) = age<br>
<font face="sans-serif"><br>
An alternative solution could be extracting the varying fields
in a different type:</font><br>
<br>
data WorkingPerson = Child School | Adult Job<br>
<br>
data Person = Person { age :: Int, workingPerson :: WorkingPerson
}<br>
<br>
<font face="sans-serif">Or to make Person a type class (which
would probably require existential types in order to be useful).<br>
<br>
But the first one looks simpler and more natural.<br>
<br>
Does this feature exist (maybe in other forms) and, if not, is
there a reason?<br>
Would it make sense to have a GHC extension (or even to make a
Template Haskell hack) for this feature?</font><br>
<br>
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