Beyond Exception Handling: Conditions and Restarts

http://www.gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html

by DanielRibeiro • 15 years ago

103 20 15 years ago

awj2• 15 years ago

This is the one feature I wish I could steal from Common Lisp in almost every language I work with. It would be much easier to put in than almost any other big lisp feature, and damn would it make a lot of things easier.

sedachv3• 15 years ago

This is something you can do in most languages if all the code that needs restarts follows a convention/protocol. Red Daly implemented CL condition handling for JavaScript for the Parenscript CL-to-JS compiler: https://github.com/gonzojive/paren-psos/commit/6578ad223515d...

The compiled CL code knows about the convention and follows it, but other JavaScript code doesn't and isn't affected by it.

awj2• 15 years ago

Yeah, that's why I talk about stealing it at the language level. Although it's possible to use this based on conventions, the real power comes from being able to use it to cover weird conditions while gluing various libraries together. If your language already has a relatively similar tool (i.e. "traditional" exceptions) there's basically no chance that anything you pick up will support conditions.

That said, it's a wonderful model to know about/follow if you come into the rare case that makes reimplementing it where you can useful.

someone13• 15 years ago

Is there any other language that implements this? I'll freely admit that I'm no expert on Lisp, so I've tried digging through the code, and I sorta get what the concept is, but I don't really understand it. Any other languages have the same idea?

briantrice2• 15 years ago

Dylan does, and my language Slate (http://www.slatelanguage.org) does, as well as its related younger siblings, Atomo (http://atomo-lang.org) and Atomy (http://atomy-lang.org). And all of these implement them in the context of an object model, rather than carefully avoiding CLOS dependencies as in CL.

bakkdoor• 15 years ago

I just added conditions & restarts today to Fancy (http://fancy-lang.org/). It's currently in a branch but it works =).

gigamonkey• 15 years ago

I believe Mesa did. On the other hand, Stroustrup claims (in the Design and Evolution of C++, I think) that the Cedar system written in Mesa basically never used the ability to restart as a reason why C++ doesn't have continuable exceptions. I've never been able to track down more information about that than his claim though.

sedachv3• 15 years ago

Kent Pitman wrote a pretty thorough history: http://www.nhplace.com/kent/Papers/Condition-Handling-2001.h...

edsrzf• 15 years ago

Here's a good explanation using pseudo-Java: http://axisofeval.blogspot.com/2011/04/whats-condition-syste...

IgorPartola5• 15 years ago

Does this basically mean that CL provides a mechanism that's lower level than what is otherwise equivalent to exceptions/warnings, and that this in turn enables things like restarts? Is this analogous to the relationship between higher level control structures such as if/for/while/break/continue vs the lower-level goto? Lastly, can some of these features not be implemented with exceptions?:

  def parse_log_file(f):
      result = []
      for line in f.readlines(): # Fails for large files
        parsed_line = None
        retries = 0
        while retries < 5:
            try:
                result.append(parse_log_entry(line))
                break
            except MalformedLogEntry:
                line = fix(line)
                retries += 1
      return result

Kototama3• 15 years ago

Your code is not equivalent. There is a separation of concerns that is possible with conditions/restarts that is not possible with exceptions.

The calling code can choose the appropriate restarts base on information that the callee code does not - and should not - have access to.

IgorPartola5• 15 years ago

> The calling code can choose the appropriate restarts base on information that the callee code does not - and should not - have access to.

In the common case, is that not equivalent to the caller catching specific errors by wrapping the callee in a try/except? Could you elaborate?

k_bx• 15 years ago

Maybe I misunderstand it, but doesn't "retries" gets increased only on MalformedLogEntries? I mean, if there's no malformed log entries -- how will it ever exit?

IgorPartola5• 15 years ago

Fixed. I changed how the data is appended to the result half way through writing it so there was no condition to break out of the loop. Now there is a break after the append.

Beyond Exception Handling: Conditions and Restarts

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