Lists and String Sequences | CodeSignal Learn

Lists and String Sequences | CodeSignal Learn

Introduction: Grouping Data in Haskell

Welcome to Unit 2! In our first unit, we focused on pattern matching and working with single, isolated values such as individual numbers and characters . However, when writing real-world programs, we rarely deal with just one of something. We have lists of user IDs, lines of text, and groups of settings . In Haskell, the primary way to group items together is by using a sequence called a list . A list allows us to store multiple pieces of data in a specific order, provided that all items share the same type . In this lesson, we will learn how to create lists, add new elements to them, combine them, and display them on the screen.

Building and Growing Lists of Numbers

Let's start by creating a simple list of whole numbers . In Haskell, we write a list by placing our items inside square brackets, separated by commas . Haskell numbers :: [Int] numbers = [1, 2, 3] Here, we define a variable named numbers . The type signature [Int] tells Haskell that this is a list containing integers . We often need to add new data to an existing list . We can add a single item to the very front of a list using the cons operator, which is written as a single colon (:). Haskell numbers :: [Int] numbers = [1, 2, 3] withZero :: [Int] withZero = 0 : numbers In this code, 0 : numbers takes the number 0 and attaches it to the front of the numbers list . Sometimes, we want to combine two complete lists rather than just adding a single item. We can join two lists together using the concatenation operator, which is written as two plus signs (++). Haskell numbers :: [Int] numbers = [1, 2, 3] withZero :: [Int] withZero = 0 : numbers moreNumbers :: [Int] moreNumbers = numbers ++ [4, 5] Here, numbers ++ [4, 5] takes our original list and attaches [4, 5] to the end of it. There is a very important rule to remember regarding how Haskell handles data: lists are permanent, or immutable . When we use : or ++, we are not changing the original numbers list . Instead, Haskell creates brand-new lists for withZero and moreNumbers . The original numbers list remains exactly [1, 2, 3] .

The Secret Identity of Strings

Now that we know how lists work, we can uncover a fundamental rule in Haskell : a standard text String is simply a list of characters behind the scenes. Let's prove this by starting with a list of individual characters, which has the type [Char] . Notice that single characters use single quotes . Haskell letters :: [Char] letters = ['H', 'i'] Because a String is identical to a [Char] list, we can use our list operators on text. Let's use the concatenation operator (++) to join our character list with a standard string literal . Haskell letters :: [Char] letters = ['H', 'i'] greeting :: String greeting = letters ++ "!" In this code, "!" is a String, which means it is treated as ['!'] . Because letters is also a list of characters, ++ works perfectly to join them into a new String called greeting .

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Displaying Lists and Strings

To see the results of our work, we need to print our lists to the screen. As a reminder from earlier lessons, we do this inside the main block of our program . We will start by using the print function to output our number lists . Haskell main :: IO () main = do print numbers print withZero print moreNumbers The print function is excellent for viewing our raw data exactly as we typed it. When we run this code, the output is as follows: text [1,2,3] [0,1,2,3] [1,2,3,4,5] Note that numbers remained unchanged, even after we used it to build withZero and moreNumbers . Now, let's add our text to the output. While we could use print for text, it would display the string with quotes around it. When we want to display a String as clean, normal text, we use the putStrLn (put string line) function instead. Haskell main :: IO () main = do print numbers print withZero print moreNumbers putStrLn greeting When we run our full program, we see our number lists alongside our clean text greeting : text [1,2,3] [0,1,2,3] [1,2,3,4,5] Hi!