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Comment by xyzzyz | Hacker News Reader

Not sure what you mean, exactly. Could you sketch a simple example with some types and typeclasses?

This is how it would be done in Java:

  class Parent {  
    public void first() {};  
    public void second() {};  
    public void third() {};   
  }

  class Child extends Parent {  
    @Override  
    public void third() {}; //change implementation of third function, reusing the first two.
  }

third = first + second

I would accomplish this in modules in the following way. I use reasonML syntax but it's OCaml under the hood

  module Parent {
    let first = () => ();
    let second = () => ();
    let third = () => ();
  };

  module Child {
    include Parent; //add parent funcs to namespace
    //@Override 
    let third = () => ();
  };

I make regular use of this pattern production.

In OCaml:

    module Foo = struct
      let first = 1
      let second = 2
      let third = 3
    end

    module Bar = struct
      include Foo    (* Include contents of [Foo] *)
      let third = 4  (* Override [third] *)
    end

OCaml has real objects and classes. Moreover, it is trivial to demonstrate that your example doesn't work in practice:

  module Foo = struct
   let first() = 1
   let second() = first() + 1
  end

  module Bar = struct
    include Foo
    let first() = 0
  end

Bar.second () will still be 2 because of static binding. Thus you cannot use this approach to replace 'first' by itself without creating inconsistencies.

  data Numberable = Numberable {
    first  :: IO ()
   ,second :: IO ()
   ,third  :: IO ()
  }

  parent = Numberable {first = return (), second = return (), third = return ()}

  child = parent { third = print 'third' }

And if you wanted to DI into a function:

  data Env = { numberable :: Numberable }

  doStuff :: (MonadIO m, MonadReader Env m) => Int -> Int -> m ()
  doStuff num1 num2 = do
    myNumberable <- reader numberable
    print (show num1)
    print (show num2)
    liftIO (third myNumberable)

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