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Comment by xyzzyz | Hacker News Reader

And #3 is also an area where Functional Programming falls completely flat. There is simply no easy way in any FP language that I know to capture such a simple mechanism as: "Reuse 90% of this piece of code but alter just this 10%".

Haskell’s typeclasses and OCaml modules solve this problem neatly. It is not as easy as in Java, because you need to spend a few minutes thinking how to abstractly specify the interface you expect, but really if your object hierarchy is supposed to satisfy Liskov’s substitution principle, you need to do that just the same.

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> Haskell’s typeclasses and OCaml modules solve this problem neatly.

Neither do, really.

Imagine you have an existing typeclass with three functions. How do you create your own typeclass that reuses two of these three functions but your own instance reimplements the third one?

Try it. It's pretty much impossible, or at least not without a lot of boilerplate and forwarding.

Do you mean your own instance of a typeclass? Because that's the direct Haskell analogy to a subclass in Java. I'm not sure where the difficulty is here, as what you're describing is quite straightforward. You can also have the same type be an instance of as many typeclasses as you want, which is something that inheritance-based subclassing and tree hierarchies (is-a rather than has-a) has always made awkward.

> Reuse 90% of this piece of code but alter just this 10%

There is no general solution to this, in any programming language, without some foresight (unless you admit some crazy features like arbitrary code re-writing or source-level patches). If you have, say, a single function of 100 lines, and 10 lines need to be changed, you're going to have to edit that code to enable the change, and if that 100 lines is in a third-party library or code you otherwise don't control, you're going to have a hard time. However, if you control the code and are able to make changes to it to support new behaviors, FP languages give you plenty of power here, just as OOP languages do. There's not a lot of objective difference here in power and plenty of differences in taste and familiarity, so please be careful about repeating "you can't do X in language Y".

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.
  }

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)

third = first + second

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