Issue #18194 has been updated by ioquatix (Samuel Williams).


One more think, thinking about the "metadata" which we attach to an exception will force us to come up with a sufficiently generic interface. I still think the concept of "source location" is a good one which is sufficiently abstract to ensure that we don't force logging systems to read files from disk in order to get source code, etc. https://bugs.ruby-lang.org/issues/6012

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Bug #18194: No easy way to format exception messages per thread/fiber scheduler context.
https://bugs.ruby-lang.org/issues/18194#change-93845

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: mame (Yusuke Endoh)
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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In the new error highlighting gem, formatting exception messages appears to be per-process which is insufficiently nuanced for existing use cases.

As in:

```ruby
class TerminalColorFormatter
  def message_for(spot)
    # How do we know the output format here? Maybe it's being written to a log file?
    "..."
  end
end

ErrorHighlight.formatter = TerminalColorFormatter.new
```

But we won't know until the time we actually write the error message whether terminal codes are suitable or available. Or an error message might be formatted for both the terminal and a log file, which have different formatting requirements. There are many consumers of error messages an some of them produce text, or HTML, or JSON, etc.

Because of this design we are effectively forcing everyone to parse the default text output if they want to do any kind of formatting, which will ossify the format and make it impossible in practice for anyone to use anything but the default `ErrorHighlight.format`. For what is otherwise a really fantastic idea, this implementation concerns me greatly.

I would like us to consider introducing sufficient metadata on the exception object so that complete formatting can be implemented by an output layer (e.g. logger, terminal wrapper, etc). This allows the output layer to intelligently format the output in a suitable way, or capture the metadata to allow for processing elsewhere.

In addition, to simplify this general usage, we might like to introduce `Exception#formatted_message`.

In order to handle default formatting requirements, we need to provide a hook for formatting uncaught exceptions. This would be excellent for many different use cases (e.g. HoneyBadger type systems), and I suggest we think about the best interface. Probably a thread-local with some default global implementation makes sense... maybe even something similar to `at_exit { ... $! ... }`.



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