From: Austin Ziegler <austin@...>
Date: 2002-10-22T06:33:22+09:00
Subject: Re: functional Ruby equiv to this perl snippet

On Tue, 22 Oct 2002 05:54:49 +0900, Bob X wrote:
> Cool..but what if serverlist.txt does not exist? The one thing I do
> like about Perl is the "open file or die" test. I believe that I am
> "stuck" with a rescue clause which is sometimes confusing for me (a
> newbie) to use when it is located within a "def".

Truth be told, you don't need to "or die". If you run the following 
code (a minor modification to parameterize the filename), you'll get 
(assuming, of course, that serverlist2.txt doesn't exist):

c:/home/Projects/Ruby/test.rb:4:in `initialize': No such file or 
directory - "se
rverlist2.txt" (Errno::ENOENT)
        from c:/home/Projects/Ruby/test.rb:4:in `open'
        from c:/home/Projects/Ruby/test.rb:4:in `load_server_list'
        from c:/home/Projects/Ruby/test.rb:13

def load_server_list(filename)
    servers = []
    File.open(filename).each do |line|
        next unless line =~ /^\S+$/
        next if line =~ /^#.*$/
        servers << line.chomp.sub(/\s+#.*$/, '')
    end
    servers
end

servers = load_server_list('serverlist2.txt')
p servers.size, servers

To handle it as a caller, just replace the last two lines with 
something like:

begin
    fn = 'serverlist2.txt'
    servers = load_server_list(fn)
rescue Errno::ENOENT => ex
    puts "Problem reading \"#{fn}\".\n\t#{ex.message}"
    exit
end

File.open automatically raises an error. Where you might want to do 
something different is to have your own FileNotFoundException, like 
so:

class FileNotFoundException < RuntimeError
end

def load_server_list(filename)
    servers = []
    File.exist?(filename) or raise FileNotFoundException, filename
    File.open(filename).each do |line|
        next unless line =~ /^\S+$/
        next if line =~ /^#.*$/
        servers << line.chomp.sub(/\s+#.*$/, '')
    end
    servers
end

begin
    servers = load_server_list('serverlist2.txt')
rescue FileNotFoundException => fnf
    puts "File '#{fnf.message}' does not exist."
    exit
end

If there is anywhere that I think that Ruby's class model is a bit 
weak, it's in the existence of and documentation of 'standard' 
exceptions. IMO, Errno::ENOENT is not what File.open should throw, 
even though that's the actual errno value received -- it should throw 
an appropriate readable exception (like FileNotFoundException) or 
something like that.

-austin
-- Austin Ziegler, austin@halostatue.ca on 2002.10.21 at 17.17.05