Wednesday, November 23, 2011

Scalaz Seven Functor Feels like Seven Samurai

Jason Zaugg and all his scalaz folks are working on scalaz seven at the moment. Although they have started quite a couple of times ago, I only had a chance to play with the library only today.

In case you don't know scalaz, it's actually a popular but often misunderstood library written in scala. It's not haskell standard library ported to scala, but it's highly inspired by the haskell standard library though. I recommend you to have a look here scalaz . One recommendation: take your time, don't be hurry to understand, otherwise you will end up associate scalaz to banana [1], to rabbit [2], or to ejb2 [3]. Of course, scalaz is not ready for production, because you know ... you cannot persist a scalaz object using Hibernate.

I start with one of the simplest scalaz type class, called Functor. It sounds scarry, right ? Don't worry, think of Functor as a way to lift a function that maps A to B to something that map "container" of A to "container" of B. If you have a function that maps, from int to int, say, a functor allows mapping from List of integer to a list of integer by mapping each element of the list.

Say, you have a function incr:

val incr:Int=>Int= x => x + 1

a Functor[List] allows you to map every element in a list using that function like this:

val xs = List(5, 6, 7)
Functor[List].map(xs, incr)   // 6, 7, 8

Well, yeah, is that all? No ! I can also have this:

val eitX:Either[String, Int] = Right(6)
val eitY:Either[String, Int] = Left("Err")
Functor[Either].map(eitX, incr) // Right(7)
Functor[Either].map(eitY, incr) // Left("Err")

It starts to be interesting right, because you don't have map for Either in Scala standard library. Fine, is it great ?  No, it sucks. Function[Either] or Function[List] suck. We don't want it. Fortunately, scalaz comes wi th some magics that simplify the thing

(wait a minute? Scalaz ? Simplifies something? You must be kidding ?)

With the super complex highly intelligent Scalaz (as it is perceived  by many), indeed life is simpler, we can directly write:

val eitX:Either[String, Int] = Right(6)
val eitY:Either[String, Int] = Left("Err")             // Right(7)             // Left("Err")

That works, because Either, List, Option are instances of Functor (you know, the container that allows you to lift a function).

Is that all ? No. There are something even more interesting. Imagine if we have a List of Option, is it a functor ? Oh yes, it turns out that the composition of functors is a functor. Let's check:

val listCompOpt = Functor[List].compose(Functor[Option])
val ys = List(Some(1), Some(3), None, Some(6))
val zs =  // List(Some(2), Some(4), None, Some(7))

Yeah! What about the product ?

val listOptProdF = Functor[List].product(Functor[Option])
val ts = (List(1,2),Some(1))
val us = (List(3,7, 2, 1),None)   // (List(2, 3),Some(2))   // (List(4, 8, 3, 2),None)

Oh, Cool.

OK, to finish this post, scalaz-seven (at least the version I was playing with) provides a couple of interesting functions like strengthL, strengthR, and fpair.

Here they are:

val xEit:Either[String, Int] = Right(3)
val yEit:Either[String, Int] = Right(4)
println(xEit.strengthL("Help"))    // Right("Help",4)
println(yEit.strengthL("Help"))    // Right(("Help",3))
val xOpt:Option[Int] = Some(4)
val yOpt:Option[Int] = None
println(xOpt.strengthR("my godness"))   // Some(4, "my godness") println(yOpt.strengthR("My godness"))  // None

println(xOpt.fpair)     // Some(4,4)
println(xEit.fpair)     // Right(3,3)

I  hope to be able to come with other posts in this series. No promise, but stay tuned.

[1] Banana in this article context is not a scala or a java library, it is a fruit name. In case you're not familiar with it, check this article. You may wonder how could one wrongly associate scalaz with banana. Well, that may happen, who knows.

[2] Rabbit in this article context is not a scala or a java library, not even a cool javascript library, it is a name of an animal. Again, wikipedia is helpful in case you're not familiar with it. Well, again, it may happen that you associate scalaz to rabbit, who knows.

[3] EJB2 is a server side component, usually managed by the application server. It is used mainly in enterprise application, and by the way the 'E' is Enterprise. You should not wonder why you might associate scalaz to EJB2, it happened !