Naming Your Domain | CodeSignal Learn

Naming Your Domain | CodeSignal Learn

Introduction: Speaking the Language of Your App

Welcome to the very first unit of our " Haskell Programming for Beginners " course! Over the next seven units, we will be learning Haskell by building a command-line task manager . When writing programs, we often use basic data types built into the language. For example, we use Int for numbers and String for text. However, if we use Int and String for everything, our code can become confusing. If you see a function that requires an Int, is that number meant to be an age, a price, or a task ID ? To solve this, we want to create a " domain vocabulary. " This means we want to name our types after the real-world concepts they represent. By the end of this lesson, you will learn how to give standard types clear, descriptive nicknames. This will make your code much easier to read and understand.

A Quick Refresher: Strings, show, and Printing

Before we create our new types, let us briefly look at a few basic tools we will use to build our code today. Since our goal is to format and display task details on the screen, we need to know how to work with text. In Haskell, you can join two pieces of text (strings) together using the ++ symbol. Haskell "Hello " ++ "World" If you have a number, you cannot directly join it to a string. First, you must convert the number into a string using a built-in tool called show . Haskell "Task number: " ++ show 1 Finally, to print text so the user can see it on the screen, we use putStrLn (which stands for "put string line"). We will use these three basic tools — ++, show, and putStrLn — to display our tasks later in the lesson.

Creating Nicknames with the type Keyword

Now, let us introduce our main topic. In our task manager, every task will have an ID number and a text title. Instead of just calling them Int and String, we can declare a type synonym using the type keyword. Haskell type TaskId = Int type Title = String In this code, we are telling Haskell: "From now on, the name TaskId means the exact same thing as Int, and Title means the exact same thing as String ." The most important rule to remember is that the type keyword does not create a brand-new type. It only creates an alias, or a nickname. A TaskId is still just a regular Int under the hood. It follows all the exact same rules as a normal number.

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Writing Clearer Function Signatures

Now that we have our domain vocabulary, we can use it to write clearer code. Let us create a function called describeTask that takes an ID and a title, and formats them into a single string. Normally, a generic signature for this function would look like this: Haskell describeTask :: Int -> String -> String While this works, it does not tell us what the Int and String are supposed to be. Let us upgrade this signature using our new type synonyms. Haskell type TaskId = Int type Title = String describeTask :: TaskId -> Title -> String This new signature is much better! It acts as its own documentation. Any programmer reading this instantly knows the function expects a Task ID first, followed by a Title and it will return a regular String . Now, let us add the actual logic to our function. We will take our two inputs, tid and title, and use the string tools we reviewed earlier to format them. Haskell type TaskId = Int type Title = String describeTask :: TaskId -> Title -> String describeTask tid title = "#" ++ show tid ++ " " ++ title Here, we add a "#" symbol to the front, convert our tid number to a string using show, add a space, and then attach the title .

Putting It All Together in main

To see our code in action, we need to put it inside the main block . The main block is where a Haskell program starts running. Let us call our describeTask function with a real number and a real string and print the results. Haskell type TaskId = Int type Title = String describeTask :: TaskId -> Title -> String describeTask tid title = "#" ++ show tid ++ " " ++ title main :: IO () main = do putStrLn (describeTask 1 "Write report") putStrLn (describeTask 2 "Review code") Notice how we pass a plain 1 and "Write report" into our function. Because TaskId and Title are just nicknames, Haskell perfectly accepts normal numbers and strings in those slots. When we run this full program, we get the following output: text #1 Write report #2 Review code Our function took the raw data, treated the numbers and text correctly based on our underlying types, and formatted them perfectly.