The popularization of the Scala programming language, apparent by the affluence of opinions and criticism on blogs and amusing networks (like this one by Nikita Ivanov from GridGain and the accepted Yammer case), abundantly added the bulk of advice about the language. However, the affection of such advice generally leaves abundant to be desired.
Whether those opinions are favorable or adverse to the Scala, they generally accommodate outdated, apparent or biased statements. The ambition of this commodity is to advice those acquirements or evaluating Scala to appear into their own conclusions. It presents the best accepted questions about accent and its ambiance and, for anniversary one, added clarifications, examples and links, benign the accumulation of a bigger appraisal or a added authentic assessment.
Scala is a aggregate language, advised to run on a managed environment, best acceptable the JVM, and offers the abutment of the anatomic and acquisitive paradigms. Scala offers anatomic and acquisitive programming, a avant-garde compiler and a blazon arrangement arrested at abridge time, as in Java, but with the alive syntax of (usually) interpreted languages, such as Groovy or Ruby. However, the aforementioned appearance that makes Scala alive can additionally advance to achievement problems and complexity. This commodity capacity area this antithesis needs to be advised and it is not an accession or tutorial, but does not assume ability in Scala.
As development affluence is a actual abstruse matter, it is all-important to decompose this issue, evaluating the appearance of Scala that commonly abutment it.
Functional Programming
The anatomic archetype expresses programs as functions in the algebraic sense, mapping from one bulk to accession (f (x) = y) afterwards ancillary effects, such as advancement the cachet of altar or input/output of data. This allows several abridge time optimizations and avoids accommodation problems, as will be presented throughout this article.
Functional programming is a software archetype based on the Lambda Calculus and has continued been allotment of the bookish ambiance in languages like Lisp, Scheme and Haskell. It is a trend of bartering languages such as Java and C # to absorb appearance from these languages, abnormally closure, but this accomplishment is bound by astern compatibility, blueprint and conflicts of interest. These limitations opens amplitude for new multi-paradigm languages to be developed, such ad as Clojure and Scala, gluttonous to action the best of both Commodity Oriented and Anatomic worlds.
Features such as abiding values, collections, higher-order functions and arrangement matching encourages Scala developers to use the anatomic style. This can accomplish developers accustomed with this archetype added productive, but may additionally alienate those who do not apperceive it. For example, a affairs that prints the thousandth aspect of the Fibonacci arrangement can be accounting as follows in Java:
Translated actually to Scala, this cipher would become:
These are the implementations of the accepted algorithm in Java and Scala, appreciably similar. However, a added bunched and anatomic version, appliance absolute sequences and tuples can be actual different:
This adaptation can be advised abridged or complex, according to alone abilities and preferences. Anatomic programming is not the alone antecedent of complication in Scala, as will be apparent throughout this article, but is accordant to the aberration in productivity.
To apprentice added about anatomic programming in Scala, this video of Nathan Hamblen introduces the archetype in accepted and this another, by Daniel Spiewak, goes a footfall further, implementing some anatomic abstracts structures. The tutorials in the official affidavit and the ones appear by the Twitter aggregation are additionally generally recommended.
Less boilerplate code
Some Scala features, such as blazon inference, absolved exceptions, another altar and absolute conversions, can abundantly abate the bulk of statements and checks in a program, afterwards alteration its meaning. Furthermore, Scala tries to be bacteria than Java, removing appearance that can be bidding in agreement of others. For example, there are no changeless references or archaic types in Scala, because the aforementioned aftereffect can be acquired appliance apparent objects.
To accept the aberration in boilerplate code, accede this affairs that prints the MAC addresses of all arrangement interfaces in the system. There are several means to address this code, but that’s the way it would apparently be accounting in anniversary language.
In Java:
In Scala:
Or, accession accomplishing in Scala appliance arrangement apperception and absolute about-face :
In this example, the afterward differences are accordant in the abolishment of boilerplate code:
Other features, like Absolute About-face and Closures, may accord to the abridgement of code. Although these appearance do not abide in Java, some of them can be apish or adapted, mostly by creating chatty APIs.
Traits
Traits are blazon definitions, agnate to Java interfaces, but may accommodate accomplishing and are cumulative, alms a altered admission to cipher reuse. A affection is agnate to the aggregate of an abstruse chic and an interface in a distinct definition. The afterward example, taken from the shows how to amalgamate ancestry to double, accession and/or clarify a chain of integers:
Once defined, ancestry can be “mixed in” during either blazon analogue or commodity instantiation:
The aforementioned appearance that can accomplish Scala added advantageous can additionally accomplish it unreadable. A affection that is oftenly questioned is the use of symbols in adjustment names, like the adjustment class. These methods, bearded as operators, may be advantageous to represent accepted operations, like account chain or accession of circuitous numbers. However, this classification may calmly be abused. Back accumulated with blazon limits, about-face and fractional functions, aerial capacity in any language, the statements may become actual difficult, like this one mentioned in this column “Opinion:Scala is the new EJB2?”:
Another adversity is that ScalaDocs, the official affidavit of the chic library, is abridged in abounding aspects. However, there is an accomplishment to advance this affidavit and added assets for learning, all actuality aggregate into a affidavit site.
This does not beggarly that there is no inherent complication in Scala. In the commodity “True Scala Complexity”, Zhang Yang presents a abundant archetype of how the blazon arrangement and conversions can get confusing, alike for an accomplished Scala developer. Some of these complexities can be prevented, alleviated or alone in the future, but others shall remain, such as those consistent from the affiliation of a alive blazon arrangement over both anatomic and acquisitive paradigms.
Writing actual and able circumstantial programs is difficult. Debugging these programs can be alike added arduous and unpredictable. Scala offers accompaniment at a aerial akin of abstraction, but so does Java, decidedly with the accommodation appearance alien by Java 7. The accommodation appearance of the two languages are agnate in their purpose, but they are actual altered in their architectonics and a abounding allegory is aloft the ambit of this article. However, this affair becomes clearer back analysing the appearance of Scala that abutment it.
Unfortunately, programmers accept begin it actual difficult to anxiously body able-bodied multi-threaded applications appliance the aggregate abstracts and locks model, abnormally as applications abound in admeasurement and complexity. The botheration is that at anniversary point in the program, you charge acumen about what abstracts you are modifying or accessing that ability be adapted or accessed by added threads, and what locks are actuality held. At anniversary adjustment call, you charge acumen about what locks it will try to hold, and argue yourself that it will not deadlock while aggravating to admission them.
Programming in Scala, Chapter 32
Scala attempts to abate these difficulties appliance the immutability and actors. Furthermore, the use of anatomic programming facilitates centralized parallelism, automatic by the compiler or libraries. The Closures angle for Java 8 attempts to absorb some of these appearance in the Java accent and its libraries.
Immutability
Synchronizing admission to aggregate arbitrary altar can aftereffect in abundant complication in the use of accommodation archaic (locks, semaphores, etc.). Although this is not a accepted affair for appliance developers, it can be alarming for developers of servers and frameworks. Scala tries to abate this botheration by appliance abiding altar and authentic functions. If an commodity is immutable, it can be aggregate or affected afterwards annoying about who is appliance it, so it is artlessly “thread-safe.”
Unlike added anatomic languages, Scala does not force altar to be immutable. Arbitrary altar are important to apparatus a cardinal of requirements, anatomic and nonfunctional. What Scala does is animate the acumen amid arbitrary and abiding appliance altered bales and statements for anniversary case.
This talk, by Rich Hickey, presents the capital account abaft the immutability and some considerations on how to cipher with it.
Actors
Low-level accompaniment controls, such as locks and synchronized blocks, are acceptable to address circumstantial programs accurately – but this assignment may not be easy. To address this blazon of affairs added productively and anticipate defects, a aerial akin accommodation ascendancy is actual desirable. Such absorption can be like Fork /Join, Software Transactional Memory, or, as featured in Scala, the Amateur Model. In this abstraction, the accompaniment is bidding as actors reacting to messages, rather than locking and absolution of threads.
The afterward archetype demonstrates actors ciphering the bulk of Pi appliance the Monte Carlo method. This adjustment generates accidental credibility in a aboveboard and calculates the arrangement of them that falling aural the inscribed circle, approximating Pi. The afterward accomplishing uses a alternating arrangement in the actors model: one amateur will be the “coordinator”, managing several “calculators” who, either alone or cooperatively, advance appear the outcome.
The “calculator” in this archetype may accept two messages: “Calculate”, which is replied with an appraisal of Pi and “ShutDown”, which shuts bottomward the actor. As these letters are simple notifications, they can be represented as connected case objects. The artful amateur can be accounting as follows:
The “coordinator” starts a account of calculators and acquaint them to account until any of them produces an authentic abundant admiration and, in this case, terminates the adding and prints the bulk begin and the beheading time:
Finally, an commodity with a capital adjustment initializes the coordinator with the cardinal of calculators to be used. The college the cardinal of coordinators, the added acceptable one of them will acquisition the acclimatized bulk sooner, abbreviation the absolute beheading time.
The actors archetypal goes aloft the advice amid threads. The Akka belvedere extends the archetypal to abutment alien actors and adds several accoutrement for developing broadcast systems with aerial scalability and accountability tolerance.
Although advantageous in abounding cases, the actors archetypal and its accomplishing in Scala is not chargeless of controversies. Abundant of its allowances are not acquired from the actors themselves, but the barter of messages, which are usually abiding and favors the absence of aggregate arbitrary state. The absence accomplishing in Scala, apparent above, binds anniversary amateur to a built-in thread, which can be problematic, as apparent by Tony Arcieri in the column “Why I Do not Like Scala”. There are another implementations, appliance event-based actors for example, but they consistently appear at some cost, at atomic of complexity.
Parallel Collections
Scala 2.9 alien alongside collections, which makes it accessible to parallelize the beheading of accepted collections operations, such as map(), clarify () and foreach(). The par() adjustment can be acclimated to admission a alongside adaptation of the collection, as apparent in this example:
Note that the adjustment of the printed elements in the additional account is unpredictable, as it depends on the scheduling of accoutrement by the operating system. Aleksandar Prokopec shows absorbing capacity of how the alongside collections were implemented in his presentation at the Scala Days 2010.
Scala allows developers to adapt the attending and feel of the language, creating new languages and altering the accumulation process. Such tasks can be arduous or alike accomplish the cipher unreadable, but abundantly extend the possibilities of language.
Domain Specific Languages
Using all-encompassing classes, abstruse types, functions as objects, methods called as operators and absolute conversions, Scala cipher can become a area specific accent (DSL). This is actual advantageous back the accent is apparent to area experts, acceptance them to adapt the arrangement at runtime, as exemplified by Debasish Ghosh in his banking DSL.
Domain-specific languages can additionally be created back a added abstruse and allegorical accent is bare by developers. For example, Apache Camel offers a Scala DSL to accomplish the agreement of account routes added abridged and correct.
The development of domain-specific languages is a accountable as abysmal as popular. For a introduction, see this presentation by Martin Fowler, who additionally wrote a book on DSLs.
Changing the compilation
Scala takes the development of languages a footfall added with parser combinators, acceptance the conception of absolutely new grammars, as apparent by Daniel Spiewak in this article. Back alike that is not enough, one can still actualize compiler plugins to change the body process. These plugins could be written, for example, to accomplish changeless cipher assay and evaluating metrics, like PMD or FindBugs. Accession achievability would be to actualize a plugin to change or optimize the behavior of a library at abridge time.
These appearance can accomplish Scala cipher attending actual altered from the aboriginal language, as apparent by Michael Fogus in his accomplishing of BASIC in Scala. These customizations of the accent can be acclimated to break circuitous problems elegantly, but can additionally be abused, alienating developers alien with the ambience of the changes.
Although it is accessible to adjure Java methods in Scala cipher and carnality versa, the alternation amid languages is not afterwards complications.
When calling Scala methods from Java, the developer needs to accept how the appearance that do not abide in Java are adapted into executable objects. For example, methods with non-alphabetic names, accepting functions and tuples as parameters, do assignment back acclimated in java code, but charge to be accounting properly, as apparent in this article.
When invoking Java from Scala, the problems is the appearance of Java that were alone or implemented abnormally in Scala, such as interfaces, annotations and collections. This commodity explains added about these differences and alternatives for assorted cases, but some of these differences may force the developer to address cipher in the two languages. For example, interfaces and comment charge be accounting in Java, back these are usable, but not declarable in Scala.
The scalac compiler does not abridge java cipher or adjure the javac compiler to do it, but it analyses the .java
The fifteen years of history and the backbone of the Java association are absolutely reflected by the affluence and ability of its tools. The Scala accoutrement are evolving faster, abnormally with the efforts from Typesafe (the aggregation abaft the accent and several accompanying projects) and association contributions, as those from Twitter.
Using Java Libraries and Frameworks in Scala
One of the allowances of appliance a JVM accent is the affluence of libraries and frameworks accessible for reuse. Considering the limitations of interoperability presented above, any library or framework accessible for Java can be acclimated in Scala. This includes all Java EE (EJB, JSF, JAX-RS, etc.) and accepted libraries, such as Hibernate and JUnit, because Java and Scala classes are around duplicate already compiled. For example, a servlet that prints the ambit of HTTP requests can be accounting in Scala as follows:
Scala Libraries and Frameworks
The botheration back appliance Java libraries in Scala is that they were not advised to the syntax improvements of Scala. In some cases this is aloof inconvenient, but sometimes it may crave the boilerplate cipher that Scala tries to avoid.
For an acute example, booty the CollectionUtils chic from the Apache Aliment Collections library. It has methods to filter, transform and iterate over collections, as does the Scala accepted library. A affairs that uses Aliment Collections to book the bifold of anniversary absolute aspect from a account of integers can be accounting as follows in Scala:
While correct, that cipher would apparently be advised actual bombastic or repetitive by a Scala developer. However, appliance built-in Scala collections, one can address the aforementioned affairs in a abundant added chatty syntax:
Although absolutely obvious, this is the acumen why several libraries end up actuality re-developed in Scala. It is the case, for example, of the Scalaz library, which appearance several abstracts structures added acceptable to anatomic programming. The aforementioned goes for web frameworks (Lift, Play), libraries for TDD and BDD (ScalaTest, Specs) and abounding others. Some of these libraries are actual agnate to their Java versions, while others are radically altered and aggressive by libraries from added languages, such as Scalatra, based on the library Sinatra, for Ruby or ScalaCheck based on QuickCheck for Haskell.
Development Tools
All above Java IDEs (Eclipse, NetBeans and IntelliJ IDEA) servers, etc.) may be abnormal or defective.
The affluence of body automation accoutrement can be both absolute and confusing. The Simple Body Apparatus (SBT) is broadly used, about a association standard. However, to accommodate with added accoutrement (IDEs, servers, etc.) and abutment the complete development action (testing, packaging, deployment, etc.) accoutrement such as maven, ant or Buildr may be added adapted and they already abutment Scala, usually accouterment a added array of plugins and integrations.
Like the libraries, abounding Scala accoutrement are aggressive in languages added than Java. The best accepted is the alternate animate (REPL), actual acceptable in scripting languages and advantageous to assay and apprentice the accent and its libraries. Such accoutrement can be abnormal to the developer accustomed with the Java environment, so the clearing of the development ambiance needs to be analyzed carefully. Not all accoutrement will accept an exact parallel, but the differences may be benign to the development cycle.
Scala has been advised to be absolute of the basal basic machine, but the ambition was acutely the JVM. At the alpha of the activity in 2004, a .NET adaptation was developed, but that never got abundant acceptance and bound became obsolete. However, in June 2011 a activity conducted by the Ecole Polytechnique Fédérale de Lausanne has accomplished cogent after-effects in adapting the Scala to the .NET platform, as explained in this article. The .NET adaptation is still in aboriginal development and has several limitations, but it is already accessible to run Scala programs on both platforms. For added advice about the Scala anchorage to .NET, see this this account with Martin Odersky and this folio of the Scala website.
Portability to added platforms, alike aloft Java and .NET, is an absorbing affection of Scala. An avant-garde activity in this faculty is the scala-llvm, an accomplishing of the Scala compiler for LLVM. However, such portability is still alone a possibility, Java actuality the alone belvedere that supports all the features, accoutrement and libraries usually accepted by action developers.
Leaps in akin of absorption are usually followed by achievement criticism and Scala is no exception. In general, Java and Scala systems accept agnate runtime performance, back both are accountable to the costs and allowances of the JVM. At abridge time, however, the Scala compiler has abundant added assignment to do and is absolutely apathetic compared to the Java compiler.
Runtime Performance
Performance differences usually appear from the appearance of Scala that are not natively accurate by the JVM. Some of these features, such as closures, are acceptable to be accurate soon, but abounding others never will. Therefore, a angular cipher at a aerial akin of absorption accounting in Scala can be aggregate to a ample bulk of bytecode, aspersing runtime performance, as apparent in this presentation.
This can be a botheration for achievement acute programs, but there are workarounds. The best accepted is to assay the generated bytecode appliance javap to accept what is accident beneath the awning and become accustomed with the achievement of the appearance and libraries. Understanding bytecode and affability low akin achievement is not a simple task, but in acceptable anatomic languages like Haskell and Scheme implementations, such assay is abundant added complex, if possible.
However, it should be acclaimed that the accent is important for the achievement of a system, but is not the alone factor. The allowances acquired from accommodation utilities and added appearance may account or alike beat the penalties from absorption back barometer the final cessation and throughput.
Compilation Performance
Some Scala features, such as the chase for absolute conversions, can booty a continued time to build, and the compiler still charge to analysis both the Scala blazon arrangement and the blazon arrangement of the basal belvedere (JVM, CLR or other). All this makes accumulation slower, but does so in the account of adaptability and belvedere independence.
The IDE or body apparatus can use incremental accumulation to allay these problems, but for circuitous abstracts or connected integration, compiler achievement can be an issue.
Maintaining bifold affinity is a difficult best for the developers of a programming language. If there are bright rules for the aliment of compatibility, as does the Chapter 13 of the Java Accent Specification, again programmers can apprehend new cipher to be accordant with old binaries afterwards recompiling everything.
However, over time, advancement astern affinity can accomplish the accent difficult to develop. The adversity to advance the accent while advancement affinity was one of the affidavit for the delays in the absolution of Java 7 and 8 and the alarming change of JavaScript.
On the added hand, not committing to astern affinity makes a accent abundant easier to evolve. This can be actual important in the aboriginal stages of the language, to acquiesce alteration of architecture decisions, acclimation bugs and affability features. The botheration is that cogent changes, as apparent change of Scala, can account incompatibilities and crave libraries and applications to be recompiled for the aforementioned adaptation of the language. This botheration can be mitigated usingbridge methods,migration tools, acceptable affidavit and processes, but there is no absolute solution.
For this reason, accepted libraries in Scala accept altered distributions, aggregate for altered versions of the language. A distinct adaptation of theBDD library specs, for example, can be begin as specs_2.9.0 specs_2.8.1, targeting these corresponding versions of Scala. Less accepted libraries may charge to be recompiled appliance aforementioned adaptation of the accent acclimated by the application. The body automation accoutrement can advice abundantly in this task. SBT, for example, can easilycross-compile and accomplish binaries for several versions of Scala in a distinct run, as able-bodied as advertence libraries accurately appliance both the adaptation of the library and the adaptation of the language.
The advantages and disadvantages of Scala are generally declared in opinions afflicted by either activity or disappointment. To appropriately appraise the technology, it is important to accept the ambience and the facts abaft such opinions, as able-bodied as the accordant architecture choices.
All cipher examples presented in this commodity are accessible in this Git Hub repository. Sincere acknowledgment to the associates of the Scala users list, which maintains a actual alive association and abundantly helped this writing. If you are or accept been a Scala developer, accord with your appraisal in the comments!
Julio Faerman is a software engineer, developer and teacher, amid added labels. Specialized in developing action software with circuitous requirements and software action improvement, mostly for government and telecom.
Currently alive at Red Hat / JBoss, afterwards Borland, NEC and absolute audience of his own company. Interested in a advanced ambit of subjects, from algebraic bold approach to music and gardening, but consistently abrogation time for association contest and publications.
How To Write A Thread In Java – How To Write A Thread In Java
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