Functional equivalent of decorator pattern?

Question

What is the functional programming equivalent of the decorator design pattern?

For example, how would you write this particular example in a functional style?

Answer 1

In Haskell, this OO pattern translates pretty much directly, you only need a dictionary. Note that a direct translation is not actually a good idea. Trying to force a OO concept into Haskell is kind of backwords, but you asked for it so here it is.

The Window Interface

Haskell has classes, which has all the functionality of an Interface and then some. So we will use the following Haskell class:

class Window w where
  draw :: w -> IO ()
  description :: w -> String

The Abstract WindowDecorator class

This one is a bit more tricky since Haskell has no concept of inheritance. Usually we would not provide this type at all and let the decorators implement Window directly, but lets follow the example completely. In this example, a WindowDecorator is a window with a constructor taking a window, lets augment this with a function giving the decorated window.

class WindowDecorator w where
   decorate :: (Window a) => a -> w a
   unDecorate :: (Window a) => w a -> a
   drawDecorated :: w a -> IO ()
   drawDecorated = draw . unDecorate
   decoratedDescription :: w a -> String
   decoratedDescription = description . unDecorate

instance (WindowDecorator w) => Window w where
   draw = drawDecorated
   description = decoratedDescription

Note that we provide a default implementation of Window, it can be replaced, and all instances of WindowDecorator will be a Window.

The decorators

Making decorators can then be done as follows:

data VerticalScrollWindow w = VerticalScrollWindow w

instance WindowDecorator VerticalScrollWindow where
  decorate = VerticalScrollWindow
  unDecorate (VerticalScrollWindow w ) = w
  drawDecorated (VerticalScrollWindow w )  = verticalScrollDraw >> draw w

data HorizontalScrollWindow w = HorizontalScrollWindow w

instance WindowDecorator HorizontalScrollWindow where
  decorate = HorizontalScrollWindow
  unDecorate (HorizontalScrollWindow w .. ) = w
  drawDecorated (HorizontalScrollWindow w ..)  = horizontalScrollDraw >> draw w

Finishing Up

Finally we can define some windows:

data SimpleWindow = SimpleWindow ...

instance Window SimpleWindow where
   draw = simpleDraw
   description = simpleDescription

makeSimpleWindow :: SimpleWindow
makeSimpleWindow = ...

makeSimpleVertical = VerticalScrollWindow . makeSimpleWindow
makeSimpleHorizontal = HorizontalScrollWindow . makeSimpleWindow
makeSimpleBoth = VerticalScrollWindow . HorizontalScrollWindow . makeSimpleWindow