What are practical uses of applicative style?

Question

I am a Scala programmer, learning Haskell now. It's easy to find practical use cases and real world examples for OO concepts, such as decorators, strategy pattern etc. Books and interwebs are filled with it.

I came to the realization that this somehow is not the case for functional concepts. Case in point: applicatives.

I am struggling to find practical use cases for applicatives. Almost all of the tutorials and books I have come across so far provide the examples of [] and Maybe. I expected applicatives to be more applicable than that, seeing all the attention they get in the FP community.

I think I understand the conceptual basis for applicatives (maybe I am wrong), and I have waited long for my moment of enlightenment. But it doesn't seem to be happening. Never while programming, have I had a moment when I would shout with a joy, "Eureka! I can use applicative here!" (except again, for [] and Maybe).

Can someone please guide me how applicatives can be used in a day-to-day programming? How do I start spotting the pattern? Thanks!

Answer 1

Warning: my answer is rather preachy/apologetic. So sue me.

Well, how often in your day-to-day Haskell programming do you create new data types? Sounds like you want to know when to make your own Applicative instance, and in all honesty unless you are rolling your own parser, you probably won't need to do it very much. Using applicative instances, on the other hand, you should learn to do frequently.

Applicative is not a "design pattern" like decorators or strategies. It is an abstraction, which makes it much more pervasive and generally useful, but much less tangible. The reason you have a hard time finding "practical uses" is because the example uses for it are almost too simple. You use decorators to put scrollbars on windows. You use strategies to unify the interface for both aggressive and defensive moves for your chess bot. But what are applicatives for? Well, they're a lot more generalized, so it's hard to say what they are for, and that's OK. Applicatives are handy as parsing combinators; the Yesod web framework uses Applicative to help set up and extract information from forms. If you look, you'll find a million and one uses for Applicative; it's all over the place. But since it's so abstract, you just need to get the feel for it in order to recognize the many places where it can help make your life easier.

Answer 2

Applicatives are great when you've got a plain old function of several variables, and you have the arguments but they're wrapped up in some kind of context. For instance, you have the plain old concatenate function (++) but you want to apply it to 2 strings which were acquired through I/O. Then the fact that IO is an applicative functor comes to the rescue:

Prelude Control.Applicative> (++) <$> getLine <*> getLine
hi
there
"hithere"

Even though you explicitly asked for non-Maybe examples, it seems like a great use case to me, so I'll give an example. You have a regular function of several variables, but you don't know if you have all the values you need (some of them may have failed to compute, yielding Nothing). So essentially because you have "partial values", you want to turn your function into a partial function, which is undefined if any of its inputs is undefined. Then

Prelude Control.Applicative> (+) <$> Just 3 <*> Just 5
Just 8

but

Prelude Control.Applicative> (+) <$> Just 3 <*> Nothing
Nothing

which is exactly what you want.

The basic idea is that you're "lifting" a regular function into a context where it can be applied to as many arguments as you like. The extra power of Applicative over just a basic Functor is that it can lift functions of arbitrary arity, whereas fmap can only lift a unary function.

Answer 3

Here is an example taken from the aeson package:

data Coord = Coord { x :: Double, y :: Double }

instance FromJSON Coord where
   parseJSON (Object v) = 
      Coord <$>
        v .: "x" <*>
        v .: "y"

Answer 4

Since many applicatives are also monads, I feel there's really two sides to this question.

Why would I want to use the applicative interface instead of the monadic one when both are available?

This is mostly a matter of style. Although monads have the syntactic sugar of do-notation, using applicative style frequently leads to more compact code.

In this example, we have a type Foo and we want to construct random values of this type. Using the monad instance for IO, we might write

data Foo = Foo Int Double

randomFoo = do
    x <- randomIO
    y <- randomIO
    return $ Foo x y

The applicative variant is quite a bit shorter.

randomFoo = Foo <$> randomIO <*> randomIO

Of course, we could use liftM2 to get similar brevity, however the applicative style is neater than having to rely on arity-specific lifting functions.

In practice, I mostly find myself using applicatives much in the same way like I use point-free style: To avoid naming intermediate values when an operation is more clearly expressed as a composition of other operations.

Why would I want to use an applicative that is not a monad?

Since applicatives are more restricted than monads, this means that you can extract more useful static information about them.

An example of this is applicative parsers. Whereas monadic parsers support sequential composition using (>>=) :: Monad m => m a -> (a -> m b) -> m b, applicative parsers only use (<*>) :: Applicative f => f (a -> b) -> f a -> f b. The types make the difference obvious: In monadic parsers the grammar can change depending on the input, whereas in an applicative parser the grammar is fixed.

By limiting the interface in this way, we can for example determine whether a parser will accept the empty string without running it. We can also determine the first and follow sets, which can be used for optimization, or, as I've been playing with recently, constructing parsers that support better error recovery.

Answer 5

I described an example of practical use of the applicative functor in a discussion, which I quote below.

Note the code examples are pseudo-code for my hypothetical language which would hide the type classes in a conceptual form of subtyping, so if you see a method call for apply just translate into your type class model, e.g. <*> in Scalaz or Haskell.

If we mark elements of an array or hashmap with null or none to indicate their index or key is valid yet valueless, the Applicative enables without any boilerplate skipping the valueless elements while applying operations to the elements that have a value. And more importantly it can automatically handle any Wrapped semantics that are unknown a priori, i.e. operations on T over Hashmap[Wrapped[T]] (any over any level of composition, e.g. Hashmap[Wrapped[Wrapped2[T]]] because applicative is composable but monad is not).

I can already picture how it will make my code easier to understand. I can focus on the semantics, not on all the cruft to get me there and my semantics will be open under extension of Wrapped whereas all your example code isn’t.

Significantly, I forgot to point out before that your prior examples do not emulate the return value of the Applicative, which will be a List, not a Nullable, Option, or Maybe. So even my attempts to repair your examples were not emulating Applicative.apply.

Remember the functionToApply is the input to the Applicative.apply, so the container maintains control.

list1.apply( list2.apply( ... listN.apply( List.lift(functionToApply) ) ... ) )

Equivalently.

list1.apply( list2.apply( ... listN.map(functionToApply) ... ) )

And my proposed syntactical sugar which the compiler would translate to the above.

funcToApply(list1, list2, ... list N)

It is useful to read that interactive discussion, because I can't copy it all here. I expect that url to not break, given who the owner of that blog is. For example, I quote from further down the discussion.

the conflation of out-of-statement control flow with assignment is probably not desired by most programmers

Applicative.apply is for generalizing the partial application of functions to parameterized types (a.k.a. generics) at any level of nesting (composition) of the type parameter. This is all about making more generalized composition possible. The generality can’t be accomplished by pulling it outside the completed evaluation (i.e. return value) of the function, analogous to the onion can’t be peeled from the inside-out.

Thus it isn’t conflation, it is a new degree-of-freedom that is not currently available to you. Per our discussion up thread, this is why you must throw exceptions or stored them in a global variable, because your language doesn’t have this degree-of-freedom. And that is not the only application of these category theory functors (expounded in my comment in moderator queue).

I provided a link to an example abstracting validation in Scala, F#, and C#, which is currently stuck in moderator queue. Compare the obnoxious C# version of the code. And the reason is because the C# is not generalized. I intuitively expect that C# case-specific boilerplate will explode geometrically as the program grows.