Understanding Haskell Recursion: Fixing the Non-exhaustive patterns in function go Error

Learn how to calculate the length of a list in Haskell using guards without running into the `Non-exhaustive patterns in function go` error. Discover the correct approach to avoid pattern matching issues in your recursive functions.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Calculating length of the list recursively using guards throws "Non-exhaustive patterns in function go"

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Haskell Recursion: Fixing the Non-exhaustive patterns in function go Error

Working with recursion in Haskell can sometimes be challenging, especially when it comes to pattern matching. A common issue arises when trying to calculate the length of a list using guards, which may throw the error "Non-exhaustive patterns in function go." In this guide, we will explore the source of this error and how to fix it while effectively calculating the length of a list.

The Problem

You may have encountered the following Haskell code while attempting to calculate the length of a list:

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

This pattern-matching approach works flawlessly. However, when you try to use guards like this:

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

...you run into an error: Non-exhaustive patterns in function go.

Why Does This Error Occur?

The key issue here lies in how you're using pattern matching and guards. When using (x : xs), you are explicitly stating that the list has at least one element (where x is the head and xs is the tail). However, checking if (x : xs) == [] can never be true since (x : xs) creates a non-empty list.

Understanding the Components

Pattern Matching:

In pattern matching, there are specific patterns that must be covered for all possible inputs. If pattern matches do not satisfy any input scenario, you will encounter a runtime error.

The go Function:

When the list is empty (represented by []), it cannot match the pattern (x : xs) since the latter requires at least one element, while [] indicates no elements.

The Solution

To properly use guards in your function without running into the non-exhaustive patterns issue, you need to ensure that every possible input is accounted for. Here's how you can modify your myLength2 function:

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

Breakdown of the Solution

Base Case:

The line go [] res = res acts as the base case, returning the result when the list is empty.

Recursive Case:

The line go (x : xs) res = go xs (res + 1) processes non-empty lists. No guards are needed here since the pattern itself covers existing elements.

Conclusion

Understanding recursion and pattern matching in Haskell is essential. When you encounter the Non-exhaustive patterns in function go error, remember to ensure all possible list configurations are addressed in your function. By correcting how you incorporate guards and making sure the base case is present, you can achieve your desired outcomes without errors.

Now that you've learned how to calculate the length of a list using Haskell guards properly, give it a try, and enjoy exploring more of Haskell’s powerful features!