Ruby has the class Proc which encapsulates a "block of code". There are 2 "flavors" of Procs:

• Those with "block semantics", called blocks or confusingly sometimes also procs
• Those with "method semantics", called lambdas

lambdas

They behave like Ruby method definitions:

• They are strict about their arguments.
• return means "exit the lambda"

How to define a lambda

1. With the lambda keyword

   test = lambda do |arg|
     puts arg
   end
   

2. With the lambda literal -> (since Ruby 1.9.1)
   ...

This class contains nerdcore things such as disassembling a piece of Ruby into VM calls or enabling tail-call optimization.

Also see this article about enabling TCO on a per-method basis.

When there's a Gemfile.lock in your working directory that you cannot remove by either checkout, reset [--hard], stash, probably Rails' Spring is the culprit and not Bundler itself.

Fix

spring stop

The author of the linked Stackoverflow post supposes Spring re-writes the Gemfile.lock on change to ensure all Spring processes are using the same gem versions. Meh.

Sometimes you just want to have a small web server that serves files to test something.

Serve the current directory

On Ruby 1.9.2+ you can do the following ("." for current directory).

ruby -run -ehttpd . -p8000

Python 2.x offers a similar way.

python -m SimpleHTTPServer 8000 .

This is the same way with Python 3.x

python -m http.server

In both cases your web server is single-threaded and will block when large files are being downloaded from you.

WEBrick also offers [a simple way](https://stackoverflow.com/quest...

When you're developing a gem, never list the same dependency as both runtime and development dependency in your .gemspec.

So don't do this:

spec.add_dependency 'activesupport'
spec.add_development_dependency 'activesupport', '~> 2.3'

If you do this, your gemspec will not validate and modern versions of Bundler will silently ignore it. This leads to errors like:

Could not find your-gem-0.1.2 in any of the sources

What to do instead

If you want to freeze a different version of a dependency for your t...

There are many solutions, but a very concise one is this:

hash.merge!(hash) do |key, old_value, new_value|
  # Return something
end

The block of merge! will be called whenever a key in the argument hash already exists in the base hash. Since hash is updated with itself, each key will conflict and thus allow you to modify the value of each key to whatever you like (by returning old_value you'd get the behavior of Rails' reverse_merge!, by re...

Sometimes you want to know exactly how many objects exist within your running Ruby process. Here is how:

stats = {}
ObjectSpace.each_object  {|o| stats[o.class].nil? ? stats[o.class] = 0 : stats[o.class] += 1 }; stats

=> {String=>192038, Array=>67690, Time=>2667, Gem::Specification=>2665, Regexp=>491, Gem::Requirement=>16323, Gem::StubSpecification=>2665, ...}

Maybe you want to sort it like this:

stats.sort_by {|k,v| v }

This is basically Ruby-native syntax for andand.

Ruby has a set of methods to convert an object to another representation. Most of them come in explicit and implicit flavor.

There may be even more.

Don't name your methods like the implicit version (most prominently to_hash) but the like the explicit one.

Explicit conversion

Explicit conversion happens when requesting it, e.g. with the splat opera...

When you make a simple TCP connection to a remote server (like telnet), your client won't normally notice when the connection is unexpectly severed on the remote side. E.g. if someone would disconnect a network cable from the server you're connected to, no client would notice. It would simply look like nothing is being sent.

You can detect remote connection loss by configuring your client socket to send TCP keepalive signals after some period of inactivity. If those signals are not acknowledged by the other side, your client will terminat...

You want to test your 1GE or 10GE internet uplink? We needed to ensure we have full 10GE to the backbone for a customer project.

Using netcat

To test whether we can achieve the bandwidth internally, you can use netcat and dd like this:

On your first server: nc -v -l 55333 > /dev/null
On your second server: dd if=/dev/zero bs=1024K count=5000 | nc -v $remote_ip 55333

You should see some output like this:

user@xxx:~ % dd if=/dev/zero bs=1024K count=5000 | nc -v removed 55333
Connection to 91.250.95.249 55333 port [...

Ruby allows multiple assignment:

a, b, c = o

In order to prove multiple values from a single object, Ruby calls #to_a on the object:

o = String.new('O')
def o.to_a
  [1,2,3]
end

a, b, c = o # Implicit call to #to_a here
a # => 1
b # => 2
c # => 3

Hence, if you want your class to support multiple assignment, make sure to define a #to_a method.

When a method has keyword arguments, Ruby offers implicit conversion of a Hash argument into keyword arguments. This conversion is performed by calling to_hash on the last argument to that method, before assigning optional arguments. If to_hash returns an instance of Hash, the hash is taken as keyword arguments to that method.

Iss...

Writing ruby methods that accept both optional and keyword arguments is dangerous and should be avoided. This confusing behavior will be deprecated in Ruby 2.7 and removed in Ruby 3, but right now you need to know about the following caveats.

Consider the following method

# DO NOT DO THIS

def colored_p(object = nil, color: 'red')
  switch_color_to(color)
  puts object.inspect
end


colored_p(['an array'])                   # ['an array'] (in red)
colored_p({ a: 'hash' }, color: 'blue')   # {:a=>'hash'} (in blue)
colored_p({ a: 'ha...

Starting with Rails 4.0, when you get an exception reported via the ExceptionNotification gem, you will only see a very short backtrace with all backtrace lines from gems or ruby libraries missing.

This happens, because the ExceptionNotification gem uses Rails' default backtrace cleaner. To get a full backtrace in exception emails, you can remove the comment from this line in config/initializers/backtrace_silencers.rb:

Rails.backtrace_cleaner.remove_silencers!

Note that this will break the "Application Trace" functionality o...

Starting with Ruby 1.9, most #each methods can be called without a block, and will return an enumerator. This is what allows you to do things like

['foo', 'bar', 'baz'].each.with_index.collect { |name, index| name * index }
# -> ["", "bar", "bazbaz"]

If you write your own each method, it is useful to follow the same practice, i.e. write a method that

• calls a given block for all entries
• returns an enumerator, if no block is given

How to write a canonical each method

To write a m...

Rails 5 will introduce ActiveRecord::Relation#or. On Rails 4 and 3.2 you can use the activerecord_any_of gem which seems to be free of ugly hacks and nicely does what you need.

Use it like this:

User.where.any_of(name: 'Alice', gender: 'female')

^
SELECT "users".* FROM "users" WHERE (("users"."name" = 'Alice' OR "users"."gender" = 'female'))

To group conditions, wrap them in hashes:

User.where.any_of({ name: 'Alice', gender: 'female' }, { name: 'Bob' }, { name: 'Charl...

To safely transport an arbitrary string within a URL, you need to percent-encode characters that have a particular meaning in URLs, like & or =.

If you are using Rails URL helpers like movies_path(:query => ARBITRARY_STRING_HERE), Rails will take care of the encoding for you. If you are building URLs manually, you need to follow this guide.

Ruby

In Ruby, use CGI.escape:

# ✅ good
CGI.escape('foo=foo&bar=bar')
=> "foo%3Dfoo%26bar%3Dbar"

Do not ever use `URI.en...

While debugging an intricate issue with failed HTTP requests I have come to appreciate the more advanced features of the httpclient Rubygem.

The gem is much more than a lightweight wrapper around Ruby's net/http. In particular:

• A single HTTPClient instance can re-use persistent connections across threads in a thread-safe way.
• Has a custom and configurable SSL certificate store (which you probably want to disable by default...

If you have problems installing a gem and get a error collect2: error: ld returned 1 exit status it's due to missing development headers of a library (ld is the linker).

For example, with this output:

$ gem install json
Building native extensions.  This could take a while...
ERROR:  Error installing json:
       ERROR: Failed to build gem native extension.

   /home/foobar/.rvm/rubies/ruby-2.2.3/bin/ruby -r ./siteconf20150915-3539-1i9layj.rb extconf.rb
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
compiling generator.c...

Ruby has Enumerable.find(&block), which returns the first item in the collection for which the block evaluates to true.

first_post_with_image = posts.find do |post|
  post.image
end

However, sometimes it's not the item you're interested in, but some value depening on it – e.g. the value the block evaluated to. You could first map the collection and then take the first truthy value, but this way you need to process the whole collection twice:

first_image_url = posts.map(&:image).find(&:present?).url

If the mapping ...

To update your Rubygems to the latest available version, type the following:

 gem update --system

Note that you have a separate Rubygems installation for each Ruby version in your RVM or rbenv setup. Updating one does not update the others.

Ruby 1.8.7

If you are using Ruby 1.8.7 you cannot use the latest version of Rubygems. Type the following to get the latest version that is compatible with 1.8.7:

 gem updat...

On Ruby 1.9+, standard ruby character classes like \w, \d will only match 7-Bit ASCII characters:

"foo" =~ /\w+/   # matches "foo"
"füü" =~ /\w+/   # matches "f", ü is not 7-Bit ASCII

There is a collection of character classes that will match unicode characters. From the documentation:

• /[[:alnum:]]/ Alphabetic and numeric character
• /[[:alpha:]]/ Alphabetic character
• /[[:blank:]]/ Space or tab
• /[[:cntrl:]]/ Control character
• /[[:digit:]]/ Digit
• /[[:graph:]]/ Non-blank character (excludes spaces...

rbenv

To check which rubygems versions your different rbenv rubys are using, you can use this small bash script:

for i in $(rbenv versions --bare); do rbenv shell "${i}"; echo -n "ruby ${i} has gem version: "; gem -v; done

RVM

rvm all do gem -v