Modeling Absence with Maybe | CodeSignal Learn

Modeling Absence with Maybe | CodeSignal Learn

The "Not Found" Problem and the Maybe Type

Welcome to Unit 4! In our previous lessons, you learned how to create descriptive type synonyms, define custom data types like our Task model, and use pattern matching to look inside those custom types. Now, we are going to tackle a very common real-world problem: searching for an item that might not exist. Imagine looking up a task by its ID . What happens if you provide an ID for a task that is not in your list? In many programming languages, trying to access a missing item causes the program to crash or silently return an unsafe empty value (like null), leading to errors later on. Haskell handles this safely by making "not found" an ordinary, expected value. It does this using a built-in type called Maybe . You can think of Maybe as a physical box. If the search is successful, the box contains exactly one item. We represent this using the word Just, followed by the item (for example, Just task). If the search fails, the box is completely empty. We represent this using the word Nothing . By using Maybe, we make absence explicit in the type of the function. That encourages us to handle both the success and failure cases. When we pattern match on both Just and Nothing, as we will in this lesson, our lookup can handle missing tasks without crashing.

Setting up a Helper Function

Before we write our safe search function, let's prepare a small helper function. As a reminder, here is the custom Task data type we built in previous lessons: type TaskId = Int type Title = String data Status = Todo | Done deriving (Show, Eq) data Task = Task TaskId Title Status deriving (Show, Eq) To search through a list of tasks, we will frequently need to check the ID of a given task. Instead of writing out the full pattern match every time, we can create a small helper function called taskId . taskId :: Task -> TaskId taskId (Task tid _ _) = tid Because we only care about the ID, we use underscores_to ignore the title and the status. This function takes a Task, looks inside it using pattern matching, and returns just the TaskId .

Writing a Safe Search Function

Now we are ready to write our findTask function. We want to provide a TaskId and a list of Task items, and we want it to return a Maybe Task (our safe box). Let's start by defining the signature and the most basic scenario: an empty list. findTask :: TaskId -> [Task] -> Maybe Task findTask _ [] = Nothing If the list is empty [], it means we have run out of tasks to check. Therefore, the task we are looking for is definitely not there. We return Nothing . Notice that we use an underscore_for the ID we are searching for, because if the list is empty, the ID we want does not matter. Next, we handle the case where the list is not empty. We separate the list into the first task t and the rest of the list. findTask :: TaskId -> [Task] -> Maybe Task findTask _ [] = Nothing findTask wanted (t : rest) | taskId t == wanted = Just t | otherwise = findTask wanted rest Here, we use guards (the | symbol) to check a condition: If the taskId of our current task t equals the wanted ID, we found a match! We safely place the task in our box by returning Just t . otherwise, if they do not match, we call findTask again to keep searching through the rest of the list.

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Unboxing the Result with Pattern Matching

Now that our search safely returns a Maybe Task, we need a way to open that box and use the result in our application. We could use print to see raw debugging output like Just (Task 1 "Write report" Todo) or Nothing, because our types have Show instances. But for a user-friendly task manager, we want a clean message, and to produce that message we must handle both possible outcomes. Let's write a report function that takes a Maybe Task and returns a standard String message. We will use pattern matching to handle the two possibilities. First, let's handle the empty box: Haskell report :: Maybe Task -> String report Nothing = "No task with that id." If the search returns Nothing, we simply return a friendly string explaining that the task was not found. Next, we handle the successful box. We will rely on a describe function that we built in a previous lesson, which takes a standard Task and turns it into a formatted string. Haskell report :: Maybe Task -> String report Nothing = "No task with that id." report (Just t) = describe t When we pattern match using (Just t), we pull the task out of the box and assign it to the variable t . We then pass that extracted task t directly into our describe function. Let's tie it all together in our main program to see how it works. We will define a small list of tasks and try to find an ID that exists, followed by an ID that does not exist. Haskell tasks :: [Task] tasks = [Task 1 "Write report" Todo, Task 2 "Review code" Done] main :: IO () main = do putStrLn (report (findTask 1 tasks)) putStrLn (report (findTask 9 tasks)) If you run this program, you will see the following output: text #1 Write report (not done) No task with that id. Notice that the program smoothly prints out the formatted task for ID 1 and elegantly handles the missing ID 9 without crashing!