Why do I get "expected type Future" error using match statement on Result?

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06-10-2024

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"Expected Type Future": Demystifying Result Matching in Dart

The Problem: You're trying to use a match statement on a Result object in your Dart code, but you encounter the dreaded "expected type Future" error. This leaves you scratching your head, wondering why the compiler can't seem to grasp what you're trying to achieve.

Scenario: Let's say you have a function that returns a Result object, representing the outcome of some operation:

Result<String, String> fetchData() {
  // ... fetch data from an API or other source
  if (success) {
    return Result.success("Data successfully fetched!");
  } else {
    return Result.failure("Failed to fetch data");
  }
}

Now, you want to use a match statement to gracefully handle both successful and unsuccessful outcomes:

void main() {
  final result = fetchData();
  match (result) {
    Result.success(String value) => print("Success! Data: $value"),
    Result.failure(String error) => print("Error: $error"),
  };
}

But instead of smoothly executing, you encounter the infamous "expected type Future" error. What's going on?

Explanation: The core issue lies in the interplay of Result and Future in Dart. While Result is a great tool for representing success or failure states, it doesn't inherently deal with asynchronous operations. The match statement, designed for exhaustive pattern matching, expects to work with values, not with potential future values.

The Solution: You need to bring the asynchronous nature of your data fetching into the picture. There are two primary approaches to achieve this:

1. Utilize async/await:

   import 'package:result/result.dart';
   
   Future<void> main() async {
     final result = await fetchData();
     match (result) {
       Result.success(String value) => print("Success! Data: $value"),
       Result.failure(String error) => print("Error: $error"),
     };
   }
   
   Future<Result<String, String>> fetchData() async {
     // ... fetch data from an API or other source
     if (success) {
       return Result.success("Data successfully fetched!");
     } else {
       return Result.failure("Failed to fetch data");
     }
   }
   

   Here, we use await to pause execution until the fetchData function completes, allowing us to directly work with the resolved Result value within the match statement.

2. Embrace Futures directly:

   import 'package:result/result.dart';
   
   void main() {
     fetchData().then((result) {
       match (result) {
         Result.success(String value) => print("Success! Data: $value"),
         Result.failure(String error) => print("Error: $error"),
       };
     });
   }
   
   Future<Result<String, String>> fetchData() async {
     // ... fetch data from an API or other source
     if (success) {
       return Result.success("Data successfully fetched!");
     } else {
       return Result.failure("Failed to fetch data");
     }
   }
   

   This approach explicitly handles the asynchronous aspect. We use then to define a callback function that will be executed when fetchData completes, and then we can directly perform the match operation on the Result value within the callback.

In Summary:

• The "expected type Future" error arises because the match statement isn't designed for asynchronous operations.
• Using async/await or then allows you to effectively deal with asynchronous results, providing the necessary synchronization for your code.
• Remember that Result is a powerful tool for representing success or failure states, but it doesn't inherently address the asynchronous nature of data fetching.

By understanding the nuances of asynchronous operations and their interplay with Result and match, you can confidently handle your asynchronous tasks with clarity and efficiency.