Why pattern matching

You should start with the Wikipedia page that gives a pretty good explanation. Then, read the relevant chapter of the Haskell wikibook.

So pattern matching is a way of assigning names to things or binding those names to those things , and possibly breaking down expressions into subexpressions at the same time as we did with the list in the definition of map. As you can see the pattern matched solution has less noise, you can clearly see what are the different cases and how easy it's to travel and de-structure our list.

I've written a more detailed blog post about it here. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What is 'Pattern Matching' in functional languages? Ask Question. Asked 11 years, 7 months ago. Active 2 years, 1 month ago. Viewed 42k times. Improve this question. Roman Roman Add a comment. Active Oldest Votes. Understanding pattern matching requires explaining three parts: Algebraic data types. What pattern matching is Why its awesome.

Algebraic data types in a nutshell ML-like functional languages allow you define simple data types called "disjoint unions" or "algebraic data types". Cons s. Pattern matching is awesome You can implement something similar to pattern matching in C using the visitor pattern , but its not nearly as flexible because you can't effectively decompose complex data structures. Improve this answer.

Community Bot 1 1 1 silver badge. Juliet Juliet Nevertheless, excellent answer! Can you point me to a primer? DavidMoles: The type system makes it possible to elide all run-time checks by proving pattern matches to be exhaustive and not redundant. If you try to feed a language like SML, OCaml or F a pattern match that is not exhaustive or contains redundancy then the compiler will warn you at compile time.

This is an extremely powerful feature because it allows you to eliminate run-time checks by rearranging your code, i. Furthermore, it is easy to understand! JonHarrop I can see how that would work effectively it's similar to dynamic message dispatch but I can't see how at run-time you select a branch without a type test.

Show 3 more comments. Xaerxess What an interesting way of thinking about the equals sign. Thanks for sharing that! What does Cons mean? Roymunson: Cons is the cons tructor that builds a linked list out of a head the a and a tail the List a.

The name comes from Lisp. In Haskell, for the built-in list type, it's the : operator which is still pronounced "cons". In Scala: import Double. Blur your eyes a little and you can imagine this in javascript. Russell Leggett Russell Leggett 8, 3 3 gold badges 29 29 silver badges 45 45 bronze badges.

Tomas Petricek Tomas Petricek k 19 19 gold badges silver badges bronze badges. It is not only a functional language feature but is available for many different languages. Also as you can see from the example the interpreter can also break up a single argument into several variables automatically e.

For many people, picking up a new concept is easier if some easy examples are provided, so here we go: Let's say you have a list of three integers, and wanted to add the first and the third element. This is a nice definition from the above wikibook: So pattern matching is a way of assigning names to things or binding those names to those things , and possibly breaking down expressions into subexpressions at the same time as we did with the list in the definition of map.

For example, we can pattern match on tuples:. And also when comparing different types, for example if matching a tuple on the left side with a list on the right side:. More interestingly, we can match on specific values. The example below asserts that the left side will only match the right side when the right side is a tuple that starts with the atom :ok :.

The [head tail] format is not only used on pattern matching but also for prepending items to a list:. Pattern matching allows developers to easily destructure data types such as tuples and lists. As we will see in the following chapters, it is one of the foundations of recursion in Elixir and applies to other types as well, like maps and binaries.

Because we have pinned x when it was bound to the value of 1 , it is equivalent to the following:. You can write patterns that examine multiple properties of an object. Consider the following Order record:. The preceding positional record type declares two members at explicit positions. Appearing first is the Items , then the order's Cost. For more information, see Records. The following code examines the number of items and the value of an order to calculate a discounted price:.

The first two arms examine two properties of the Order. The third examines only the cost. The next checks against null , and the final matches any other value. If the Order type defines a suitable Deconstruct method, you can omit the property names from the pattern and use deconstruction to examine properties:. The preceding code demonstrates the positional pattern where the properties are deconstructed for the expression.

This article provided a tour of the kinds of code you can write with pattern matching in C. The following articles show more examples of using patterns in scenarios, and the full vocabulary of patterns available to use. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.

Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode.

Null checks One of the most common scenarios for pattern matching is to ensure values aren't null. You can test and convert a nullable value type to its underlying type while testing for null using the following example: int? That makes it an ideal way to check null reference values, adding the not pattern: string? Type tests Another common use for pattern matching is to test a variable to see if it matches a given type. Skip halfLength.

Compare discrete values You can also test a variable to find a match on specific values. Relational patterns You can use relational patterns to test how a value compares to constants. Multiple inputs All the patterns you've seen so far have been checking one input.

Consider the following Order record: public record Order int Items, decimal Cost ; The preceding positional record type declares two members at explicit positions.