Solving Expression.Error in Excel Power Query: Passing a Table to a Function

Discover how to resolve the `Expression.Error` in Excel Power Query when passing a table to a function. Get step-by-step guidance and solutions tailored for beginners.
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Understanding the Challenge: Passing a Table to a Function in Power Query

In the world of data analysis, Excel’s Power Query offers powerful tools for transforming and processing data. However, new users often encounter hiccups in their journey. One common error message that arises is: "Expression.Error: We cannot convert a value of type Table to type Function." This error can be frustrating, particularly for those who are still learning the intricacies of Excel's M language.

The core of the problem lies in the attempt to pass a query that returns a table into a function, which leads to type mismatch issues. In this guide, we will delve into how to fix this problem and pass tables to functions successfully in Power Query.

The Scenario: Encountering the Error

Let’s break down the situation to understand the components involved.

You are trying to use the following code:

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

In this example, fxRetry is a recursive function designed to repeatedly call the Insider([Value]) query until it successfully retrieves information. Unfortunately, you are getting an expression error because Insider([Value]) is not recognized as a function.

What’s Going Wrong?

The main issue here is that the variable Insider([Value]) is actually returning a table instead of being treated as a callable function. The fxRetry function expects a function (as indicated by its signature), but you're giving it a table, thereby causing a type mismatch.

The Solution: Correcting Type Mismatches

Step 1: Check the Nature of Insider([Value])

First and foremost, it's essential to ensure that Insider([Value]) is a function and not a table. In the Power Query M language, functions require specific structure. Here’s an example format typically used for a function:

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

If Insider is defined as a complete query that returns a table but it is not wrapped in a function, you will have to redefine it.

Step 2: Redefine Insider as a Function

To resolve the issue, make sure that Insider is defined correctly as a function. For instance:

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

This creates a definable function that can be passed to fxRetry correctly.

Step 3: Refactor Your fxRetry Function

While maintaining your existing logic, ensure that the call to fxRetry within Table.AddColumn matches the signature expected. Here’s a full refactor:

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

This modification ensures that you are passing a function ((() => Insider([Value]))) instead of a table, thereby avoiding the conversion error.

Conclusion

While working with Power Query M functions can initially be challenging, approaching and solving the problems step by step can significantly simplify the process. By ensuring that you pass the correct types to your functions, you can avoid common pitfalls like the Expression.Error encountered here. Always validate the structure of your data and ensure you're using functions correctly.

By following the guidance outlined in this guide, you should be well on your way to mastering the nuances of Excel's Power Query and effective data handling. Happy querying!

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