Call rescue_from with a method instead of a block

[robertopedroso]

This PR adds a simple DSL for passing methods to rescue_from, much like Rails permits:

rescue_from Sequel::NoMatchingRow, with: rescue_missing_record

def rescue_missing_record
  Rack::Response.new('Missing Record', 404)
end

This means you can implement and call custom methods in your rescue handlers. At the moment, this is impossible because the passed block is executed via an instance_exec in the context of Grape::Middleware::Error.

The only problem is that this implementation cannot incorporate exception messages into the rescue method. Rails overcomes this limitation by passing the method as a symbol. It then calls the method with something like:

handler = method(rescuer)
handler.arity != 0 ? handler.call(exception) : handler.call

I'm not quite sure what changes would have to be made to Grape::Middleware::Error to enable this kind of functionality, but I'm hoping this PR is at least a start.

[dblock]

I like it. We do allow methods elsewhere, like for error_formatter and the argument is not a hash, but just a proc. I like the with syntax, but I think we should be consistent and just check whether the argument is a lambda, not a hash with a with option.

I would be open to support with if you want to make this change everywhere and make it backwards compatible, btw.

Also, please update the CHANGELOG.

Thanks!

[robertopedroso]

I'd be happy to make the changes either way, but I'd just like to clarify what you mean. You're saying I should either make the syntax of rescue_from consistent with error_formatter, like this:

rescue_from ArgumentError, lambda { rescue_missing_record }

Or instead, that I change error_formatter (in a backwards-compatible way) to take a 'with' option? Like so:

# New format, takes an optional hash to assign a method or class
error_formatter :custom, with: CustomFormatter

# But retains the old functionality
error_formatter :custom, CustomFormatter
error_formatter :custom, lambda { ... }

Have I understood you correctly?

[dblock]

Correct. For this PR, start by doing the first part (make the syntax of rescue_from consistent with error_formatter. Make another PR that extends both to support with:.

[robertopedroso]

Okay -- I reverted the first commit and made rescue_from consistent with error_formatter. I'll submit a PR for with support later tonight or tomorrow morning.

[dblock]

Merged, thank you very much.