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Functional Programming in Object-Oriented Programming in Functional Programming in Swift

The objects that I’ve been building up over the last few posts have arbitrarily broad behaviours. They can respond to any selector drawn from the set of all possible strings.

As with all art, beauty is produced by imposing constraints. An important class (pardon the pun) of objects only has a meaningful response to one selector (value or, if it takes an additional argument, value:) which causes the object to do the one thing it knows how and return a result.

In many object-oriented programming circles, including the Smalltalk world, these objects are called blocks. They’re useful because they enable multiple uses of the same algorithm to be extracted into a single place, with the block customising the significant activity that happens in the algorithm. As an example, iterating an array to square numbers and iterating an array to load images from paths both involve iterating an array. The algorithm that iterates an array can be expressed in a form that accepts a block that does the work: squaring some numbers or loading some images.

Without going in to the details of the implementation (which is a collection of classes in the Objective-Swift system), here’s how a block could be used:

let myArray = newObject(NSArray(Integer(1, proto: o), Integer(2, proto: o),
  Integer(3, proto: o), Integer(4, proto: o)))
let evens = (myArray,"filter:") ✍ OneArgBlock({ obj in
  return (ℹ︎(obj)! % 2) == 0 ? True : False
  }, proto: o)
ℹ︎(evens→"count") // 2
📓((evens,"objectAtIndex:") ✍ Integer(0, proto: o)) // "2"
📓((evens,"objectAtIndex:") ✍ Integer(1, proto: o)) // "4"

The NSArray→filter: selector takes a block, and for each element in the array it sends the value: message with the element as the argument. In fact, it passes another block to the result of this block. Both True and False respond to the ifTrue: message. True→ifTrue: executes its argument, False→ifTrue: doesn’t.

Now we already have a name for the concept of an object that does exactly one thing, returning some result based on its input parameters. That’s a function. Yes, we finally got there:

Swiftception

So just as Object-Oriented Programming was a restricted application of a subset of Functional Programming, we can see Functional Programming as a restricted application of a subset of Object-Oriented Programming.

And that’s pretty much the point of this series of posts. These two ways of thinking about computer programs get you to the same place, as long as you apply the thinking. Neither is a silver bullet, neither is subsumed nor obsoleted by the other.

Coda

Now imagine a block that takes a selector and returns another block…

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