How to Pattern Match In Haskell?

11 minutes read

Pattern matching is a fundamental concept in Haskell that allows you to deconstruct data structures and extract values based on their structure. It helps in creating more concise and elegant code by efficiently handling different cases of input.


To pattern match in Haskell, you typically use the case statement or define functions using pattern matching. Here's an overview of how pattern matching works:

  1. The case statement: The case statement allows you to match a value against a series of patterns and execute corresponding code based on the matched pattern. It has the following syntax: case expression of pattern1 -> code1 pattern2 -> code2 ... patternN -> codeN Each pattern is checked one by one from top to bottom, and the code corresponding to the first matching pattern is executed. You can use patterns like literals, wildcards ("_"), variables, tuples, lists, and constructors of custom data types.
  2. Function definition with pattern matching: In Haskell, you can define functions using pattern matching to handle different input patterns efficiently. Each function clause consists of a pattern and the corresponding code to be executed when the input matches the pattern. Here's an example: myFunction :: Int -> String myFunction 0 = "Zero" myFunction 1 = "One" myFunction n = "Other: " ++ show n In this example, myFunction takes an Int as input and pattern matches against different values. If the input is 0, it returns the string "Zero". If the input is 1, it returns the string "One". For any other value, it returns a string concatenation of "Other: " and the string representation of the input.


By utilizing pattern matching, you can easily handle different cases and decompose complex data structures in Haskell. It helps you write robust and concise code that takes advantage of the powerful type system and functional programming features of the language.

Best Haskell Books to Read in 2024

1
Haskell in Depth

Rating is 5 out of 5

Haskell in Depth

2
Programming in Haskell

Rating is 4.9 out of 5

Programming in Haskell

3
Get Programming with Haskell

Rating is 4.8 out of 5

Get Programming with Haskell

4
Parallel and Concurrent Programming in Haskell: Techniques for Multicore and Multithreaded Programming

Rating is 4.7 out of 5

Parallel and Concurrent Programming in Haskell: Techniques for Multicore and Multithreaded Programming

5
Haskell from the Very Beginning

Rating is 4.6 out of 5

Haskell from the Very Beginning

6
The Haskell School of Expression: Learning Functional Programming through Multimedia

Rating is 4.5 out of 5

The Haskell School of Expression: Learning Functional Programming through Multimedia


What is the role of pattern matching in recursive data types in Haskell?

Pattern matching plays a crucial role in working with recursive data types in Haskell. Recursive data types are often defined using algebraic data types (ADTs), where a type can have one or more constructors. When working with recursive data types, pattern matching allows us to deconstruct the data type by matching against its constructors, extracting the values contained within.


For example, let's consider a simple recursive data type called List:

1
data List a = Empty | Cons a (List a)


Here, List has two constructors: Empty, representing an empty list, and Cons, representing a non-empty list with a head value and a tail that is another List. With pattern matching, we can write functions to manipulate List values by matching against the constructors:

1
2
3
4
5
6
7
length :: List a -> Int
length Empty = 0
length (Cons _ tail) = 1 + length tail

reverse :: List a -> List a
reverse Empty = Empty
reverse (Cons head tail) = append (reverse tail) (Cons head Empty)


In the length function, pattern matching is used to distinguish between the Empty and Cons constructors and compute the length accordingly. In the reverse function, pattern matching is used to extract the head and tail of the list, recursively reverse the tail, and append the reversed tail with the head.


Pattern matching allows us to drill down into the structure of recursive data types, handling different cases separately, and performing recursive operations on the components. It is an essential tool for working with and manipulating recursive data types in Haskell.


What is pattern matching in Haskell?

Pattern matching in Haskell is a powerful feature that allows you to match data structures against specific patterns and bind variables to the matched parts. It is a way of deconstructing complex data structures by examining their structure and contents. It is used in function definitions, where different patterns of input can be matched to different function implementations.


In Haskell, pattern matching is primarily used with algebraic data types such as lists, tuples, and user-defined data types. It is done using a combination of patterns and guards. Patterns can be simple, like matching a specific value or a variable, or they can be complex, like matching a list with specific elements.


Pattern matching is often used in function definitions to handle different cases or combinations of input values. It allows you to define different function implementations for different patterns, making the code more concise and readable.


Here is an example of pattern matching in Haskell:

1
2
3
isZero :: Int -> Bool
isZero 0 = True
isZero _ = False


In this example, the function isZero takes an Int as input. If the input is 0, it matches the pattern 0, and the function returns True. If the input is any other value, represented by the wildcard pattern _, the function returns False.


Pattern matching can also be used to destructure complex data structures, like lists or tuples. For example:

1
2
3
sumList :: [Int] -> Int
sumList []     = 0
sumList (x:xs) = x + sumList xs


In this example, the sumList function takes a list of integers as input. If the list is empty, it matches the pattern [] and returns 0. If the list is not empty, it matches the pattern (x:xs) where x is the head of the list and xs is the tail. The function then recursively calls itself (sumList xs) to sum the rest of the list elements.


Pattern matching is a fundamental concept in Haskell and is widely used to write concise and expressive code. It allows you to match data structures against patterns, extract values, and handle different cases or combinations of input.


How to match against guards and conditions in Haskell pattern matching?

In Haskell pattern matching, guards and conditions are used to filter out certain patterns and perform additional checks on the values being matched. Here's how you can use them:

  1. Basic pattern matching: foo :: Int -> String foo 0 = "Zero" foo 1 = "One" foo _ = "Other"
  2. Using guards: bar :: Int -> String bar x | x < 0 = "Negative" | x == 0 = "Zero" | otherwise = "Positive" In this example, the guard | x < 0 checks if x is negative, and if true, returns "Negative". Similarly, other guards x == 0 and otherwise are used to check for zero and positive numbers, respectively.
  3. Combining guards and pattern matching: baz :: (Int, String) -> String baz (x, y) | x < 0 = "Negative " ++ y | x == 0 = "Zero " ++ y | otherwise = "Positive " ++ y Here, the pattern (x, y) matches a tuple, and the guards are used to perform additional checks based on the value of x. The result is combined with the value of y in each case.
  4. Using conditions within guards: qux :: Int -> String qux x | odd x && x > 0 = "Positive odd" | even x && x > 0 = "Positive even" | otherwise = "Other" In this example, the conditions odd x and even x are used within the guards to perform further checks along with the value of x.


Remember that guards should be indented further than the corresponding pattern and start with a pipe symbol (|). They are evaluated from top to bottom, and the first matching guard with a true condition is executed. If none of the guards are true, a runtime error occurs.


What are irrefutable patterns and how are they used in Haskell pattern matching?

Irrefutable patterns in Haskell are patterns that match any value and never fail. In other words, they always succeed in binding one or more variables. They are typically written using a tilde (~) prefix.


Irrefutable patterns are used in Haskell pattern matching when you want to bind variables without checking the structure of the matched value. For example, consider the following function definition:

1
2
3
foo :: [a] -> [a]
foo ~(x:xs) = x:foo xs
foo [] = []


In the first pattern, ~(x:xs), the tilde signifies an irrefutable pattern where the function foo binds the head element x and the tail xs without checking if the input list is empty or not. This allows foo to handle infinite lists lazily.


If we were to define foo without the tilde, like foo (x:xs), it would only match non-empty lists, and applying foo to an empty list would result in a pattern match failure.


Using irrefutable patterns in pattern matching can be beneficial when you know that the pattern will always succeed, or when you want to create a lazy data structure. However, using them indiscriminately can lead to runtime errors if the assumption of non-empty values is violated.

Facebook Twitter LinkedIn Telegram Whatsapp Pocket

Related Posts:

Pattern matching is a fundamental concept in Erlang, allowing developers to match and manipulate values in a concise and powerful way. Here&#39;s a brief explanation of pattern matching in Erlang:Basic Syntax: In Erlang, pattern matching is performed using the...
Function evaluation in Haskell follows a process known as lazy evaluation. Unlike eager evaluation, which immediately evaluates all expressions, lazy evaluation delays evaluation until the value is actually needed. This leads to increased efficiency by avoidin...
To use PHP pattern regex in Delphi, you can follow these steps:Add the RegularExpressions unit to your Delphi project by including System.RegularExpressions in the uses section of your code. Create a new instance of the TRegEx class in Delphi. This class provi...