The Importance of Exhaustive Case Statements: Why Every Possibility Matters

#elm #webdev

In programming, unexpected runtime errors frequently hide in unhandled branches of logic. Exhaustive case statements force developers to consider every possible input explicitly, resulting in safer, more maintainable, and robust code. In this post, we’ll explore what makes a case statement exhaustive, see concrete examples in Elm, discuss how TypeScript handles similar patterns, and highlight the extra verbosity and convention-dependence of achieving exhaustiveness in TypeScript.

What Is an Exhaustive Case Statement?

An exhaustive case statement (or pattern matching) requires that every possible variant of a data type is explicitly handled. Imagine designing a function that operates on a union type where disregarding even one possibility could lead to unexpected behavior at runtime. Enforcing exhaustiveness is like ensuring you’ve catered to every dietary need at a dinner party—it prevents a scenario where some guest’s requirement is accidentally overlooked.

Exhaustive Case Statements in Elm

Elm is renowned for its robust type system and for insisting that functions be total—meaning every possible input must yield an output. When writing a case statement in Elm, the compiler automatically checks that every variant of your union type is handled.

For example, consider a simple union type for fruits:

type Fruit
    = Apple
    | Banana
    | Orange

A corresponding function to describe each fruit might look like this:

describeFruit : Fruit -> String
describeFruit fruit =
    case fruit of
        Apple ->
            "A crisp apple that's both sweet and tart."

        Banana ->
            "A smooth, ripe banana perfect for a quick snack."

        Orange ->
            "An orange bursting with vitamin C!"

Since every declared variant—Apple, Banana, and Orange—has been addressed, the Elm compiler is content. Should you later extend the type:

type Fruit
    = Apple
    | Banana
    | Orange
    | Grape

the compiler will immediately flag that describeFruit lacks a branch for Grape, enforcing a timely update and preventing potential runtime issues.

Exhaustive Pattern Matching in TypeScript

TypeScript, while powerful with union types, does not provide native exhaustive pattern matching. By default, it will not force you to account for every possibility in a switch statement. Instead, you must rely on established patterns to simulate this behavior.

Here’s a similar example in TypeScript:

type Fruit =
  | { kind: "apple" }
  | { kind: "banana" }
  | { kind: "orange" };

function describeFruit(fruit: Fruit): string {
  switch (fruit.kind) {
    case "apple":
      return "A crisp apple with a tangy flavor.";
    case "banana":
      return "A smooth, ripe banana, ready to eat.";
    case "orange":
      return "An orange bursting with vitamin C!";
    default: {
      // The `never` type here ensures that if a new variant is added, a compile-time error occurs.
      const exhaustiveCheck: never = fruit;
      throw new Error(`Unhandled fruit: ${JSON.stringify(exhaustiveCheck)}`);
    }
  }
}

In this TypeScript code, the default case uses an assignment to a variable of type never. This manual check will catch any missing cases during the development process, but it relies entirely on the developer to include this convention. Without this extra clause, TypeScript will not enforce exhaustiveness, and errors could slip past unnoticed.

The Verbosity of TypeScript

One of the key differences between Elm and TypeScript in this context is the verbosity and discipline required on the TypeScript side. Whereas Elm’s compiler automatically enforces that every branch is covered, TypeScript demands the following:

Extra Code: You need to write additional boilerplate (such as the default case with the never check) for every switch statement that you want to be exhaustive.
Developer Discipline: Without a built-in mechanism to ensure exhaustiveness, the safety net entirely depends on coding conventions. If the convention isn’t applied consistently, unhandled cases may lead to runtime issues.
Manual Maintenance: As your union types evolve, ensuring that the manual pattern matching remains exhaustive requires vigilant maintenance, whereas Elm handles these changes for you automatically.

This reliance on conventions makes TypeScript more verbose and, potentially, more error-prone compared to languages with built-in exhaustive checking. Developers need to remember to incorporate and maintain these safeguards in every relevant part of their code.

Comparing Exhaustiveness: Elm vs. TypeScript

Let’s break down the performance of exhaustiveness in both languages:

Aspect	Elm	TypeScript
Compiler Guarantees	Automatically verifies totality of pattern matching.	Relies on manual implementation (e.g., using the `never` pattern).
Code Safety	High – Missing cases trigger compile-time errors automatically.	Moderate – Missing cases require discipline to catch using conventions.
Ease of Refactoring	Changes in union types immediately enforce necessary updates.	Developers must manually update the switch statements, which can lead to oversights if not managed carefully.

Elm’s built-in exhaustive checks provide a seamless experience, whereas TypeScript offers developer-driven methods that require extra attention.

Why Does Exhaustiveness Matter?

Enhanced Reliability: By forcing every possible input to be accounted for, exhaustive case statements reduce the risk of runtime errors.
Improved Maintainability: As your data types evolve, exhaustive checks act as a guide, highlighting areas needing updates, thus easing the maintenance and refactoring process.
Clear Design Intent: Code that explicitly handles every possibility naturally communicates its intent and logic, making it easier for future developers to understand and extend.
Compiler Assistance: In languages like Elm, the compiler’s enforcement of exhaustiveness ensures that no case is accidentally omitted. In TypeScript, while this is simulated through conventions, the extra boilerplate in turn serves as a constant reminder to consider every possibility.

Looking Ahead

While we’ve spotlighted Elm's built-in exhaustive checking and TypeScript's more verbose, convention-driven approach, the principles remain broadly applicable. Several modern languages—including Haskell, Rust, and Swift—offer their own strategies to enforce complete handling of possible cases. Delving into these aspects can provide fresh insights into designing robust, maintainable software.

For additional exploration, you might consider:

Advanced Pattern Matching Techniques: Study how nested patterns and guards can further enforce logical integrity in your functions.
Leveraging Compiler Feedback: Discover how different languages use compiler feedback to drive safer coding practices.
Designing for Scalability: Learn how ensuring exhaustive coverage can make your codebase easier to extend as new features are added.

By embracing exhaustive case statements—whether automatically enforced like in Elm or manually maintained as in TypeScript—you’re not merely writing code; you're constructing a robust framework that stands the test of time and evolving requirements.

Are you intrigued by these patterns? How do you balance the trade-off between brevity and safety in your projects? Whether you’re leaning into Elm’s compiler guarantees or meticulously applying manual checks in TypeScript, every extra line of code is an opportunity to make your logic more resilient and your development process more predictable.