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Comment by vikeri | Hacker News Reader

One thing that seemingly is missing is the ability to tag a specific error with an error code. You typically want to know that all of a sudden the ”failed to get user” error is being returned a lot. Since the message is a dynamic string you can’t just group by the string so unless you build it as part of your abstraction it becomes very hard to do.

Edit: looking more carefully at the lib I assume that ”tag” is the concept that is supposed to cover this?

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in standard go you'd have errors implement:

    func (myError) Is(err error) bool

and it can match different sentinel errors. Or you can make your own wrapper to have the error chain match.

In standard go you would not implement Is but would use the errors.Is call. How would this work if it is a sentinel error as you would use with errors.Is??

https://pkg.go.dev/errors#Is

> An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.

by default errors.Is matches the exact error variable, but you can use it to match other errors as well.

I stand corrected. Still you cannot implement an Is on a sentinel error and is only applicable to concrete error types. And if I'm using concrete types I'm going to us errors.As

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