In this question from 2013, Mr. Odersky notes that "it's too early to tell" whether libraries like Scalaz will be able to exist (at least in their current state) under Dotty, due to the castration of higher-kinded and existential types.
In the time passed, has Dotty's implications for Scalaz & Cats been elucidated? Will proposed features like built-in Effects and Records change the scope of these projects?
I understand that Dotty is still a ways off from replacing scalac, but as I am considering investing time applying purely functional constructs and methodologies to my work, I believe it important to consider the future of its flagship libraries.
One example of the latest on Dotty is "Scaling Scala" By Chris McKinlay (December 15, 2016) (the same article also mention the Scalaz and Cats situation)
Martin Odersky has been leading work on Dotty, a novel research compiler based on the Dependent Object Types (DOT) calculus (basically a simplified version of Scala) and ideas from the functional programming (FP) database community.
The team working on Dotty development has shown some remarkable improvements over the state of the art, most notably with respect to compilation times. I asked Odersky what he thought was novel about the Dotty architecture and would help end users. Here’s what he said:
Two things come to mind:
first, it's closely related to formal foundations, giving us better guidance on how to design a sound typesystem. This will lead to fewer surprises for users down the road.
Second, it has an essentially functional architecture. This makes it easier to extend, easier to get correct, and will lead to more robust APIs where the compiler is used as a service for IDEs and meta programming.
Although Dotty opens up a number of interesting language possibilities (notably full-spectrum dependent types, a la Agda and Idris), Odersky has chosen to prioritize making it immediately useful to the community. Language differences are fairly small, and most of them are in order to either simplify the language (like removing procedure syntax) or fix bugs (unsound pattern matching) or both (early initializers).
Still, I couldn’t resist asking him if there is any chance of full-spectrum dependent types ending up in Scala at some point. Here is what he said:
Never say never :-). In fact, we are currently working with Viktor Kuncak on integrating the Leon program prover with Scala, which demands richer dependent types than we have now. But it's currently strictly research, with a completely open outcome.
The Scala and Dotty teams are working closely toward convergence for Scala 2.x and Dotty, and they’ve indicated that they take continuity very seriously. Scala 2.12 and 2.13 have language flags that unlock features being incubated in Dotty (e.g., existential types), and the Dotty compiler has a Scala 2 compatibility mode. There’s even a migration tool.
In hist recent talk at Strange Loop Martin Odersky shed the light on his vision of Scala's future version called Dotty. I understand this is work-in-progress and it even may not flow into Scala (at least not very fast) due to many possible backward-compatibility issues. But if it happens, how should we program in Scala today to be forward-compatible with Dotty? I didn't get all the ideas from the talk so I'd like someone more profound to summarize the changes and describe how can we prepare to them.
The only other advice I would give is to also stay away from structural types containing a type members. Those type members are the elements that make type-checking in Scala turing complete, and I am not yet sure whether we want to support that in Dotty. Structural types that contain only vals and defs will likely stay supported.
Towards the end of the talk, he says "if your program is compiled without any language feature imports for higher-kinded and existentials then you should be ok".
So if I understand it correctly the only thing you would need to be aware of is any possible usage of existential types or higher-kinded types in your code.
I've heard people claim that:
Scala's type system is amazing (existential types, variant, co-variant)
Because of the power of macros, everything is a library in Clojure: (pattern matching, logic programming, non-determinism, ..)
Question:
If both assertions are true, why is Scala's type system not a library in Clojure? Is it because:
types are one of these things that do not work well as a library? [i.e. the changes would somehow have to threaded through every existing clojure library, including clojure.core?]
is Scala's notion of types fundamentally incompatible with clojure protocol / records?
... ?
It's an interesting question.
You are certainly right about Scala having an amazing type system, and about Clojure being phenomenal for meta-programming and extension of the language (although that is about more than just macros....).
A few reasons I can think of:
Clojure is a dynamically typed language while Scala is a statically typed language. Having powerful type inference isn't so much use in a language where you can assume relatively little about the types of your inputs.
Clojure already has a very interesting project to add typing as a library (Typed Clojure) which looks very promising - however it's very different in approach to Scala as it is designed for a dynamic language from the start (inspired more by Typed Racket, I believe).
Clojure philosophy actually discourages certain OOP concepts (particularly implementation inheritance, mutable objects, and data encapsulation). A type system that supports these things (as Scala does) wouldn't be a good fit for Clojure idioms - at best they would be ignored, but they could easily encourage a style of development that would cause people to run into severe problems later.
Clojure already provides tools that solve many of the problems you would typically solve with types in other languages - e.g. the use of protocols for polymorphism.
There's a strong focus in the Clojure community on simplicity (in the sense of the excellent video "Simple Made Easy" - see particularly the slide at 39:30). While Scala's type system is certainly amazing, I think it's a stretch to describe it as "Simple"
Putting in a Scala-style type system would probably require a complete rewrite of the Clojure compiler and make it substantially more complex. Nobody seems to have signed up so far to take on that particular challenge... and there's a risk that even if someone were willing and able to do this then the changes could be rejected for the various cultural / technical reasons covered above.
In the absence of a major change to Clojure itself (which I think would be unlikely) then one interesting possibility would be to create a DSL within Clojure that provided Scala-style type inference for a specific domain and compiled this DSL direct to optimised Java bytecode. I could see that being a useful approach for specific problem domains (large scale numerical data crunching with big matrices, for example).
To simply answer your question "... why is Scala's type system not a library in Clojure?":
Because the type system is part of the scala compiler and not of the scala library. The whole power of scalas type system only exists at compile time. The JVM has no support for things like that, because of type erasure and also, because it would simply slow down execution. And also there is no need for it. If you have a statically typed language, you don't need type information at runtime, unless you want to do dirty stuff.
edit:
#mikera the jvm is sure capable of running the scala compiler, I did not say anything like that. I just said, that the jvm has no support for type systems like that. It does not even support generics. At runtime all these types are gone. The compiler checks for the correctness of a program and removes all the higher kinded types / generics.
example:
val xs: List[Int] = List(1,2,3,4)
val x1: Int = xs.head
will at runtime look like this:
val xs: List = List.apply(1,2,3,4)
val x1: Int = xs.head.asInstanceOf[Int]
But it doesn't matter, because the compiler checked it before. You can only get in trouble here, when you use reflection, because you could put any value in the list and it would break at runtime exactly where the value is casted to Int.
And this is one of the reasons, why the scala type system is not part of the scala library, but built into the compiler.
And also the question of the OP was "... why is Scala's type system not a library in Clojure?" and not "Is it possible to create a type system such as scalas for clojure?" and I perfectly answered that question.
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.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 9 years ago.
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..