Implementing joins using extensible pattern matching the clip lab. Feb 14, 2011 playing with scalas pattern matching how many times have you been stuck in your frustration because you were unable to use strings as entries in switchcase statements. Pattern matching can also be used to check the type of an instance, rather than using isinstanceofb val anyref. This video explores a powerful feature of scala called pattern matching. Sep 22, 2014 in his latest video, senior software developer and lead trainer alejandro lujan explains pattern matching, a very powerful and elegant feature of scala, using a series of examples. Match expressions and pattern matching are a major feature of the scala programming language, and you want to see examples of the many ways to use them. A match expression has a value, the match keyword, and at least.
Scala provides great support for pattern matching, in processing the messages. Scala pattern match is not exhaustive on nested case classes. Scala has been designed to interoperate seamlessly with java. See also effective scala has opinions about partialfunction. It is a clear sign of the missing language feature. Scala is a modern multiparadigm programming language designed to express common programming patterns in a concise, elegant, and typesafe way.
Scala allows guards to be placed in patterns to test for particular conditions that cannot be tested in the pattern declaration itself. Thus, we can write our fibonacci calculator to return 0 if a negative number is passed in as in the following examples. Because woodpecker is defined as a case object and has no name, it is also matched as shown. Try the following example program where we will try to find out word scala from a statement. Scala provides a lightweight syntax for defining anonymous functions, it supports higherorder functions, it allows functions to be nested, and it supports currying. Indepth documentation covering many of scalas features. When matching a cat, i want to ignore the name, so i use the syntax shown to match any cat instance. The following commands are used to compile and execute this program. The following example function takes a character and a list of tuples and returns a new list of tuples. A list of frequentlyasked questions about scala language features and their answers. Such an ability would be really useful for example to analyze the arguments of your application or to parse a file, or any content of a string. This tutorial is sponsored by the scala cookbook, which i wrote for oreilly, and which you can find on.
It reduces the number of lines and helps the programmer to code in a typesafe way. If youre coming to scala from java, the most obvious uses are. It works same as switch case in other programming languages. Scala language pattern matching with stable identifier. Pattern matching over class hierarchies unifies functional and object oriented data access. The first thought which comes to mind is if else operator or java switch statement. Pattern matching over class hierarchies unifies functional and object oriented data. Basically a pattern match in scala has the following syntax 12. The singleton pattern restricts the instantiation of a class to one object, and provides a global point of access to it singleton is probably the most wellknown design pattern in java. Ssccaallaa ppaatttteerrnn mmaattcchhiinngg pattern matching is the second most widely used feature of scala, after function values and closures. Scala smoothly integrates the features of objectoriented and functional languages. A pattern match includes a sequence of alternatives, each starting with the keyword case. An arrow symbol separates the pattern from the expressions. Pattern matching tour of scala scala documentation.
It also has an extensible system of userdefinable pattern extractors. Rubin, phd director, center of excellence for big data graduate programs in software university of st. Sometimes it is necessary to match on the enclosing scopes variable. Find first match of a pattern of length m in a text stream of length n. Scala is also a functional language in the sense that every function is a value. Scala language pattern matching on a seq scala tutorial. This so post shows other ways to try to use string patternmatching in match expressions, but kaleidoscope looks like the easiest solution.
It has the potential to make code a lot cleaner and make ruby feel a bit more modern and exciting. How to use case classes in scala match expressions. Problem you need to match one or more patterns in a scala match expression, and the pattern may be a constant pattern, variable pattern, constructor pattern, sequence pattern, tuple pattern, or type pattern. Scala language pattern matching on tuples scala tutorial. A successful match can also deconstruct a value into its constituent parts. An indepth guide on how to write idiomatic scala code. It is a more powerful version of the switch statement in java and it can likewise be used in place of a series of ifelse statements. This is an excerpt from the scala cookbook partially modified for the internet. You need to match one or more patterns in a scala match expression, and the pattern may be a constant pattern, variable pattern, constructor pattern, sequence pattern, tuple pattern, or type pattern. Scala provides concise direct realization of the singleton pattern in the language. Scala language pattern matching types scala tutorial. Pattern matching is a mechanism for checking a value against a pattern. Many existing companies, who depend on java for business critical applications, are turning.
You need to match one or more patterns in a scala match expression, and the pattern may be a constant pattern, variable pattern, constructor. This book is available in both paper and pdf ebook form. Pattern matching tests whether a given value or sequence of values has the shape defined by a pattern, and, if it does, binds the variables in the pattern to the corresponding components of the value or sequence of values. Pdf download scala language for free previous next.
A handy cheatsheet covering the basics of scala s syntax. The apply method can be defined for both objects and classes. Write a regular expression matching all words with an upper case z. For each pattern provide an improved implementation or possibly a componentized pattern. Scala has been created by martin odersky and he released the first version in 2003. To check for a precise number of elements in the collection. Pattern matching princeton university computer science. Scalas case classes and its builtin support for pattern matching provide the functionality of. Scala pattern matching with case statement journaldev. Playing with scalas pattern matching kerflyns blog.
A design pattern is generally thought of as a reusable solution to a commonly occurring design problem in objectoriented software. An arrow symbol separates pattern from the expressions. Scala multiple type pattern matching stack overflow. Scala enables you to use all the classes of the java sdk and also your own custom java classes, or your favorite java open source projects. In standard pattern matching, the identifier used will shadow any identifier in the enclosing scope. Each alternative includes a pattern and one or more expressions, which will be evaluated if the pattern matches. Scala is an emerging generalpurpose, typesafe language for the java platform that combines objectoriented and functional programming. Scala is a functional language in the sense that every function is a value. Pavel fatin blog archive design patterns in scala blog.
Introduction to scala and spark sei digital library. With the questions from the motivation in mind, present and identify. Personally, i think pattern matching is an exciting new feature for ruby developers. In addition, we will discuss the types of scala pattern matching.
Scala smoothly integrates features of objectoriented and functional languages and scala is compiled to run on the java virtual machine. Analyse a collection of design patterns in the context of scala. A class hiercharchy like this is expressed as follows we use scala as programming. Regular expressions, scala, derivatives introduction based on slides by. Scala provides great support for pattern matching for processing the messages. Each includes a pattern or one or more expression that shall be evaluated if the pattern matches. The case statements define a function that is invoked when there is a match. The use of the case statement is actually similar to that of scalas implementation of pattern matching. The ability to use string patternmatching code in a scala match expression is a very cool and useful ability, and i didnt realize how important it was when the kaleidoscope project was released. Net in the past, pattern matching may at first sight appear similar to switch statements. Concurrent programming, join patterns, chords, actors. Scala supports inbuilt pattern matching mechanism which is one of the more powerful feature. How to use pattern matching in scala matchcase expressions. Scala also supports a general notion of pattern matching which can model the algebraic types used in many functional languages.
This tutorial is sponsored by the scala cookbook, which i wrote for oreilly. You can find the scala cookbook at these locations. Scala extractors apply, unapply and pattern matching journaldev. This chapter explains how scala supports regular expressions through regex class available in the scala. May 25, 2009 summary martin odersky talks with bill venners and frank sommers about the mechanics and purpose of pattern matching in scala. Regular expression patterns tour of scala scala documentation. While java language includes static keyword, static members are not associated with any object, and static member classes cannot implement an. In the above example, the numberpattern is a regex regular expression which we use to make sure a password contains a number you can also search for groups of regular expressions using parentheses. A pattern is built from constants, constructors, variables and type tests. But scala offers more powerful and elegant approach for handling such scenarios. Pattern matching can be used with scala maps to useful effect. Scala extractor pattern matching if the instance of the class contains a list of zero or more parameters, compiler calls apply method on that instance. Match expressions matchcase statements and pattern matching are a major feature of the scala language. String symbol match case notename, duration, octave duration case restduration duration the above match expression first checks if the given symbol is a note, and if it is the case it extracts its fields name, duration and octave and evaluates the.
Pdf scalas pattern matching framework supports algebraic data types. Browse other questions tagged scala patternmatching or ask your own question. Browse other questions tagged scala pattern matching or ask your own question. Nesting of function definitions and higherorder functions are naturally supported. In this tutorial, we will show a functional feature of scala which is pattern matching. Hence, in this tutorial, we learned the introduction to scala pattern matching with examples and syntax.934 1331 77 1114 1318 283 1468 1421 912 1277 63 767 1487 145 1193 944 781 892 619 1130 1211 1365 1033 1352 1492 547 531 1350 773 1 925 10 1141 15 608 1395 9 771 1481 1248 721 1185 1366 863 1396 918