What is the best way to convert a tuple into an array in Scala? Here "best" means in as few lines of code as possible. I was shocked to search Google and StackOverflow only to find nothing on this topic, which seems like it should be trivial and common. Lists have a a toArray function; why don't tuples?
Use productIterator, immediately followed by toArray:
(42, 3.14, "hello", true).productIterator.toArray
gives:
res0: Array[Any] = Array(42, 3.14, hello, true)
The type of the result shows the main reason why it's rarely used: in tuples, the types of the elements can be heterogeneous, in arrays they must be homogeneous, so that often too much type information is lost during this conversion. If you want to do this, then you probably shouldn't have stored your information in tuples in the first place.
There is simply almost nothing you can (safely) do with an Array[Any], except printing it out, or converting it to an even more degenerate Set[Any]. Instead you could use:
Lists of case classes belonging to a common sealed trait,
shapeless HLists,
a carefully chosen base class with a bit of inheritance,
or something that at least keeps some kind of schema at runtime (like Apache Spark Datasets)
they would all be better alternatives.
In the somewhat less likely case that the elements of the "tuples" that you are processing frequently turn out to have an informative least upper bound type, then it might be because you aren't working with plain tuples, but with some kind of traversable data structure that puts restrictions on the number of substructures in the nodes. In this case, you should consider implementing something like Traverse interface for the structure, instead of messing with some "tuples" manually.
Can anyone please share insight into the trait "Immutable" in scala? At first glance I thought this would be a nice control structure to limit a class I'm building, but oddly I noticed that primitive types do not extend this. Is there a reason for this? Is there a way to bind the syntax to Immutable or AnyVal?
class Test {
def test[T<:Immutable](x:T)={
println("passes "+x)
}
case class X(s:String) extends Immutable
test(X("hello")) //passes
// test("fail") - does not pass compiler
}
The only direct subtypes of Immutable in the Scala core library are:
collection.immutable.Traversable
collection.parallel.immutable.ParIterable
Nothing else refers to Immutable at all.
Immutable hasn't been changed since it was added in 2009 in Martin Odersky's "massive new collections checkin". I'm searching through that commit, and it looks like Immutable was never even used as a bound when it was first introduced either.
Honestly, I doubt there's much intent behind these traits anymore. Odersky probably planned to use Immutable to bound the type arguments on immutable collections, and then thought better of it. But that's just my speculation.
So-called primitive types (Boolean, Byte, Char, Short, Int, Long, Float, Double) are intrinsically immutable. 5 is 5 is 5. You cannot do anything to 5 to turn it into anything that is not 5.
Otherwise, immutability is a property of how a value is stored. If stored in a var, that var may be replaced freely with a new value (of a compatible type). By extension, constructed types (classes, traits and objects) may be either immutable or mutable depending on whether they allow any of their internal state to be altered following construction.
Java's String (also used as Scala's String) is immutable.
However, none of this has anything to do with you example, since you did not demonstrate mutability. You simply showed what happens when one applies the + method of one value to another value.
While it is certainly possible that one can implement a + method that mutates its (apparent) left-hand operand, one rarely does that. If there's a need for that kind of mutation, one would conventionally define the += method instead.
+ is somewhat special in that it may be applied to any value (if the argument / right-hand operand) is a String by virtue of an implicit conversion to a special class that defines +(s: String) so that the string concatenation interpretation of + may be applied. In other words, if you write e1 + "e2" and the type of the expression e1 does not define +, then Scala will convert e1 to String and concatenate it with "e2".
I've been working with Scala for a while now and have written a 10,000+ line program with it, but I'm still confused by some of the inner workings. I came to Scala from Python after already having intimate familiarity with Java, C and Lisp, but even so it's been slow going, and a huge problem is the frustrating difficulty I've often found when trying to investigate the inner workings of objects/types/classes/etc. using the Scala REPL as compared with Python. In Python you can investigate any object foo (type, object in a global variable, built-in function, etc.) using foo to see what the thing evaluates to, type(foo) to show its type, dir(foo) to tell you the methods you can call on it, and help(foo) to get the built-in documentation. You can even do things like help("re") to find out the documentation on the package named re (which holds regular-expression objects and methods), even though there isn't an object associated with it.
In Scala, you can try and read the documentation online, go look up the source code to the library, etc., but this can often be very difficult for things where you don't know where or even what they are (and it's often a big chunk to bite off, given the voluminous type hierarchy) -- stuff is floating around in various places (package scala, Predef, various implicit conversions, symbols like :: that are nearly impossible to Google). The REPL should be the way to explore directly, but in reality, things are far more mysterious. Say that I've seen a reference to foo somewhere, but I have no idea what it is. There's apparently no such thing as a "guide to systematically investigating Scala thingies with the REPL", but the following is what I've pieced together after a great deal of trial and error:
If foo is a value (which presumably includes things stored in variables plus companion objects and other Scala objects), you can evaluate foo directly. This ought to tell you the type and value of the result. Sometimes the result is helpful, sometimes not.
If foo is a value, you can use :type foo to get its type. (Not necessarily enlightening.) If you use this on a function call, you get the type of the return value, without calling the function.
If foo is a value, you can use foo.getClass to get its class. (Often more enlightening than the previous, but how does an object's class differ from its type?)
For a class foo, you can use classOf[foo], although it's not obvious what the result means.
Theoretically, you can use :javap foo to disassemble a class -- which should be the most useful of all, but fails entirely and uniformly for me.
Sometimes you have to piece things together from error messages.
Example of failure using :javap:
scala> :javap List
Failed: Could not find class bytes for 'List'
Example of enlightening error message:
scala> assert
<console>:8: error: ambiguous reference to overloaded definition,
both method assert in object Predef of type (assertion: Boolean, message: => Any)Unit
and method assert in object Predef of type (assertion: Boolean)Unit
match expected type ?
assert
^
OK, now let's try a simple example.
scala> 5
res63: Int = 5
scala> :type 5
Int
scala> 5.getClass
res64: java.lang.Class[Int] = int
Simple enough ...
Now, let's try some real cases, where it's not so obvious:
scala> Predef
res65: type = scala.Predef$#3cd41115
scala> :type Predef
type
scala> Predef.getClass
res66: java.lang.Class[_ <: object Predef] = class scala.Predef$
What does this mean? Why is the type of Predef simply type, whereas the class is scala.Predef$? I gather that the $ is the way that companion objects are shoehorned into Java ... but Scala docs on Google tell me that Predef is object Predef extends LowPriorityImplicits -- how can I deduce this from the REPL? And how can I look into what's in it?
OK, let's try another confusing thing:
scala> `::`
res77: collection.immutable.::.type = ::
scala> :type `::`
collection.immutable.::.type
scala> `::`.getClass
res79: java.lang.Class[_ <: object scala.collection.immutable.::] = class scala.collection.immutable.$colon$colon$
scala> classOf[`::`]
<console>:8: error: type :: takes type parameters
classOf[`::`]
^
scala> classOf[`::`[Int]]
res81: java.lang.Class[::[Int]] = class scala.collection.immutable.$colon$colon
OK, this left me hopelessly confused, and eventually I had to go read the source code to make sense of this all.
So, my questions are:
What's the recommended best way from the true Scala experts of using the REPL to make sense of Scala objects, classes, methods, etc., or at least investigate them as best as can be done from the REPL?
How do I get :javap working from the REPL for built-in stuff? (Shouldn't it work by default?)
Thanks for any enlightenment.
You mentioned an important point which Scala lacks a bit: the documentation.
The REPL is a fantastic tool, but it is not as fantastic at it can be. There are too much missing features and features which can be improved - some of them are mentioned in your post. Scaladoc is a nice tool, too, but is far away to be perfect. Furthermore lots of code in the API is not yet or too less documented and code examples are often missing. The IDEs are full ob bugs and compared to the possibilities Java IDEs show us they look like some kindergarten toys.
Nevertheless there is a gigantic difference of Scalas current tools compared to the tools available as I started to learn Scala 2-3 years ago. At that time IDEs compiled permanently some trash in the background, the compiler crashed every few minutes and some documentation was absolutely nonexistent. Frequently I got rage attacks and wished death and corruption to Scala authors.
And now? I do not have any of these rage attacks any more. Because the tools we currently have are great although the are not perfect!
There is docs.scala-lang.org, which summarizes a lot of great documentation. There are Tutorials, Cheat-sheets, Glossaries, Guides and a lot of more great stuff. Another great tools is Scalex, which can find even the weirdest operator one can think of. It is Scalas Hoogle and even though it is not yet as good as his great ideal, it is very useful.
Great improvements are coming with Scala2.10 in form of Scalas own Reflection library:
// needs Scala2.10M4
scala> import scala.reflect.runtime.{universe => u}
import scala.reflect.runtime.{universe=>u}
scala> val t = u.typeOf[List[_]]
t: reflect.runtime.universe.Type = List[Any]
scala> t.declarations
res10: Iterable[reflect.runtime.universe.Symbol] = SynchronizedOps(constructor List, method companion, method isEmpty, method head, method tail, method ::, method :::, method reverse_:::, method mapConserve, method ++, method +:, method toList, method take, method drop, method slice, method takeRight, method splitAt, method takeWhile, method dropWhile, method span, method reverse, method stringPrefix, method toStream, method removeDuplicates)
Documentation for the new Reflection library is still missing, but in progress. It allows one to use scalac in an easy way inside of the REPL:
scala> u reify { List(1,2,3) map (_+1) }
res14: reflect.runtime.universe.Expr[List[Int]] = Expr[List[Int]](immutable.this.List.apply(1, 2, 3).map(((x$1) => x$1.$plus(1)))(immutable.this.List.canBuildFrom))
scala> import scala.tools.reflect.ToolBox
import scala.tools.reflect.ToolBox
scala> import scala.reflect.runtime.{currentMirror => m}
import scala.reflect.runtime.{currentMirror=>m}
scala> val tb = m.mkToolBox()
tb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] = scala.tools.reflect.ToolBoxFactory$ToolBoxImpl#32f7fa37
scala> tb.parseExpr("List(1,2,3) map (_+1)")
res16: tb.u.Tree = List(1, 2, 3).map(((x$1) => x$1.$plus(1)))
scala> tb.runExpr(res16)
res18: Any = List(2, 3, 4)
This is even greater when we want to know how Scala code is translated internally. Formerly wen need to type scala -Xprint:typer -e "List(1,2,3) map (_+1)"
to get the internally representation. Furthermore some small improvements found there way to the new release, for example:
scala> :type Predef
scala.Predef.type
Scaladoc will gain some type-hierarchy graph (click on type-hierarchy).
With Macros it is possible now, to improve error messages in a great way. There is a library called expecty, which does this:
// copied from GitHub page
import org.expecty.Expecty
case class Person(name: String = "Fred", age: Int = 42) {
def say(words: String*) = words.mkString(" ")
}
val person = Person()
val expect = new Expecty()
// Passing expectations
expect {
person.name == "Fred"
person.age * 2 == 84
person.say("Hi", "from", "Expecty!") == "Hi from Expecty!"
}
// Failing expectation
val word1 = "ping"
val word2 = "pong"
expect {
person.say(word1, word2) == "pong pong"
}
/*
Output:
java.lang.AssertionError:
person.say(word1, word2) == "pong pong"
| | | | |
| | ping pong false
| ping pong
Person(Fred,42)
*/
There is a tool which allows one to find libraries hosted on GitHub, called ls.implicit.ly.
The IDEs now have some semantic highlighting, to show if a member is a object/type/method/whatever. The semantic highlighting feature of ScalaIDE.
The javap feature of the REPL is only a call to the native javap, therefore it is not a very featue-rich tool. You have to fully qualify the name of a module:
scala> :javap scala.collection.immutable.List
Compiled from "List.scala"
public abstract class scala.collection.immutable.List extends scala.collection.AbstractSeq implements scala.collection.immutable.LinearSeq,scala.Product,scala.collection.LinearSeqOptimized{
...
Some time ago I have written a summary of how Scala code is compiled to Bytecode, which offers a lot of things to know.
And the best: This is all done in the last few months!
So, how to use all of these things inside of the REPL? Well, it is not possible ... not yet. ;)
But I can tell you that one day we will have such a REPL. A REPL which shows us documentation if we want to see it. A REPL which let us communicate with it (maybe like lambdabot). A REPL which let us do cool things we still cannot imagine. I don't know when this will be the case, but I know that a lot of stuff was done in the last years and I know even greater stuff will be done in the next years.
Javap works, but you are pointing it to scala.Predef.List, which is a type, not a class. Point it instead to scala.collection.immutable.List.
Now, for the most part just entering a value and seeing what the result's type is is enough. Using :type can be helpful sometimes. I find that use getClass is a really bad way of going about it, though.
Also, you are sometimes mixing types and values. For example, here you refer to the object :::
scala> `::`.getClass res79: java.lang.Class[_ <: object
scala.collection.immutable.::] = class
scala.collection.immutable.$colon$colon$
And here you refer to the class :::
scala> classOf[`::`[Int]] res81: java.lang.Class[::[Int]] = class
scala.collection.immutable.$colon$colon
Objects and classes are not the same thing, and, in fact, there's a common pattern of objects and classes by the same name, with a specific name for their relationship: companions.
Instead of dir, just use tab completion:
scala> "abc".
+ asInstanceOf charAt codePointAt codePointBefore codePointCount
compareTo compareToIgnoreCase concat contains contentEquals endsWith
equalsIgnoreCase getBytes getChars indexOf intern isEmpty
isInstanceOf lastIndexOf length matches offsetByCodePoints regionMatches
replace replaceAll replaceFirst split startsWith subSequence
substring toCharArray toLowerCase toString toUpperCase trim
scala> "abc".compareTo
compareTo compareToIgnoreCase
scala> "abc".compareTo
def compareTo(String): Int
If you enter the power mode, you'll get way more information, but that's hardly for beginners. The above shows types, methods, and method signatures. Javap will decompile stuff, though that requires you to have a good handle on bytecode.
There's other stuff in there -- be sure to look up :help, and see what's available.
Docs are only available through the scaladoc API. Keep it open on the browser, and use its search capability to quickly find classes and methods. Also, note that, as opposed to Java, you don't need to navigate through the inheritance list to get the description of the method.
And they do search perfectly fine for symbols. I suspect you haven't spent much time on scaladoc because other doc tools out there just aren't up to it. Javadoc comes to mind -- it's awful browsing through packages and classes.
If you have specific questions Stack Overflow style, use Symbol Hound to search with symbols.
Use the nightly Scaladocs: they'll diverge from whatever version you are using, but they'll always be the most complete. Besides, right now they are far better in many respects: you can use TAB to alternate between frames, with auto-focus on the search boxes, you can use arrows to navigate on the left frame after filtering, and ENTER to have the selected element appear on the right frame. They have the list of implicit methods, and have class diagrams.
I've made do with a far less powerful REPL, and a far poorer Scaladoc -- they do work, together. Granted, I skipped to trunk (now HEAD) just to get my hands on tab-completion.
Note that scala 2.11.8 New tab-completion in the Scala REPL can facilitate the type exploration/discovery.
It now includes:
CamelCase completion:
try:
(l: List[Int]).rroTAB,
it expands to:
(l: List[Int]).reduceRightOption
Find members by typing any CamelCased part of the name:
try:
classOf[String].typTAB,
to get getAnnotationsByType, getComponentType and others
Complete bean getters without typing get:
try:
(d: java.util.Date).dayTAB
Press TAB twice to see the method signature:
try:
List(1,2,3).partTAB,
which completes to:
List(1,2,3).partition;
press TAB again to display:
def partition(p: Int => Boolean): (List[Int], List[Int])
You need to pass fully qualified class name to javap.
First take it using classOf:
scala> classOf[List[_]]
res2: java.lang.Class[List[_]] = class scala.collection.immutable.List
Then use javap (doesn't work from repl for me: ":javap unavailable on this platform.") so example is from a command line, in repl, I believe, you don't need to specify classpath:
d:\bin\scala\scala-2.9.1-1\lib>javap -classpath scala-library.jar "scala.collection.immutable.List"
But I doubt this will help you. Probably you're trying to use techniques you used to use in dynamic languages. I extremely rarely use repl in scala (while use it often in javascript). An IDE and sources are my all.
I'm integrating with ZeroMQ and Akka to send case classes from different instances. Problem is that when I try to compile this code:
def persistRelay(relayEvent:String, relayData:Any) = {
val relayData = ser.serialize(relayData).fold(throw _, identity)
relayPubSocket ! ZMQMessage(Seq(Frame(relayEvent), Frame(relayData)))
}
The Scala compilers throws back recursive value relayData needs type.
The case classes going in are all different and look like Team(users:List[Long], teamId:Long) and so on.
Is there a method to allow any type in the serializer or a workaround? I'd prefer to avoid writing a serializer for every single function creating the data unless absolutely necessary.
Thanks!
This isn't really a typing issue. The problem is:
val relayData = ser.serialize(relayData).fold(throw _, identity)
You're declaring a val relayData in the same line that you're making a reference to the method parameter relayData. The Scala compiler doesn't understand that you have/want two variables with the same name, and, instead, interprets it as a recursive definition of val relayData. Changing the name of one of those variables should fix the error.
Regardless, since you didn't quite follow what the Scala compiler was asking for, I think that it would also be good to fill you in on what it is that the compiler even wanted from you (even though it's advice that, if followed, probably would have just led to you getting yet another error that wouldn't seem to make a lot of sense, given the circumstances).
It said "recursive value relayData needs type". The meaning of this is that it wanted you to simply specify the type of relayData by having
val relayData = ...
become something like
val relayData: Serializable = ...
(or, in place of Serializable, use whatever type it was that you wanted relayData to have)
It needs this information in order to create a recursive definition. For instance, take the simple case of
val x = x + 1
This code is... bizarre, to say the least, but what I'm doing is defining x in a (shallowly) recursive way. But there's a problem: how can the compiler know what type to use for the inner x? It can't really determine the type through type inference, because type inference involves leveraging the type information of other definitions, and this definition requires x's type information. Now, we might be able to infer that I'm probably talking about an Int, but, theoretically, x could be so many things! In fact, here's the ambiguity in action:
val x: Int = x + 1 // Default value for an Int is '0'
x: Int = 1
val y: String = y + 1 // Default value for a String is 'null'
y: String = null1
All that really changed was the type annotation, but the results are drastically different–and this is only a very simple case! So, yeah, to summarize all this... in most cases, when it's complaining about recursive values needing types, you should just have some empathy on the poor compiler and give it the type information that it so direly craves. It would do the same for you, DeLongey! It would do the same for you!
So for example why does List(1,2,3,4).contains("wtf") even compile? Wouldn't it be nice if the compiler rejected this?
Lots of interesting answers, but here's my own theory: if contains did not receive an Any, then Seq could not be co-variant.
See, for instance, Set, which is not co-variant and whose contains take an A instead of an Any.
The reasons for that is left as an exercise to the reader. ;-) But here is a hint:
scala> class Container[+A](elements: A*) {
| def contains(what: A): Boolean = elements exists (what ==)
| }
<console>:7: error: covariant type A occurs in contravariant position in type A of value what
def contains(what: A): Boolean = elements exists (what ==)
^
"contains" is fundamentally about equality testing, and equality in Scala (as in Java before it) is untyped. The practical value of having untyped-equality is small, but not zero. There are, for instance, a few occasions where it makes sense for two objects of different classes to be equal to one another. For instance, you might wish an object of type RGBColor to be equal to a PantoneColor if they define the same hue, or an immutable HashSet and an immutable TreeSet to be equal if they contain the same elements. That said, untyped-equality also causes a bunch of headaches, and the fact that the compiler could easily catch that List(1,2,3,4).contains("wtf") is nonsensical but won't is one of them.
Most Java bug-finding tools include tests to detect the presence of improbable untyped-equality uses. (I wrote the inspections to do this in IntelliJ IDEA.) I have no doubt that when Scala bug-finding tools come online, these will be among the first bugs detected.
SeqLike.contains checks whether a value is present by checking for an element in the sequence that is equal to the value (using ==). == takes an Any so I suspect that this is the reason.