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Comment by tome | Hacker News Reader

What's a "first-class property"?

   class Person {
      def age;
   }

There is a second-class "age" property: the only way to specify it or use it is to first declare the ADT/Person class. (In this example, I use a dynamic language; that is, the property can be given any type of value.)

Here is the usage of a first-class age property:

   public boolean isOld(Map<String, Object> entity) {
      return entity.getAs("age", int.class) > 30;
   }

Note that in this example, age's semantics are independent of any Person/ADT. Now a statically-typed language that would let me define a vocabulary that include "age" and its type independent of an ADT would be supporting first-class properties.

I could cheat and do this in a typed language like Java like so:

   class Age { int val; } // using class to define a property
   class Entity { Age age; ...and list all other properties here... }

But this is absurdly cumbersome and obviously Java isn't conducive to this.

> Note that in this example, age's semantics are independent of any Person/ADT. Now a statically-typed language that would let me define a vocabulary that include "age" and its type independent of an ADT would be supporting first-class properties.

What you're referring to is known as row polymorphism or structural typing and it exists in languages such as Elm, Purescript, Scala and Go (and I believe Haskell has these as an extension). For instance, this is valid Scala:

    def isOld(thing: {def age: Int}): Boolean =
      thing.age > 30

    case class Person(name: String, age: Int)
    case class Dog(age: Int)
    println(isOld(Person("Jim", 25))
    println(isOld(Dog(31))

And here's Elm:

    isOld : {age: Int | a} -> Bool
    isOld thing = thing.age > 30

    Debug.log (isOld {name: "Jim", age: 42})
    Debug.log (isOld {age: 10})

This is pretty cool. The Scala version is a little weird. Why do I need a separate Person and Dog class? Can't a dog have a name as well? Why do I need to declare that upfront or ensure that no other part of code can give a Dog a name? That's the problem with ADTs; there's all this impedance.

The Elm version is much better. Except of course I have to upfront at the method signature declare my property requirements -- while that is also a burden, it is a much lesser burden than having ADT/taxonomy of Person, Dog, etc.

> The Elm version is much better. Except of course I have to upfront at the method signature declare my property requirements -- while that is also a burden, it is a much lesser burden than having ADT/taxonomy of Person, Dog, etc.

Elm (and Purescript) should be able to fully infer the type of that function w/o forcing you to declare which fields you want to use up-front.

How would it infer:

   isOld(parseJson(json))

When parseJson may or may not produce an entity with the "age" property?

Sorry, I wasn't clear. My point was that you don't have to declare the type of `thing` up front in that example. It would be able to infer that it's a record with an `age` field from the function's definition.

In your example, `parseJson` would have to return a record with an age field, otherwise the compiler would complain. Personally, I think that represents a reasonable tradeoff in ease of use for safety.

I think the Scala version does exactly what you want it to do, but you seem to be confused about something that I can't really pin-point.

> Why do I need a separate Person and Dog class?

You don't. It's an example.

> Can't a dog have a name as well?

It can.

> Why do I need to declare that upfront or ensure that no other part of code can give a Dog a name?

I have trouble understanding the meaning of that sentence.

> I believe Haskell has these as an extension

Not really ... Haskell has libraries that claim to implement this but we Haskellers have to admit Haskell's support for row types is pretty poor. A Clojurist's objections to Haskell on those grounds are valid (in practice though not in theory).

Does this meet your criteria of "first-class" property? It works on any entity that "has" and "age", at least according to my definition of those things!

    -- The class definition can often be entirely elided
    -- with Generics or Template Haskell.  Instance   
    -- definitions can generally require minimal or no
    -- implementation either
    class HasAge r age | r -> age where
        getAge :: r -> age
        setAge :: r -> age -> r
    
    isOld :: HasAge r => r -> Bool
    isOld r = getAge r > 30

Firs-class generally means something you can put in a variable and compose with higher order operations. Think first class functions.

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