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# Chapter 2: Designing Classes with Single Responsability
Single Responsability
* Ask Each Method: 'Class whats your :method?'
* Hide Instance Variables
* Hide Data Structures
* Extract Extra Responsability From Methods

Dependencies
* Must known no Constant
* Must _not_ call methods on other objects (constants)

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Install Multiple Crystal Lang Versions on OSX (Homebrew)

brew unlink crystal-lang
brew install <url to formula> # check more on appendix below

If you'd like to check for installed crystal versions, just run:

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# Generates ctags for a Crystal source
# Based on asterite script https://github.com/crystal-lang/crystal/issues/577#issuecomment-97054600
# This works on Crystal 0.17.4 [9d258f6] (2016-06-05)
#
require "compiler/crystal/**"
require "option_parser"

include Crystal

class CTagsVisitor < Visitor

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#!/usr/bin/env ruby
# frozen_string_literal: true
#
# This file was generated by Bundler.
#
# The application 'blade' is installed as part of a gem, and
# this file is here to facilitate running it.
#
# Put this file on bin/blade

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QUnit.config.hidepassed = false;

var timeout = QUnit.config.testTimeout;
console.log(timeout);
QUnit.config.testTimeout = 10000;

QUnit.test( "VirtualMachines#index", function( assert ) {
  assert.expect( 2 );
  var done = assert.async(2);
  assert.equal( sum(1, 1), 2, "Passed!" );

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$ trailblazer generate operation Thing::Create
class Thing::Create < Trailblazer::Operation
end

$ trailblazer generate operation Thing::Create --steps
class Thing::Create < Trailblazer::Operation
  step     Model( Song, :new )
  step     :assign_current_user!
end

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xcode-select --install
brew install \
  bdw-gc \
  gmp \
  libevent \
  libpcl \
  libxml2 \
  libyaml \
  llvm
git clone https://github.com/crystal-lang/crystal.git

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require 'trailblazer/endpoint'
require 'trailblazer/endpoint/app_matcher'

class Api::Controller < ApplicationController
  protect_from_forgery with: :null_session

  protected

    def default_handler
      ->(m) do

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import React from 'react'
import ReactDOM from 'react-dom'
import PropTypes from 'prop-types'
import styled, {ThemeProvider} from 'styled-components';

const Button = styled.button`
  font-size: 1em;
  margin: 1em;
  padding: 0.25em 1em;
  border-radius: 3px;

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import test from 'ava'
import fs from 'fs'
import mock from 'mock-fs'

test.before(t => {
  mock({
    'dir': {
      'document': 'mocked content'
    }    
  })