[dry-operation] on_failure handler's first argument

1

Hello, I’m trying to implement #on_failure handler for my operation and I was expecting the first argument to be a monad, instead I’m getting unwrapped result passed to Failure.

def on_failure(failure, step_name)
  @failure_handler.call(failure:, step_name:)
end

Inside the handler I wanted to do pattern matching on the failure and apply proper behavior. But IDK if should expect the Failure or its value.

2

Also I have some doubts about the second argument as from what I noticed it always returns :call, no matter which step actually failed.

3

Hey @buszu, yeah, failure is the unwrapped result and not the actual Monad. This is because that won’t ever be a Success, so you shouldn’t need to pattern match based on that condition. Does it make sense?

4

About the second argument, it turns out docs are wrong and the method received is the one wrapping the steps, which is :call by default (I’ll fix the docs). This is because you can have different operations defined in a single class (e.g. by using operate_on :call, :run). If you want to reference a particular step, you should include it in the unwrapped result you give to Failure.

5

Fixing docs: Fix docs on error handling by waiting-for-dev · Pull Request #32 · dry-rb/dry-operation · GitHub

6

I think it makes sense, anyway I can still match arrays or simply ‚failure.first’ with normal case assuming that I return failures’ values as arrays where the first element is a symbol describing the failure.

Also this pattern should be enough to determine where the failure happened, so probably I don’t need to use the second argument of the error handler.

Thank you for your reply and fixing the documentation!

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