Shapeless Monads
Small retrospection
In my previous blog post we talked about shapeless and how it could be applied to enhance how you work with Futures
. Even though we were focused on Future
s, our goal wasn’t to provide the best and ultimate util to deal with them, but to demonstrate how shapeless can help us build functions that are more flexible than almost everything we are used to work with.
So in last post we have created a kind of varargs function that is able to adjust its return type to the arguments passed in. Today we’ll take it much farther by adding scalaz, ApplicativeBuilder
and Monad
s into the soup. Let’s talk shapeless Monads!
Scalaz? Applicative Builder?
Yes, Scalaz, Applicative Builder… Scalaz is library that, to quote the definition the authors have given, “provides purely functional data structures to complement those from the Scala standard library. It defines a set of foundational type classes (e.g. Functor, Monad) and corresponding instances for a large number of data structures”.
Most people associate it with cryptic operators to work with these “purely functional data structures”: |@|
(so called Macaulay Culkin), <*>
, <|
etc. and it’s what scares the hell out of many people. I find it unfair. In fact, scalaz provides many very useful utils (some of which are actually very basic), type classes and other concepts and I highly encourage you to get familiar with it. This is probably one of the best training resources on the web.
We won’t even try to walk through every aspect of it (is there even a man who would be able to do it?). I’m mentioning it, because some people pointed out that the same or similar effect could be obtained using scalaz’s ApplicativeBuilder
pattern. This is an excerpt from comment by @caente (an engineer at x.ai, a blogger and a guy always willing to help:) ) aiming to support this opinion:
https://gist.github.com/tomaszperek/b5bb02332bf566e0c00b
Well, my answer for this claim is “yes, but no” :) It’s not exactly the same thing, and it’s more verbose from one point of view, yet more flexible from the other, as it allows you to apply a function right away. Important thing is that this comment actually gave me an idea for this post. We’ll focus on ApplicativeBuilder
to prove some thesis.
Thesis
So here’s the thesis: With shapeless you can create code, that is much more flexible, and much more compact than it would be if you didn’t use shapeless. It won’t introduce new design patterns or paradigms nor will make any old ones obsolete, yet there will be big benefit in much less lines of code and much bigger flexibility. In order to advocate for this thesis we will try to implement our own version of ApplicativeBuilder
First steps
Before we start, let’s say what’s wrong with the code from last post. It’s only for Futures
. The advantage ApplicativeBuilder
has over our hsequence
/ zip
functions is that it works with any class belonging to Applicative
type class. So it will work with Option
s, List
s and Future
s, while zip
is only for Future
s. So let’s remove this constraint.
Our base trait will now look as follows:
https://gist.github.com/tomaszperek/adc5dfbcc41026fe3818
What has changed? Besides the name, which now reflects that the code is no longer intended to work only with Future
s, the trait has one more type parameter, M[_]
, a type constructor.
Also the return value of hsequence
method has changed from Future[Out]
to M[Out]
. Let’s ignore ToMonadOps
for a while. It’s an useful tool providing implicit conversions for instances of M, but these details aren’t needed in this discussion. Now, as a base case, implementation for HNil
:
https://gist.github.com/tomaszperek/136404741c38c27b9986
First of all, since object
s can’t have type parameters, we must use def
, but that’s not a big deal. The other thing new is evidence m: Monad[M]
. This puts a constraint on M
, we want it to belong to Monad
type class.
Why Monad
? It’s because we need access to bind
, pure
, and map
functions. That’s making our life a bit easier, as original ApplicativeBuilder
works with Applicative
s, but not having access to flatMap
would disperse our focus. In the code above we use m.pure(HNil)
to construct M[HNil]
. We couldn’t use constructor, because we simply don’t know what exact type M
will be.
What about longer HList
s? Here’s the code:
https://gist.github.com/tomaszperek/6e06f8aa1cf63a54e0c2
It’s getting a bit crowded in type parameters section, and in implicit params too, but there’s only one new thing: M[_]
parameter and evidence that it is a Monad
. The implementation of hsequence
isn’t really that much different from what we had for Future
s. We use flatMap
(again, that’s why we needed M
to be a Monad
, not just Applicative
) and map
just as we had with Future
s. Let’s put it together:
https://gist.github.com/tomaszperek/4765f33adae56250c5e1
All in all it’s just a little bit different than it was before. The same goes for implementations of hsequence
and zip
functions. We are introducing one more type parameter, M[_]
and providing one more evidence, that M
is a Monad
:
https://gist.github.com/tomaszperek/7917f660173a37c0950b
But now we won’t be limited to Futures
, as following ScalaTest
spec shows:
https://gist.github.com/tomaszperek/4ebeafb121bc8e8ead56
It may be a bit surprising how it works with List
s, but’s exactly the same as if you used |@|
from scalaz. One challenge solved, we have our zip
/hsequence
working with everything that belongs to Monad
type class.
Our Own Applicative Builder
We are now just one step away from giving life to our own ApplicativeBuilder
. But before we do it, let’s recall what it actually is. It’s a trait, that allows you to combine Applicative
s using |@|
operator and then either return tuple with combined applicative or execute some function on it, just as it was demonstrated in @caente’s comment.
Now, why are we reimplementing it?. This is why: its implementation. The code is ~170 lines long and it’s a pyramid that is just begging to be simplified. You have just one mean to do it, and it’s type-level programming. I won’t make you wait any longer – here’s the code:
https://gist.github.com/tomaszperek/994cced1161d826d24a1
We just only need an implicit def
to convert Monad
s to single-element ScalacApplicativeBuilder
:
https://gist.github.com/tomaszperek/c66359b1341767e7ef94
and that’s actually it.How does it work? First important thing is that it maintains a HList
of Monad
s in values
field. Method :@:
just adds one more item this list. asTuple
is calling our old friend hsequece
on values and turns the result into Tuple
. apply
transforms given function argument into HList
equivalent and then maps results of hsequence
with that equivalent.
All these ‘shapeless magic tricks’ are nothing new, as they were discussed in our last post. And that’s that, there’s nothing else.
Now, did we prove our thesis? Together with imports, the full and standalone implementation of our ScalacApplicativeBuilder
is only 42 lines long:
https://gist.github.com/tomaszperek/fc4b0de272da095c3da4
It allows doing the same things as original Applicative Builder and is not limited to 12 elements. Let’s just take a quick look on specs:
https://gist.github.com/tomaszperek/9e6630029699982180be
As we can see, we can do what we could with original ApplicativeBuilder
and more.So in my humble opinion that’s a Q.E.D. :) I hope you enjoyed reading this post as much as I did writing it, but what I hope even more, is that I have managed to convince at least some people to look kindly on shapeless.
Thank you!
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