How does the Cats library in Scala relate to scalaz?

How does the Cats library in Scala relate to scalaz? - scala

How does the Cats library relate to scalaz? The Cats project mentions it is descended from scalaz.

I would like to keep this from getting too political*, but cats is for all intents and purposes scalaz. It has not reached full parity as of yet, but keep in mind it was only created a few months ago. The goal is for it to be a more pragmatic approach and more democratic when it comes to its evolution. So, naming of operators and classes is hopefully going to be a little more straightforward, as well as it has no qualms with using mutable data within a method if it means better performance. Last, they are HOPING to have better documentation....all of this means that it may end up becoming a replacement for scalaz with a better beginner's approach for those not embroiled in the math world. If you want a fuller answer, then maybe head over to their gitter board and Erik (non) could answer it himself :)
*The gist is that scalaz has some social baggage with it that causes a number of big names to shy away from using and/or contributing.

just want to mention here that fairly recently scalaz became as well a namespace for a bunch of new, fp libraries, like testz, scalaz-zio, scalaz-metrics, scalaz-http, scalaz-analytics and more. And ScalaZ 8 is on the way!

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Pure FP in Scala?

I was under the impression that there are folks out there that do write pure applications using Scalaz, but based on this example: [ stacking StateT in scalaz ], it looks like anything real would also be impossibly hairy.
Are there any guidelines or examples of real, modular, loosely-coupled, pure applications in Scala? I'm expecting that this means scalaz.effect.SafeApp and RWST over IO, but I'd like to hear from folks who have done it.
Thanks.
Edit: In the absence of an answer, I've started collecting resources as an answer below. If you have any examples or related links to contribute, please do.

i think you are mixing two different things. one is pure functional programming and second is scala type system. you can do 'pure' programming in any language, even in java. if the language is funvtional than you will have pure functional programming.
does it make your programs work faster? depends on the program - it scales better but for single threaded parts you will rather loose performance.
does it 'save your cognition'? it depends on how good you are in what you are doing. if you work with FP, monads, arrows etc on the daily basis then i assume it may help significantly. if you show the code to the OO developer he probably won't understand anything.
does it save the development time? as previously, i think it may but to be honest it doesn't matter that much. you more often read the code rather than write it
can you do useful stuff in PFP? yes, some companies makes money on haskell
and now, can it be done in scala? for sure. will anyone do it in scala? probably not because it's too easy to break the purity, because type system is too weak and because there are better, 'more pure' tools for it (but currently not on jvm)

I guess I will start collecting resources here, and update as I find more.
Functional Reactive Programming: stefan hoeck's blog, github, examples
Monadic effect worlds for interacting safely with mutable data. (tpolecat)
Mellow database access for Scala (tpolecat)
Dependency Injection without the Gymnastics (tony, rúnar)
Google search for "extends SafeApp"

How to enforce Functional Programming on Scala

I'm starting to learn Functional Programming and would like to do so with Scala, not Haskell or Lisp.
But some people claim that learning Scala as the first functional language slows down your learning of Functional Programming, because Scala allows you to program both ways, and one tends to program the procedural way when confronted with a hard problem.
How can i make sure that 'm programming in a purely functional way? Maybe, due to not being able to properly distinguish both styles, I'll inadvertently program procedurally).
I know, for example, that I should only use vals and not vars.

The other answers have made some good points, but for an attempt to quickly get down some guidelines, here's how I'd start:
Firstly, some things to completely avoid:
Don't use the var keyword.
Don't use the while keyword.
Don't use anything in the scala.collection.mutable package.
Don't use the asInstanceOf method.
Don't use null. If you ever come across null (in someone else's code), immediately wrap it in a more appropriate datatype (usually Option will do nicely).
Then, a couple of things to generally avoid:
Be wary of calling anything with a return type of Unit. A function with a return type of Unit is either doing nothing, or acting only by side-effects. In some cases you won't be able to avoid this (IO being the obvious one), but where you see it elsewhere it's probably a sign of impurity.
Be wary of calling into Java libraries - they are typically not designed with functional programming in mind, and will often require you to abandon the functional approach.
Once you've avoided these things, what can you do to move your code to being more functional?
When you're performing direct recursion, look for opportunities to generalise it through the use of higher order combinators. fold is likely your biggest candidate here - most operations on lists can be implemented in terms of a suitable fold.
When you see destructuring operations on a data structure (typically through pattern matching), consider whether instead you can lift the computation into the structure and avoid destructuring it. An obvious example is the following code snippet:
foo match {
case Some(x) => Some(x + 2)
case None => None
}
can be replaced with:
foo map ( _ + 2 )

I dare say that your goal is already misleading:
I'm starting to learn Function Programming, and I really wanna learn
Scala, not Haskell or Lisp.
If you are really interested in learning concepts of function programming, then why not use a language such as Haskell that (more or less) does not allow you to use procedural or object-oriented concepts? In the end, the language is "just" a tool that helps you learning concepts of FP, you could just as well read loads of papers about FP. At least theoretically, I think it is usually easier to learn concepts of computer science with concrete tools at hand.
On the other hand, if you are interested in learning the language Scala, then why not use all features that it offers, regardless of whether they stem from the FP or the OO world?
In order to conclude with a somewhat practical advise: You could search for FP tutorials that use Scala or for blog entries etc. that describe how to realise certain FP-concepts in Scala and try to follow them. This way, it is less likely that you make use of non-FP concepts.

You don't buy a Ferarri to deliver furniture. Scala's fundamental strength is the fact that in your words, it goes both ways:). Whether or not you are programming in a functional style is decided by the techniques you use.
The best thing you can do is thoroughly review fundamental concepts of functional programming and seek the appropriate Scala implementation of the respective concepts. But if you want to program purely functional style, then go for Haskell, Lisp, Erlang, OCaml, or whatever other purely functional dialect.
Functional programming
Introduction
Functional thinking
Scala
If you want to learn Scala, then make sure to include both OO and FP in your learning curve. Lambda expressions + OO concepts + syntactic sugar made possible by IMHO the most advanced compiler on the face of the planet lead to something quite amazing. Take advantage of it!

I think learning is non linear process, it helps to see lots of ways of doing the same thing, also be opportunistic and use any learning resources that are available for you. For example Martin Odersky the creator of Scala offers a free course called "Functional Programming Principles in Scala" https://class.coursera.org/progfun-002/class/index there are some very high quality video lectures, and some really good assignments where the automated grader will tell you that your code is not functional enough and you loose style points because you are using var instead of val
I think the thing you want to focus on is learning the Functional Programming Paradigm and for me learning a paradigm is about learning what types of problems are easy to solve in one paradigm and are hard to solve in another paradigm. Focus on the paradigm and I think you will find that learning both about Haskell and Scala will teach you the functional paradigm faster, because you will be able to ask the question what are the common features between Scala and Haskell, what are the differences .... etc

I know, for example, that I should only use vals and not vars.
That's already a good start, other non-so-functional things to avoid are mutable collections and loops.
Have a look at immutable collections and recursion instead.
Of course, once you are familiar with the functional concepts, there might also be good reasons to use scala's non-functional features.

Real World Functional Programming in Scala

Soooo...
Semigroups, Monoids, Monads, Functors, Lenses, Catamorphisms, Anamorphisms, Arrows... These all sound good, and after an exercise or two (or ten), you can grasp their essence. And with Scalaz, you get them for free...
However, in terms of real-world programming, I find myself struggling to find usages to these notions. Yes, of course I always find someone on the web using Monads for IO or Lenses in Scala, but... still...
What I am trying to find is something along the "prescriptive" lines of a pattern. Something like: "here, you are trying to solves this, and one good way to solve it is by using lenses this way!"
Suggestions?
Update: Something along these lines, with a book or two, would be great (thanks Paul): Examples of GoF Design Patterns in Java's core libraries

The key to functional programming is abstraction, and composability of abstractions. Monads, Arrows, Lenses, these are all abstractions which have proven themselves useful, mostly because they are composable. You've asked for a "prescriptive" answer, but I'm going to say no. Perhaps you're not convinced that functional programming matters?
I'm sure plenty of people on StackOverflow would be more than happy to try and help you solve a specific problem the FP way. Have a list of stuff and you want to traverse the list and build up some result? Use a fold. Want to parse XML? hxt uses arrows for that. And monads? Well, tons of data types turn out to be Monads, so learn about them and you'll discover a wealth of ways you can manipulate these data types. But its kind of hard to just pull examples out of thin air and say "lenses are the Right Way to do this", "monoids are the best way to do that", etc. How would you explain to a newbie what the use of a for loop is? If you want to [blank], then use a for loop [in this way]. It's so general; there are tons of ways to use a for loop. The same goes for these FP abstractions.
If you have many years of OOP experience, then don't forget you were once a newbie at OOP. It takes time to learn the FP way, and even more time to unlearn some OOP tendencies. Give it time and you will find plenty of uses for a Functional approach.

I gave a talk back in September focused on the practical application of monoids and applicative functors/monads via scalaz.Validation. I gave another version of the same talk at the scala Lift Off, where the emphasis was more on the validation. I would watch the first talk until I start on validations and then skip to the second talk (27 minutes in).
There's also a gist I wrote which shows how you might use Validation in a "practical" application. That is, if you are designing software for nightclub bouncers.

I think you can take the reverse approach and instead when writing a small piece of functionality, ask yourself whether any of those would apply: Semigroups, Monoids, Monads, Functors, Lenses, Catamorphisms, Anamorphisms, Arrows... A lots of those concepts can be used in a local way.
Once you start down that route, you may see usage everywhere. For me, I sort of get Semigroups, Monoids, Monads, Functors. So take the example of answering this question How do I populate a list of objects with new values. It's a real usage for the person asking the question (a self described noob). I am trying to answer in a simple way but I have to refrain myself from scratching the itch "there are monoids in here".
Scratching it now: using foldMap and the fact that Int and List are monoids and that the monoid property is preserved when dealing with tuple, maps and options:
// using scalaz
listVar.sliding(2).toList.foldMap{
case List(prev, i) => Some(Map(i -> (1, Some(List(math.abs(i - prev))))))
case List(i) => Some(Map(i -> (1, None)))
case _ => None
}.map(_.mapValues{ case (count, gaps) => (count, gaps.map(_.min)) })
But I don't come to that result by thinking I will use hard core functional programming. It comes more naturally by thinking this seems simpler if I compose those monoids combined with the fact that scalaz has utility methods like foldMap. Interestingly when looking at the resulting code it's not obvious that I'm totally thinking in terms of monoid.

You might like this talk by Chris Marshall. He covers a couple of Scalaz goodies - namely Monoid and Validation - with many practical examples. Ittay Dror has written a very accessible post on how Functor, Applicative Functor, and Monad can be useful in practice. Eric Torreborre and Debasish Gosh's blogs also have a bunch of posts covering use cases for categorical constructs.
This answer just lists a few links instead of providing some real substance here. (Too lazy to write.) Hope you find it helpful anyway.

I understand your situation, but you will find that to learn functional programming you will need to adjust your point of view to the documentation you find, instead of the other way around. Luckily in Scala you have the possibility of becoming a functional programmer gradually.
To answer your questions and explain the point-of-view difference, I need to distinguish between "type classes" (monoids, functors, arrows), mathematically called "structures", and generic operations or algorithms (catamorphisms or folds, anamorphisms or unfolds, etc.). These two often interact, since many generic operations are defined for specific classes of data types.
You look for prescriptive answers similar to design patterns: when does this concept apply? The truth is that you have surely seen the prescriptive answers, and they are simply the definitions of the different concepts. The problem (for you) is that those answers are intrinsically different from design patterns, but it is so for good reasons.
On the one hand, generic algorithms are not design patterns, which suggest a structure for the code you write; they are abstractions defined in the language which you can directly apply. They are general descriptions for common algorithms which you already implement today, but by hand. For instance, whenever you are computing the maximum element of a list by scanning it, you are hardcoding a fold; when you sum elements, you are doing the same; and so on. When you recognize that, you can declare the essence of the operation you are performing by calling the appropriate fold function. This way, you save code and bugs (no opportunity for off-by-one errors), and you save the reader the effort to read all the needed code.
On the other hand, structures concern not the goal you have in mind but properties of the entities you are modeling. They are more useful for bottom-up software construction, rather than top-down: when defining your data, you can declare that it is a e.g. a monoid. Later, when processing your data, you have the opportunity to use operations on e.g. monoids to implement your processing. In some cases it is useful to strive to express your algorithm in terms of the predefined ones. For instance, very often if you need to reduce a tree to a single value, a fold can do most or all of what you need. Of course, you can also declare that your data type is a monoid when you need a generic algorithm on monoids; but the earlier you notice that, the earlier you can start reusing generic algorithms for monoids.
Last advice is that probably most of the documentation you will find about these concepts concerns Haskell, because this language has been around for much more time and supports them in a quite elegant way. Quite recommended here are Learn you a Haskell for Great Good, a Haskell course for beginners, where among others chapters 11 to 14 focus on some type classes, and Typeclassopedia (which contains links to various articles with specific examples). EDIT: Finally, an example of applications of Monoids, taken from Typeclassopedia, is here: http://apfelmus.nfshost.com/articles/monoid-fingertree.html. I'm not saying there is little documentation for Scala, just that there is more in Haskell, and Haskell is where the application of these concepts to programming was born.

Can you suggest any good intro to Scala philosophy and programs design?

In Java and C++ designing program's objects hierarchy is pretty obvious. But beginning Scala I found myself difficult to decide what classes to define to better employ Scala's syntactic sugar facilities (an even idealess about how should I design for better performance). Any good readings on this question?

I have read 4 books on Scala, but I have not found what you are asking for. I guess you have read "Programming in Scala" by Odersky (Artima) already. If not, this is a link to the on-line version:
http://www.docstoc.com/docs/8692868/Programming-In-Scala
This book gives many examples how to construct object-oriented models in Scala, but all examples are very small in number of classes. I do not know of any book that will teach you how to structure large scale systems using Scala.
Imperative object-orientation has
been around since Smalltalk, so we
know a lot about this paradigm.
Functional object-orientation on the
other hand, is a rather new concept,
so in a few years I expect books
describing large scale FOO systems to
appear. Anyway, I think that the PiS
book gives you a pretty good picture
how you can put together the basic
building blocks of a system, like
Factory pattern, how to replace the
Strategy pattern with function
literals and so on.
One thing that Viktor Klang once told me (and something I really agree upon) is that one difference between C++/Java and Scala OO is that you define a lot more (smaller) classes when you use Scala. Why? Because you can! The syntactic sugar for the case class result in a very small penalty for defining a class, both in typing and in readability of the code. And as you know, many small classes usually means better OO (fewer bugs) but worse performance.
One other thing I have noticed is that I use the factory pattern a lot more when dealing with immutable objects, since all "changes" of an instance results in creating a new instance. Thank God for the copy() method on the case class. This method makes the factory methods a lot shorter.
I do not know if this helped you at all, but I think this subject is very interesting myself, and I too await more literature on this subject.
Cheers!

This is still an evolving matter. For instance, the just released Scala 2.8.0 brought support of type constructor inference, which enabled a pattern of type classes in Scala. The Scala library itself has just began using this pattern. Just yesterday I heard of a new Lift module in which they are going to try to avoid inheritance in favor of type classes.
Scala 2.8.0 also introduced lower priority implicits, plus default and named parameters, both of which can be used, separately or together, to produce very different designs than what was possible before.
And if we go back in time, we note that other important features are not that old either:
Extractor methods on case classes object companions where introduced February 2008 (before that, the only way to do extraction on case classes was through pattern matching).
Lazy values and Structural types where introduced July 2007.
Abstract types support for type constructors was introduced in May 2007.
Extractors for non-case classes was introduced in January 2007.
It seems that implicit parameters were only introduced in March 2006, when they replaced the way views were implemented.
All that means we are all learning how to design Scala software. Be sure to rely on tested designs of functional and object oriented paradigms, to see how new features in Scala are used in other languages, like Haskell and type classes or Python and default (optional) and named parameters.
Some people dislike this aspect of Scala, others love it. But other languages share it. C# is adding features as fast as Scala. Java is slower, but it goes through changes too. It added generics in 2004, and the next version should bring some changes to better support concurrent and parallel programming.

I don't think that there are much tutorials for this. I'd suggest to stay with the way you do it now, but to look through "idiomatic" Scala code as well and to pay special attention in the following cases:
use case classes or case objects instead of enums or "value objects"
use objects for singletons
if you need behavior "depending on the context" or dependency-injection-like functionality, use implicits
when designing a type hierarchy or if you can factor things out of a concrete class, use traits when possible
Fine grained inheritance hierarchies are OK. Keep in mind that you have pattern matching
Know the "pimp my library" pattern
And ask as many questions as you feel you need to understand a certain point. The Scala community is very friendly and helpful. I'd suggest the Scala mailing list, Scala IRC or scala-forum.org

I've just accidentally googled to a file called "ScalaStyleGuide.pdf". Going to read...

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