Understanding Non-exhaustive Patterns Error in Haskell

Learn the reason behind the `Non-exhaustive patterns in function` error in Haskell and how to fix it using multiline definitions and file loading techniques.
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Understanding Non-exhaustive Patterns Error in Haskell: A Comprehensive Guide

When working with Haskell, you may encounter the dreaded "Non-exhaustive patterns in function" error. This error can be frustrating, especially if you believe you've defined your function correctly to cover all necessary cases. But why does this occur, and how can you resolve it? Let's break down the problem and explore the effective solutions.

The Root of the Problem

Consider the following code snippets for calculating the product and sum of a list:

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When you run this code, you might see an error indicating non-exhaustive patterns for both the sum and product functions. This can be confusing, especially if you've included cases for both an empty list and a list with one or more elements.

What Makes Patterns Non-exhaustive?

The error message typically suggests that there are cases that your function does not cover. In the case above, the culprit lies in how multiline definitions are structured in GHCi (the Glasgow Haskell Compiler interactive environment).

Solution: Properly Defining Multiline Functions

To define a multiline function correctly, GHCi requires a specific format. Here’s how to do it:

Using GHCi Commands

Use Block Braces: Enclose your multiline definitions between :{ and :}. Here’s how to rewrite the product function:

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When you enter the command :{, notice the change in the prompt from > to |. This indicates you are now in a multiline definition mode.

Loading from a File: Alternatively, you can write your functions in a Haskell source file (with a .hs extension), then load it into GHCi. This ensures that all lines are treated as a single function definition without the need for special commands.

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Load it in GHCi:

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Conclusion

Understanding how Haskell treats function definitions in GHCi is essential to avoiding the "Non-exhaustive patterns in function" error. By using the multiline definition commands or, preferably, creating a source file, you can effectively avoid this issue.

Now, don't let this pesky error disrupt your coding flow! Happy coding in Haskell!