F# for Fun and Profit
Part of the "Why use F#?" series (link)

Functions as interfaces

OO design patterns can be trivial when functions are used
15 Apr 2012 This post is over 3 years old

An important aspect of functional programming is that, in a sense, all functions are “interfaces”, meaning that many of the roles that interfaces play in object-oriented design are implicit in the way that functions work.

In fact, one of the critical design maxims, “program to an interface, not an implementation”, is something you get for free in F#.

To see how this works, let’s compare the same design pattern in C# and F#. For example, in C# we might want to use the “decorator pattern” to enhance some core code.

Let’s say that we have a calculator interface:

interface ICalculator
{
   int Calculate(int input);
}

And then a specific implementation:

class AddingCalculator: ICalculator
{
   public int Calculate(int input) { return input + 1; }
}

And then if we want to add logging, we can wrap the core calculator implementation inside a logging wrapper.

class LoggingCalculator: ICalculator
{
   ICalculator _innerCalculator;

   LoggingCalculator(ICalculator innerCalculator)
   {
      _innerCalculator = innerCalculator;
   }

   public int Calculate(int input)
   {
      Console.WriteLine("input is {0}", input);
      var result  = _innerCalculator.Calculate(input);
      Console.WriteLine("result is {0}", result);
      return result;
   }
}

So far, so straightforward. But note that, for this to work, we must have defined an interface for the classes. If there had been no ICalculator interface, it would be necessary to retrofit the existing code.

And here is where F# shines. In F#, you can do the same thing without having to define the interface first. Any function can be transparently swapped for any other function as long as the signatures are the same.

Here is the equivalent F# code.

let addingCalculator input = input + 1

let loggingCalculator innerCalculator input =
   printfn "input is %A" input
   let result = innerCalculator input
   printfn "result is %A" result
   result

In other words, the signature of the function is the interface.

Generic wrappers

Even nicer is that by default, the F# logging code can be made completely generic so that it will work for any function at all. Here are some examples:

let add1 input = input + 1
let times2 input = input * 2

let genericLogger anyFunc input =
   printfn "input is %A" input   //log the input
   let result = anyFunc input    //evaluate the function
   printfn "result is %A" result //log the result
   result                        //return the result

let add1WithLogging = genericLogger add1
let times2WithLogging = genericLogger times2

The new “wrapped” functions can be used anywhere the original functions could be used – no one can tell the difference!

// test
add1WithLogging 3
times2WithLogging 3

[1..5] |> List.map add1WithLogging

Exactly the same generic wrapper approach can be used for other things. For example, here is a generic wrapper for timing a function.

let genericTimer anyFunc input =
   let stopwatch = System.Diagnostics.Stopwatch()
   stopwatch.Start()
   let result = anyFunc input  //evaluate the function
   printfn "elapsed ms is %A" stopwatch.ElapsedMilliseconds
   result

let add1WithTimer = genericTimer add1WithLogging

// test
add1WithTimer 3

The ability to do this kind of generic wrapping is one of the great conveniences of the function-oriented approach. You can take any function and create a similar function based on it. As long as the new function has exactly the same inputs and outputs as the original function, the new can be substituted for the original anywhere. Some more examples:

  • It is easy to write a generic caching wrapper for a slow function, so that the value is only calculated once.
  • It is also easy to write a generic “lazy” wrapper for a function, so that the inner function is only called when a result is needed

The strategy pattern

We can apply this same approach to another common design pattern, the “strategy pattern.”

Let’s use the familiar example of inheritance: an Animal superclass with Cat and Dog subclasses, each of which overrides a MakeNoise() method to make different noises.

In a true functional design, there are no subclasses, but instead the Animal class would have a NoiseMaking function that would be passed in with the constructor. This approach is exactly the same as the “strategy” pattern in OO design.

type Animal(noiseMakingStrategy) =
   member this.MakeNoise =
      noiseMakingStrategy() |> printfn "Making noise %s"

// now create a cat
let meowing() = "Meow"
let cat = Animal(meowing)
cat.MakeNoise

// .. and a dog
let woofOrBark() = if (System.DateTime.Now.Second % 2 = 0)
                   then "Woof" else "Bark"
let dog = Animal(woofOrBark)
dog.MakeNoise
dog.MakeNoise  //try again a second later

Note that again, we do not have to define any kind of INoiseMakingStrategy interface first. Any function with the right signature will work. As a consequence, in the functional model, the standard .NET “strategy” interfaces such as IComparer, IFormatProvider, and IServiceProvider become irrelevant.

Many other design patterns can be simplified in the same way.

The "Why use F#?" series


Written by ScottW. Found a typo or error? Follow me on Twitter.

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