*This is the third in a series of posts about monads (part 1, part 2). I happen to be a .NET developer, so I use C# in my examples, but the concept applies to any language that has first-class functions. Code for the series is at github.*

In the last post, we covered over half of the items in Wikipedia's formal definition of a monad:

~~Formally, a monad consists of a type constructor~~*M*and two operations,*bind*~~and~~*return*.- The operations must fulfill several properties to allow the correct composition of
*monadic*functions (i.e. functions that use values from the monad as their arguments or return value). ~~In most contexts, a value of type~~*M a*can be thought of as an action that returns a value of type*a*.~~The~~*return*operation takes a value from a plain type*a*and puts it into a monadic container of type*M a*.- The
*bind*operation chains a monadic value of type*M a*with a function of type*a ? M b*to create a monadic value of type*M b*, effectively creating an action that chooses the next action based on the results of previous actions.

Let's break down the requirement for the *Bind* function. It

...chains a monadic value of typeThat says to me that we need to have an instance method in our monad. Or, we could use an extension method to achieve the same result. I prefer to use an extension method, so that's what we'll do. Let's create an extension method:M a...

public static ??? Bind<T>(this Monad<T> monad, ???)

Not a bad start. Let's skip to the return type. We'll need

...to create a monadic value of typeThat doesn't look so hard. We've already used ourM b...

*Monad<T>*class in place of

*M a*. But, since we already used

*T*we won't be able to use it again. But, since it's a good idea to use standard C# naming conventions when we're writing C# code (when in Rome...), our generic type should at least start with the letter 'T'. How about

*TResult*, since it is the result of our

*Bind*method?

public static Monad<TResult> Bind<T, TResult>( this Monad<T> monad, ???)

But what of the second parameter? It looks like it needs we need to be connecting

...with a function of typeWhy, if I didn't know better, I'd say that looks an awful like a lambda expression (spoiler alert: it is). And we already know whata ? M b...

*a*and

*M b*mean to us:

*T*and

*Monad<TResult>*. It would be great if we were able to pass in a lambda expression that takes an

*T*and returns a

*Monad<TResult>*. But what is a lambda expression? Just a convenient way of creating a function. Hmmmm, function. I know - why don't we take a

*Func<T, Monad<TResult>>*as our parameter type! We just put the

*function*in

*functional*. So what does our extension method look like now?

public static Monad<TResult> Bind<T, TResult>( this Monad<T> monad, Func<T, Monad<TResult>> resultSelector)

Finally, we'll need to fill in the body. Our requirements state that we need to take our *Monad<T>* and transform it into a *Monad<TResult>* by using a *Func<T, TResult>*. That function needs a <T> argument passed to it. Haven't we seen an instance of *<T>* before? Of course - in the *Value* property of our *Monad<T>* class. With that, I think we have got enough information to fill out our *Bind* method:

public static Monad<TResult> Bind<T, TResult>( this Monad<T> monad, Func<T, Monad<TResult>> resultSelector) { return resultSelector(monad.Value); }

I think we're really getting somewhere now! How would we use our spiffy new *Bind* function? I'd think that we would use it something like this:

Monad<int> m1 = 128.ToMonad(); Monad<Type> m2 = m1.Bind(x => x.GetType().ToMonad()); Console.WriteLine( "The type of the original monad is: {0}.", m2.Value);

We've taken a Monad of type *int*, and bound it to a Monad of type *Type* by calling *Bind* and passing in a lambda expression that takes a *T* and returns a *Monad<Type>*. There's one thing I don't like about this usage though: the call to *ToMonad()*. I'd prefer it if our *Bind* function had, as its parameter, a *Func<T, TResult>*. But I also want to meet the requirement for being a monad. Why don't we create an overload of *Bind* to do just that? I'd like our existing *Bind* method to remain the source of truth, so we'll have our new one call it.

public static Monad<TResult> Bind<T, TResult>( this Monad<T> monad, Func<T, TResult> resultSelector) { return monad.Bind(m => resultSelector(m).ToMonad()); }

This is getting a little funky. We're doing a few things here: 1) we're passing a new lambda expression to the other *Bind* method; 2) in the body of the new lambda expression, we're executing the function that was passed in; and 3) we're turning the result of into a *Monad<TResult>* by calling our extension method, *ToMonad*. We're not actually executing the passed-in function, we're passing it into another function that we pass to the other *Bind* method. Clear as mud, right? So why go through all this trouble? To me, it expresses intent better, and I think this makes it worth it. We can use it like this:

Monad<int> m1 = 128.ToMonad(); Monad<Type> m2 = m1.Bind(x => x.GetType()); Console.WriteLine( "The type of the original monad is: {0}.", m2.Value);

This is all well and good, but it seems kind of tedious to have to create a new variable each time we call *Bind*. Didn't the requirement mention something about chaining? As in method chaining? Why don't we try that?

var m = 128.ToMonad() .Bind(x => x.GetType()) .Bind(x => new string(x.ToString().Reverse().ToArray())); Console.WriteLine( "The backwards string representation of " + "the type of the original monad is: {0}.", m.Value);

We're taking the number *128* wrapping it in *Monad<int>*, binding it to a *Monad<Type>* with a function that takes any value and returns its type, and binding **that** to a *Monad<string>* with a function that takes any value and returns its string representation backwards. Pretty cool, huh?

We're headed for the home stretch now. It's time to address the final bullet point:

The operations must fulfill several properties to allow the correct composition ofIt seems to me like we're already doing this. We're composing a monad from a series of monadic functions. But perhaps we can make our intent more clear - callingmonadicfunctions (i.e. functions that use values from the monad as their arguments or return value).

*Bind*over and over again doesn't exactly look pretty. What if we extracted the calls to

*Bind*into methods that expressed their intent more clearly? How about using some extensions methods? Something like this?

public static Monad<Type> ToType<T>(this Monad<T> monad) { return monad.Bind(m => m.GetType()); } public static Monad<string> ToReversedString<T>( this Monad<T> monad) { return monad.Bind(m => new string(m.ToString().Reverse().ToArray())); }

Now we can replace our *Bind* statements:

var m = 128.ToMonad() .ToType() .ToReversedString(); Console.WriteLine( "The backwards string representation of " + "the type of the original monad is: {0}.", m.Value);

That about wraps it up for today. Next time, we'll discuss the three monadic laws.(Hopefully we follow them!)

Great post Brian. I had just bought a functional book. That book has moved up my reading queue because of your series on Monads. Can you give an example of the type of problem, Monads are solving for you? Looking forward to your next blog post.

ReplyDeleteClearly I needed more coffee before posting comments. I thought I was uploading a profile image.

ReplyDeleteFor the last couple of days (since DevLink ended), I've been playing around with monadic parser combinators. Specifically, I'm pre-parsing SendKeys data for my "auto-typer" app - the one I used for my monad talk. What I have works, but it was quick and dirty. I'm looking for a more lasting solution. The parser combinators will allow me to provide for a richer data interface, allowing for better commands and more control over both the "clicky" sounds and the delay between keystrokes. Instead of writing my own parser library, I'm using https://github.com/sprache/Sprache, and, while doing so, I've been dissecting how it works. So far, it's pretty cool - I'd recommend checking it out if you're interested in functional programming.

ReplyDeleteI'll probably be doing a series of tutorials on Sprache in the coming months, since the documentation is a little sparse...

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