I have a very simple question. This is not only true with spray-json but I have read similar claims with argonaut and circe. So please enlighten me.
In spray-json, I have come across the statement saying There is no reflection involved. I understand for type class based approach, if the user provides JsonFormat then all is well. But is this claim also true when it comes to using DefaultJsonProtocol?
Because when we you look at this, you can see the usage of clazz.getMethods, clazz.getDeclaredFields, etc. Isn't this the usage of reflection? Though of course thanks to object#apply that we do not need to worry about setting unlike in Java world using reflection. But at least for reading the field names, I do not understand on how reflection can be overlooked.
I'm not very familiar with spray-json, so I won't defend its claims about reflection, which definitely seem to be at odds with the parts of ProductFormats you point to.
I do know more about circe and Argonaut and argonaut-shapeless and Play JSON, all of which do use a kind of reflection to derive codecs for case classes and other user-defined types. The important point is that these libraries don't use runtime reflection—they determine the field names and other information they need at compile time through Scala's macro system.
Generally when people talk about "reflection" in the context of Java or Scala, they mean runtime reflection, but macros also support a kind of reflection, so when I personally talk about how derivation works in these libraries, I try to be careful to specify that there's no runtime reflection involved.
You can argue that compile-time reflection (or metaprogramming, or whatever you want to call it) is much less bad than runtime reflection. It may make your code more complex, and it's very easy to abuse, but it doesn't introduce the same kinds of fragility as runtime reflection, and it doesn't undermine your ability to reason about your code in the same ways that runtime reflection does. If you understand what the macro does (which is a big if), you'll never be surprised at runtime.
Types are fundamentally about rejecting bad potential programs before you run them, and introspection on types at runtime muddles this all up (as Erik Osheim says, "If you meet a Type in the Runtime, kill it"). On the other hand, introspection on types at compile-time is exactly what compilers do, and macros just give you as the programmer a clean way of getting involved in that process (or at least relatively clean, compared to writing compiler plugins, etc.).
There may also be performance benefits to avoiding runtime reflection, but for me personally that's generally a secondary concern—I hate runtime reflection because I've wasted too much of my life debugging horrible Java code that uses horrible Java libraries that depend heavily on runtime reflection—not because runtime reflection might make my programs marginally slower.
That's all a very long-winded way to say that you should read "there is no reflection involved" in this context as "there is no runtime reflection involved" (and even then you shouldn't take the author at their word, I guess, given all that getMethods stuff in spray-json).
I'm wondering why Scala does not have an IO Monad like Haskell.
So, in Scala the return type of method readLine is String whereas in Haskell the comparable function getLine has the return type IO String.
There is a similar question about this topic, but its answer it not satisfying:
Using IO is certainly not the dominant style in scala.
Can someone explain this a bit further? What was the design decision for not including IO Monads to Scala?
Because Scala is not pure (and has no means to enforce that a function is pure, like D has) and allows side effects. It interoperates closely with Java (e.g. reuses big parts of the Java libraries). Scala is not lazy, so there is no problem regarding execution order like in Haskell (e.g. no need for >> or seq). Under these circumstances introducing the IO Monad would make life harder without gaining much.
But if you really have applications where the IO monad has significant advantages, nothing stops you from writing your own implementation or to use scalaz. See e.g. http://apocalisp.wordpress.com/2011/12/19/towards-an-effect-system-in-scala-part-2-io-monad/
[Edit]
Why wasn't it done as a lazy and pure language?
This would have been perfectly possible (e.g. look at Frege, a JVM language very similar to Haskell). Of course this would make the Java interoperability more complicate, but I don't think this is the main reason. I think a lazy and pure language is a totally cool thing, but simply too alien to most Java programmers, which are the target audience of Scala. Scala was designed to cooperate with Java's object model (which is the exact opposite of pure and lazy), allowing functional and mixed functional-OO programming, but not enforcing it (which would have chased away almost all Java programmers). In fact there is no point in having yet another completely functional language: There is Haskell, Erlang, F# (and other MLs) and Clojure (and other Schemes / Lisps), which are all very sophisticated, stable and successful, and won't be easily replaced by a newcomer.
I've been hearing a lot about different JVM languages, still in vaporware mode, that propose to implement reification somehow. I have this nagging half-remembered (or wholly imagined, don't know which) thought that somewhere I read that Scala somehow took advantage of the JVM's type erasure to do things that it wouldn't be able to do with reification. Which doesn't really make sense to me since Scala is implemented on the CLR as well as on the JVM, so if reification caused some kind of limitation it would show up in the CLR implementation (unless Scala on the CLR is just ignoring reification).
So, is there a good side to type erasure for Scala, or is reification an unmitigated good thing?
See Ola Bini's blog. As we all know, Java has use-site covariance, implemented by having little question marks wherever you think variance is appropriate. Scala has definition-site covariance, implemented by the class designer. He says:
Generics is a complicated language feature. It becomes even more
complicated when added to an existing language that already has
subtyping. These two features don’t play very well together in the
general case, and great care has to be taken when adding them to a
language. Adding them to a virtual machine is simple if that machine
only has to serve one language - and that language uses the same
generics. But generics isn’t done. It isn’t completely understood how
to handle correctly and new breakthroughs are happening (Scala is a
good example of this). At this point, generics can’t be considered
“done right”. There isn’t only one type of generics - they vary in
implementation strategies, feature and corner cases.
...
What this all means is that if you want to add reified generics to the
JVM, you should be very certain that that implementation can encompass
both all static languages that want to do innovation in their own
version of generics, and all dynamic languages that want to create a
good implementation and a nice interfacing facility with Java
libraries. Because if you add reified generics that doesn’t fulfill
these criteria, you will stifle innovation and make it that much
harder to use the JVM as a multi language VM.
i.e. If we had reified generics in the JVM, most likely those reified generics wouldn't be suitable for the features we really like about Scala, and we'd be stuck with something suboptimal.
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What are the most commonly held misconceptions about the Scala language, and what counter-examples exist to these?
UPDATE
I was thinking more about various claims I've seen, such as "Scala is dynamically typed" and "Scala is a scripting language".
I accept that "Scala is [Simple/Complex]" might be considered a myth, but it's also a viewpoint that's very dependent on context. My personal belief is that it's the very same features that can make Scala appear either simple or complex depending oh who's using them. Ultimately, the language just offers abstractions, and it's the way that these are used that shapes perceptions.
Not only that, but it has a certain tendency to inflame arguments, and I've not yet seen anyone change a strongly-held viewpoint on the topic...
Myth: That Scala’s “Option” and Haskell’s “Maybe” types won’t save you from null. :-)
Debunked: Why Scala's "Option" and Haskell's "Maybe" types will save you from null by James Iry.
Myth: Scala supports operator overloading.
Actually, Scala just has very flexible method naming rules and infix syntax for method invocation, with special rules for determining method precedence when the infix syntax is used with 'operators'. This subtle distinction has critical implications for the utility and potential for abuse of this language feature compared to true operator overloading (a la C++), as explained more thoroughly in James Iry's answer to this question.
Myth: methods and functions are the same thing.
In fact, a function is a value (an instance of one of the FunctionN classes), while a method is not. Jim McBeath explains the differences in greater detail. The most important practical distinctions are:
Only methods can have type parameters
Only methods can take implicit arguments
Only methods can have named and default parameters
When referring to a method, an underscore is often necessary to distinguish method invocation from partial function application (e.g. str.length evaluates to a number, while str.length _ evaluates to a zero-argument function).
I disagree with the argument that Scala is hard because you can use very advanced features to do hard stuff with it. The scalability of Scala means that you can write DSL abstractions and high-level APIs in Scala itself that otherwise would need a language extension. So to be fair you need to compare Scala libraries to other languages compilers. People don't say that C# is hard because (I assume, don't have first hand knowledge on this) the C# compiler is pretty impenetrable. For Scala it's all out in the open. But we need to get to a point where we make clear that most people don't need to write code on this level, nor should they do it.
I think a common misconception amongst many scala developers, those at EPFL (and yourself, Kevin) is that "scala is a simple language". The argument usually goes something like this:
scala has few keywords
scala reuses the same few constructs (e.g. PartialFunction syntax is used as the body of a catch block)
scala has a few simple rules which allow you to create library code (which may appear as if the language has special keywords/constructs). I'm thinking here of implicits; methods containing colons; allowed identifier symbols; the equivalence of X(a, b) and a X b with extractors. And so on
scala's declaration-site variance means that the type system just gets out of your way. No more wildcards and ? super T
My personal opinion is that this argument is completely and utterly bogus. Scala's type system taken together with implicits allows one to write frankly impenetrable code for the average developer. Any suggestion otherwise is just preposterous, regardless of what the above "metrics" might lead you to think. (Note here that those who I've seen scoffing at the non-complexity of Java on Twitter and elsewhere happen to be uber-clever types who, it sometimes seems, had a grasp of monads, functors and arrows before they were out of short pants).
The obvious arguments against this are (of course):
you don't have to write code like this
you don't have to pander to the average developer
Of these, it seems to me that only #2 is valid. Whether or not you write code quite as complex as scalaz, I think it's just silly to use the language (and continue to use it) with no real understanding of the type system. How else can one get the best out of the language?
There is a myth that Scala is difficult because Scala is a complex language.
This is false--by a variety of metrics, Scala is no more complex than Java. (Size of grammar, lines of code or number of classes or number of methods in the standard API, etc..)
But it is undeniably the case that Scala code can be ferociously difficult to understand. How can this be, if Scala is not a complex language?
The answer is that Scala is a powerful language. Unlike Java, which has many special constructs (like enums) that accomplish one particular thing--and requires you to learn specialized syntax that applies just to that one thing, Scala has a variety of very general constructs. By mixing and matching these constructs, one can express very complex ideas with very little code. And, unsurprisingly, if someone comes along who has not had the same complex idea and tries to figure out what you're doing with this very compact code, they may find it daunting--more daunting, even, than if they saw a couple of pages of code to do the same thing, since then at least they'd realize how much conceptual stuff there was to understand!
There is also an issue of whether things are more complex than they really need to be. For example, some of the type gymnastics present in the collections library make the collections a joy to use but perplexing to implement or extend. The goals here are not particularly complicated (e.g. subclasses should return their own types), but the methods required (higher-kinded types, implicit builders, etc.) are complex. (So complex, in fact, that Java just gives up and doesn't try, rather than doing it "properly" as in Scala. Also, in principle, there is hope that this will improve in the future, since the method can evolve to more closely match the goal.) In other cases, the goals are complex; list.filter(_<5).sorted.grouped(10).flatMap(_.tail.headOption) is a bit of a mess, but if you really want to take all numbers less than 5, and then take every 2nd number out of 10 in the remaining list, well, that's just a somewhat complicated idea, and the code pretty much says what it does if you know the basic collections operations.
Summary: Scala is not complex, but it allows you to compactly express complex ideas. Compact expression of complex ideas can be daunting.
There is a myth that Scala is non-deployable, whereas a wide range of third-party Java libraries can be deployed without a second thought.
To the extent that this myth exists, I suspect it exists among people who are not accustomed to separating a virtual machine and API from a language and compiler. If java == javac == Java API in your mind, you might get a little nervous if someone suggests using scalac instead of javac, because you see how nicely your JVM runs.
Scala ends up as JVM bytecode, plus its own custom library. There's no reason to be any more worried about deploying Scala on a small scale or as part of some other large project as there is in deploying any other library that may or may not stay compatible with whichever JVM you prefer. Granted, the Scala development team is not backed by quite as much force as the Google collections, or Apache Commons, but its got at least as much weight behind it as things like the Java Advanced Imaging project.
Myth:
def foo() = "something"
and
def bar = "something"
is the same.
It is not; you can call foo(), but bar() tries to call the apply method of StringLike with no arguments (results in an error).
Some common misconceptions related to Actors library:
Actors handle incoming messages in a parallel, in multiple threads / against a thread pool (in fact, handling messages in multiple threads is contrary to the actors concept and may lead to racing conditions - all messages are sequentially handled in one thread (thread-based actors use one thread both for mailbox processing and execution; event-based actors may share one VM thread for execution, using multi-threaded executor to schedule mailbox processing))
Uncaught exceptions don't change actor's behavior/state (in fact, all uncaught exceptions terminate the actor)
Myth: You can replace a fold with a reduce when computing something like a sum from zero.
This is a common mistake/misconception among new users of Scala, particularly those without prior functional programming experience. The following expressions are not equivalent:
seq.foldLeft(0)(_+_)
seq.reduceLeft(_+_)
The two expressions differ in how they handle the empty sequence: the fold produces a valid result (0), while the reduce throws an exception.
Myth: Pattern matching doesn't fit well with the OO paradigm.
Debunked here by Martin Odersky himself. (Also see this paper - Matching Objects with Patterns - by Odersky et al.)
Myth: this.type refers to the same type represented by this.getClass.
As an example of this misconception, one might assume that in the following code the type of v.me is B:
trait A { val me: this.type = this }
class B extends A
val v = new B
In reality, this.type refers to the type whose only instance is this. In general, x.type is the singleton type whose only instance is x. So in the example above, the type of v.me is v.type. The following session demonstrates the principle:
scala> val s = "a string"
s: java.lang.String = a string
scala> var v: s.type = s
v: s.type = a string
scala> v = "another string"
<console>:7: error: type mismatch;
found : java.lang.String("another string")
required: s.type
v = "another string"
Scala has type inference and refinement types (structural types), whereas Java does not.
The myth is busted by James Iry.
Myth: that Scala is highly scalable, without qualifying what forms of scalability.
Scala may indeed be highly scalable in terms of the ability to express higher-level denotational semantics, and this makes it a very good language for experimentation and even for scaling production at the project-level scale of top-down coordinated compositionality.
However, every referentially opaque language (i.e. allows mutable data structures), is imperative (and not declarative) and will not scale to WAN bottom-up, uncoordinated compositionality and security. In other words, imperative languages are compositional (and security) spaghetti w.r.t. uncoordinated development of modules. I realize such uncoordinated development is perhaps currently considered by most to be a "pipe dream" and thus perhaps not a high priority. And this is not to disparage the benefit to compositionality (i.e. eliminating corner cases) that higher-level semantic unification can provide, e.g. a category theory model for standard library.
There will possibly be significant cognitive dissonance for many readers, especially since there are popular misconceptions about imperative vs. declarative (i.e. mutable vs. immutable), (and eager vs. lazy,) e.g. the monadic semantic is never inherently imperative yet there is a lie that it is. Yes in Haskell the IO monad is imperative, but it being imperative has nothing to with it being a monad.
I explained this in more detail in the "Copute Tutorial" and "Purity" sections, which is either at the home page or temporarily at this link.
My point is I am very grateful Scala exists, but I want to clarify what Scala scales and what is does not. I need Scala for what it does well, i.e. for me it is the ideal platform to prototype a new declarative language, but Scala itself is not exclusively declarative and afaik referential transparency can't be enforced by the Scala compiler, other than remembering to use val everywhere.
I think my point applies to the complexity debate about Scala. I have found (so far and mostly conceptually, since so far limited in actual experience with my new language) that removing mutability and loops, while retaining diamond multiple inheritance subtyping (which Haskell doesn't have), radically simplifies the language. For example, the Unit fiction disappears, and afaics, a slew of other issues and constructs become unnecessary, e.g. non-category theory standard library, for comprehensions, etc..
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...