From: "Jörg W Mittag" <JoergWMittag+Usenet@...>
Date: 2009-02-26T05:19:14+09:00
Subject: Re: Common Lisp conditions

Gavin Sinclair wrote:
> So far as the search engine on this newsgroup is concerned, there's
> been no discussion about including a Common Lisp-style condition
> system in a future version of Ruby.
[...]
> For those who _do_ know about Common Lisp conditions, is there any
> appetite for them in Ruby?  I see great positives and no negatives.
> They don't seem at all un-Ruby-like; in fact the relationship between
> high- and low-level code reminds me of the elegance of blocks and
> yield.

They seem to work very well in Ola Bini's new language Ioke
(<http://Ioke.Org/>), which borrows heavily from Ruby. So, I agree:
they would fit very well. Replacing Exceptions with Conditions could
probably even be done in a (somewhat) backwards-compatible way. Maybe
they could replace throw/catch, too.

> After a small amount of thought, I haven't come up with a suitable
> syntax.  That's probably because I haven't used the things, only read
> about them.

I just tried converting the example from Practical Common Lisp, but it
isn't too pretty. I hope that's because I mis-translated the original
procedural Common Lisp into pseudo-OO Ruby and not because my chosen
syntax is ugly.

    class MalformedLogEntryError < RuntimeError
      def initialize text
        @text = text
      end
      attr_reader :text
    end

    class LogEntryParser
      def parse_log_entry text
        error MalformedLogEntryError.new(text) unless text.wellformed_log_entry?
        LogEntry.new( ... )
      restart use_value(value)
        value
      restart reparse_entry(fixed_text)
        parse_log_entry fixed_text
      end
    end

    class LogfileParser
      def parse_log_file file
        File.each_line(file).map do |line|
          begin
            @log_entry_parser.parse_log_entry text
          restart skip_log_entry
            nil
          end
        end
      end
    end

    class LogAnalyzer
      def main
        find_all_logs.each do |log|
          analyze_log log
        end
      handle MalformedLogEntryError
        restart skip_log_entry
      end

      def analyze_log log
        @logfile_parser.parse_log_file(log).each do |entry|
          analyze_entry entry
        end
      end
    end

    class StrictLogAnalyzer # Annotates malformed entries instead of ignoring
      def main
        super
      handle MalformedLogEntryError => c
        MalformedLogEntry.new(c.text)
      end
    end

jwm