How to Handle Non-Exhaustive Patterns Error in Haskell for String Comparison

Learn how to resolve the `Non-exhaustive patterns` error in Haskell when comparing two strings and identify their first different character.
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Resolving Non-Exhaustive Patterns Error in Haskell for String Comparison

When working with Haskell, you might encounter a common issue while trying to match elements in lists or strings, particularly when checking for the first different character between two strings. This guide will explore the challenge of getting a Non-exhaustive patterns error and how to effectively fix it through pattern matching.

The Problem

You want to create a function in Haskell called match, which should take two strings as inputs and return the first different character from the first string. If both strings are identical or if the first string is a prefix of the second, the function should return Nothing. Here's an example of how you expect it to behave:

[[See Video to Reveal this Text or Code Snippet]]

However, upon running your implementation, you encounter a frustrating Non-exhaustive patterns error. This suggests that your code isn’t handling all possible input cases. Let's break down how to address this issue.

Analyzing the Current Code

You may have already implemented code similar to the following:

[[See Video to Reveal this Text or Code Snippet]]

While this implementation covers several base cases, it fails to account for cases where the first string is longer than the second, resulting in the dreaded Non-exhaustive patterns error.

Key Insights to Fix the Issue

The error occurs because there are scenarios where the input strings can differ in their lengths, particularly when one string is empty while the other has characters. To solve this, you can simplify your pattern matching by focusing on two key cases:

When the first string is empty: If the first string is [], there can’t be any characters to return; thus, you can return Nothing.

When the second string is empty: If you reach this point with some characters left in the first string, you want to return the first character.

The Fixed Code

By adjusting the pattern match and removing unnecessary singleton list checks, you achieve a more straightforward implementation. Here's how your match function can look now:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Revised Code

match [] _ = Nothing: This handles the case where the first string is empty. No difference can be found, so it safely returns Nothing.

match (x:_) [] = Just x: If the second string is empty, the first string still has characters. Here, Just x returns the first character of the first string.

match (x:xs) (y:ys): This recursive case compares the current characters from both strings. If they match, it calls itself with the remaining characters. If they don’t match, it returns the first character of the first string where the difference occurs.

Conclusion

By understanding and addressing the concept of pattern matching in Haskell, you can easily solve the Non-exhaustive patterns error and create a reliable function to find the first different character between two strings. By reducing unnecessary complexity in your conditions, you not only make your code cleaner but also more efficient.

Now you can confidently use and adapt this approach in your future Haskell endeavors!