How to Perform Type Assertions for Custom Errors in Go?

Type Assertion for Custom Error Handling in Go

In Go, custom error types allow for more specific error handling. However, checking for the exact type of an error can be challenging. This article addresses the question of how to perform type assertions for custom errors.

The question highlights the use of a custom ModelMissingError type to represent a missing model error. The caller of the method that throws this error wishes to check its type and act accordingly.

The snippet provided in the question uses the equality operator (==) to compare the error variable to the ModelMissingError type, which leads to the error "type model.ModelMissingError is not an expression."

The solution lies in using the "comma ok idiom" in a type assertion. The following code demonstrates how to do this:

serr, ok := err.(*model.ModelMissingError)

This syntax asserts that the interface variable err holds a concrete value of type *model.ModelMissingError and assigns the underlying value to serr. It also assigns a boolean value to ok to indicate the success of the assertion.

The "comma ok idiom" allows you to check for the exact type of an error without triggering a panic if the type assertion fails. If ok is true, the error is of the expected type, and you can take appropriate action based on serr.

By leveraging type assertions and the "comma ok idiom," you can effectively check for custom error types in Go and handle them accordingly, providing more flexibility and precision in error handling.

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