Configuration pattern in Ruby

Sometimes you need to add flexibility to your code and this pattern is a simple and clear way to accomplish this.It is widely used in gems also.

This is how it’s used:

Zuper.configure do |config|
  config.name = 'ACME'
  config.platform = 'Unix'
end

You can then access the values:

Zuper.configuration.name
Zuper.configuration.platform

And this is the implementation:

module Zuper
  class Configuration
    attr_accessor :name, :platform
  end

  class << self
    attr_writer :configuration
  end

  def self.configuration
    @configuration ||= Configuration.new
  end

  def self.configure
    yield configuration
  end
end
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Class instances equality

The Problem:

We have the following Color class:

class Color
  def initialize name
    @name = name
  end
end

and we want a Set of uniq Color instances, by their name.

The Issue:

colors = Set.new([Color.new(:red), Color.new(:red)])
=> #<Set: {#<Color:0x007fa9a4239e70 @name=:red>, #<Color:0x007fa9a4239e20 @name=:red>}>

Ouch!
As you can see the Set contains 2 red colors, which is not what we want.

Solution:

From Set class documentation:

Equality of elements is determined according to Object#eql? and Object#hash

So we need to implement these 2 tiny methods in order to make things work:

class Color
  attr_reader :name

  def initialize name
    @name = name
  end

  def eql?(other)
    name == other.name
  end
  alias_method :==, :eql?

  def hash
    name.hash
  end
end

Result:

colors = Set.new([Color.new(:red), Color.new(:red)])
=> #<Set: {#<Color:0x007fa9a389be90 @name=:red>}>

Yey!

Install MongoDB on OS X (10.6 Snow Leopard)

Here are the steps:

  1. Download the pre-compiled binaries from:
    http://www.mongodb.org/display/DOCS/Downloads

  2. Create mongodb folder and unpack the files there
    sudo mkdir /usr/local/mongodb

  3. Create the configuration file
    sudo touch /usr/local/mongodb/mongod.conf
    and paste these lines in it:
    dbpath = /usr/local/mongodb
    logpath = /var/logs/mongodb/output.log
    bind_ip = 127.0.0.1
    # DEFAULTS
    # port = 27017
    # noauth = true

    More configurations options you can find here:
    http://www.mongodb.org/display/DOCS/Command+Line+Parameters

  4. Add “/usr/local/mongodb/bin” to your $PATH
    or
    only for Leopard versions create a file “/etc/paths.d/mongodb” where you paste this path.

  5. Make mongodb starts when system starts
    touch /Library/LaunchDaemons/org.mongodb.mongod.plist
    and paste these lines in it:<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
    "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
    <dict>
    <key>Label</key>
    <string>org.mongodb.mongod</string>
    <key>ProgramArguments</key>
    <array>
    <string>/usr/local/mongodb/bin/mongod</string>
    <string>run</string>
    <string>--config</string>
    <string>/usr/local/mongodb/mongod.conf</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <true/>
    <key>WorkingDirectory</key>
    <string>/usr/local/mongodb</string>
    <key>StandardErrorPath</key>
    <string>/var/log/mongodb/output.log</string>
    <key>StandardOutPath</key>
    <string>/var/log/mongodb/output.log</string>
    </dict>
    </plist>

    and then
    sudo launchctl load /Library/LaunchDaemons/org.mongodb.mongod.plist

  6. See if it works (type in a terminal):

    $> mongo
    => MongoDB shell version: 1.2.4
    url: test
    connecting to: test

That’s it.

%Q, %q, %W, %w, %x, %r, %s

%Q | %q | %W | %w | %x | %r | %s

 
%Q
This is an alternative for double-quoted strings, when you have more quote characters in a string.Instead of putting backslashes in front of them, you can easily write:

>> %Q(Joe said: "Frank said: "#{what_frank_said}"")
=> "Joe said: "Frank said: "Hello!"""

The parenthesis “()” can be replaced with any other non-alphanumeric characters and non-printing characters (pairs), so the following commands are equivalent:

>> %Q!Joe said: "Frank said: "#{what_frank_said}""!
>> %Q[Joe said: "Frank said: "#{what_frank_said}""]
>> %Q+Joe said: "Frank said: "#{what_frank_said}""+

You can use also:

>> %/Joe said: "Frank said: "#{what_frank_said}""/
=> "Joe said: "Frank said: "Hello!"""


%q
Used for single-quoted strings.The syntax is similar to %Q, but single-quoted strings are not subject to expression substitution or escape sequences.

>> %q(Joe said: 'Frank said: '#{what_frank_said} ' ')
=> "Joe said: 'Frank said: '\#{what_frank_said} ' '"

 
%W
Used for double-quoted array elements.The syntax is similar to %Q

>> %W(#{foo} Bar Bar\ with\ space)
=> ["Foo", "Bar", "Bar with space"]

 
%w
Used for single-quoted array elements.The syntax is similar to %Q, but single-quoted elements are not subject to expression substitution or escape sequences.

>> %w(#{foo} Bar Bar\ with\ space)
=> ["\#{foo}", "Bar", "Bar with space"]

 
%x
Uses the ` method and returns the standard output of running the command in a subshell.The syntax is similar to %Q.

>> %x(echo foo:#{foo})
=> "foo:Foo\n"

 
%r
Used for regular expressions.The syntax is similar to %Q.

>> %r(/home/#{foo})
=> "/\\/home\\/Foo/"

 
%s
Used for symbols.It’s not subject to expression substitution or escape sequences.

>> %s(foo)
=> :foo

>> %s(foo bar)
=> :"foo bar"

>> %s(#{foo} bar)
=> :"\#{foo} bar"