Extracting a specific error from the error tree

How to do it...

Use the errors.As function to descend an error tree, find a particular error, and extract it.

How it works...

Similar to the errors.Is function, errors.As(err error, target any) bool descends the error tree of err until an error that is assignable to target is found. That is done by the following:

1. It checks if the value pointed to by target is assignable to the value pointed to by err.
2. If that fails, it checks if err has an As(error) bool method by calling err.As(target). If it returns true, then an error is found.
3. If not, it checks if err has an Unwrap() error method and err.Unwrap() is not nil, descending the tree.
4. Otherwise, it checks if err has an Unwrap() []error method, and if it returns a non-empty slice, it descends the tree for each of those until a match is found.

In other words, errors.As copies the error that can be assigned to target into target.

The following example...