I'm new to Scala. And I noticed that List exists in both scala and scala.collection.immutable. At the beginning, I thought scala.List is just an alias for scala.collection.immutable.List. But later I found that:
scala> typeOf[scala.List[A]]
res1: reflect.runtime.universe.Type = scala.List[A]
scala> typeOf[scala.collection.immutable.List[A]]
res2: reflect.runtime.universe.Type = List[A]
As shown above, the typeof on these two Lists give different results, which make me doubious about my judgement.
I would like to know:
Are scala.List and scala.collection.immutable.List the same thing?
If yes, why typeOf gives different results as shown above?
If no, what are the differences?
Thanks!
As #Patryk mentioned in comment, it is an alias for scala.collection.immutable.List.
If you look at scala\package.scala:
type List[+A] = scala.collection.immutable.List[A]
val List = scala.collection.immutable.List
So fundamentally they are the same:
scala> implicitly[scala.List[Int] =:= scala.collection.immutable.List[Int]]
res19: =:=[List[Int],List[Int]] = <function1>
scala> :type List
scala.collection.immutable.List.type
scala> :type scala.collection.immutable.List
collection.immutable.List.type
What you are using is I believe typeOf from import scala.reflect.runtime.universe._. Which is a reflective representation of the type parameter and hence it gave you just scala.List (which is correct behavior in my opinion)
scala.List is type alias for scala.collection.immutable.List
package scala
type List[+A] = scala.collection.immutable.List[A]
Anything under the package scala is by default available you don't have to explicitly import it.
As List is something which is widely used it makes perfect sense if it is available by default in the context rather than importing. That is why a type alias of the scala.collection.immutable.List is in package scala.
typeOf must be showing the higher preference package by default.
Prior to version 2.10 of Scala sequence types had methods like toList and toArray for converting from one type to another. As of Scala 2.10 we have to[_], e.g. to[List], which appears to subsume toList and friends and also give us the ability to convert to new types like Vector and presumably even to our own collection types. And of course it gives you the ability to convert to a type which you know only as a type parameter, e.g. to[A] -- nice!
But why weren't the old methods deprecated? Are they faster? Are there cases where toList works but to[List] does not? Should we prefer one over the other where both work?
toList is implemented in TraversableOnce as to[List], so there won't be any noticeable performance difference.
However, toArray is (very slightly) more efficient than to[Array] as the former allocates an array of the right size while the latter first creates an array and then sets the size hint (as it does for every target collection type). This should not make a difference in a real application unless you are converting data to arrays in a tight loop.
The old methods could easily be deprecated, and I bet they will in the future, but people are so used to them that deprecating them right away would probably make some people angry.
On issue seems to be that you cannot use to[] in postfix notation:
scala> Array(1,2) toList
res2: List[Int] = List(1, 2)
scala> Array(1,2) to[List]
<console>:1: error: ';' expected but '[' found.
Array(1,2) to[List]
scala> Array(1,2).to[List]
res3: List[Int] = List(1, 2)
Following along with chapters and examples in "Scala for the Impatient", there's an exercise related to using a Java TreeMap as a Scala SortedMap. In the scala shell, I tried this:
var t: scala.collection.SortedMap[String,Int] = new java.util.TreeMap[String,Int]()
but I get an error message about type mismatch. Is there a simple way to do this?
Note: I did an import of scala.collection.JavaConversions._ and then did this:
var t: SortedMap[String,Int] = TreeMap[String,Int]()
This works, but variable t has type java.util.SortedMap, not scala.collection.SortedMap.
I haven't read that book, but you need to make your mind up. Is t a Scala SortedMap or is it a Java TreeMap?
A TreeMap isn't a SortedMap, so you can't assign one to a SortedMap variable without converting it. JavaConversions will do some conversions for you, including:
implicit def mapAsScalaMap [A, B] (m: Map[A, B]): Map[A, B]
Implicitly converts a Java Map to a Scala mutable Map.
However there's nothing to convert to a SortedMap.
If this conversion seems mysterious to you, the library designers would agree, so JavaConversions is deprecated in Scala 2.10, in favour of JavaConverters, which requires a specific .asScala method to do a conversion.
Unfortunately JavaConverters doesn't have anything to produce a SortedMap either (.asScala gives you a mutable.Map). So you need to rebuild the collection using its elements.
import collection.JavaConverters._
import collection.SortedMap
var t: SortedMap[String, Int] =
SortedMap[String, Int]() ++ new java.util.TreeMap[String,Int].asScala
Producing a new TreeMap in the code above is obviously a bit pointless because it's empty, but you should get the idea of how to deal with an existing one. If you don't have an existing one, just produce a new SortedMap in Scala.
import collection._
import JavaConverters._
var t: scala.collection.SortedMap[String,Int] =
new java.util.TreeMap[String,Int]().asScala.map(identity)(breakOut)
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.