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+---
+language: crystal
+filename: learncrystal.cr
+contributors:
+    - ["Vitalii Elenhaupt", "http://veelenga.com"]
+    - ["Arnaud Fernandés", "https://github.com/TechMagister/"]
+
+---
+
+```crystal
+
+# This is a comment
+
+# Everything is an object
+nil.class  #=> Nil
+100.class  #=> Int32
+true.class #=> Bool
+
+# Falsey values are: nil, false and null pointers
+!nil   #=> true  : Bool
+!false #=> true  : Bool
+!0     #=> false : Bool
+
+# Integers
+
+1.class #=> Int32
+
+# Four signed integer types
+1_i8.class  #=> Int8
+1_i16.class #=> Int16
+1_i32.class #=> Int32
+1_i64.class #=> Int64
+
+# Four unsigned integer types
+1_u8.class  #=> UInt8
+1_u16.class #=> UInt16
+1_u32.class #=> UInt32
+1_u64.class #=> UInt64
+
+2147483648.class          #=> Int64
+9223372036854775808.class #=> UInt64
+
+# Binary numbers
+0b1101 #=> 13 : Int32
+
+# Octal numbers
+0o123 #=> 83 : Int32
+
+# Hexadecimal numbers
+0xFE012D #=> 16646445 : Int32
+0xfe012d #=> 16646445 : Int32
+
+# Floats
+
+1.0.class #=> Float64
+
+# There are two floating point types
+1.0_f32.class #=> Float32
+1_f32.class   #=> Float32
+
+1e10.class    #=> Float64
+1.5e10.class  #=> Float64
+1.5e-7.class  #=> Float64
+
+# Chars
+
+'a'.class #=> Char
+
+# Octal codepoint
+'\101' #=> 'A' : Char
+
+# Unicode codepoint
+'\u0041' #=> 'A' : Char
+
+# Strings
+
+"s".class #=> String
+
+# Strings are immutable
+s = "hello, "  #=> "hello, "        : String
+s.object_id    #=> 134667712        : UInt64
+s += "Crystal" #=> "hello, Crystal" : String
+s.object_id    #=> 142528472        : UInt64
+
+# Supports interpolation
+"sum = #{1 + 2}" #=> "sum = 3" : String
+
+# Multiline string
+"This is
+   multiline string"
+
+# String with double quotes
+%(hello "world") #=> "hello \"world\""
+
+# Symbols
+# Immutable, reusable constants represented internally as Int32 integer value.
+# They're often used instead of strings to efficiently convey specific,
+# meaningful values
+
+:symbol.class #=> Symbol
+
+sentence = :question?     # :"question?" : Symbol
+
+sentence == :question?    #=> true  : Bool
+sentence == :exclamation! #=> false : Bool
+sentence == "question?"   #=> false : Bool
+
+# Arrays
+
+[1, 2, 3].class         #=> Array(Int32)
+[1, "hello", 'x'].class #=> Array(Int32 | String | Char)
+
+# Empty arrays should define a type
+[]               # Syntax error: for empty arrays use '[] of ElementType'
+[] of Int32      #=> [] : Array(Int32)
+Array(Int32).new #=> [] : Array(Int32)
+
+# Arrays can be indexed
+array = [1, 2, 3, 4, 5] #=> [1, 2, 3, 4, 5] : Array(Int32)
+array[0]                #=> 1               : Int32
+array[10]               # raises IndexError
+array[-6]               # raises IndexError
+array[10]?              #=> nil             : (Int32 | Nil)
+array[-6]?              #=> nil             : (Int32 | Nil)
+
+# From the end
+array[-1] #=> 5
+
+# With a start index and size
+array[2, 3] #=> [3, 4, 5]
+
+# Or with range
+array[1..3] #=> [2, 3, 4]
+
+# Add to an array
+array << 6  #=> [1, 2, 3, 4, 5, 6]
+
+# Remove from the end of the array
+array.pop #=> 6
+array     #=> [1, 2, 3, 4, 5]
+
+# Remove from the beginning of the array
+array.shift #=> 1
+array       #=> [2, 3, 4, 5]
+
+# Check if an item exists in an array
+array.includes? 3 #=> true
+
+# Special syntax for an array of string and an array of symbols
+%w(one two three) #=> ["one", "two", "three"] : Array(String)
+%i(one two three) #=> [:one, :two, :three]    : Array(Symbol)
+
+# There is a special array syntax with other types too, as long as
+# they define a .new and a #<< method
+set = Set{1, 2, 3} #=> [1, 2, 3]
+set.class          #=> Set(Int32)
+
+# The above is equivalent to
+set = Set(typeof(1, 2, 3)).new
+set << 1
+set << 2
+set << 3
+
+# Hashes
+
+{1 => 2, 3 => 4}.class   #=> Hash(Int32, Int32)
+{1 => 2, 'a' => 3}.class #=> Hash(Int32 | Char, Int32)
+
+# Empty hashes should specify a type
+{}                     # Syntax error
+{} of Int32 => Int32   # {}
+Hash(Int32, Int32).new # {}
+
+# Hashes can be quickly looked up by key
+hash = {"color" => "green", "number" => 5}
+hash["color"]        #=> "green"
+hash["no_such_key"]  #=> Missing hash key: "no_such_key" (KeyError)
+hash["no_such_key"]? #=> nil
+
+# Check existence of keys hash
+hash.has_key? "color" #=> true
+
+# Special notation for symbol and string keys
+{key1: 'a', key2: 'b'}     # {:key1 => 'a', :key2 => 'b'}
+{"key1": 'a', "key2": 'b'} # {"key1" => 'a', "key2" => 'b'}
+
+# Special hash literal syntax with other types too, as long as
+# they define a .new and a #[]= methods
+class MyType
+  def []=(key, value)
+    puts "do stuff"
+  end
+end
+
+MyType{"foo" => "bar"}
+
+# The above is equivalent to
+tmp = MyType.new
+tmp["foo"] = "bar"
+tmp
+
+# Ranges
+
+1..10                  #=> Range(Int32, Int32)
+Range.new(1, 10).class #=> Range(Int32, Int32)
+
+# Can be inclusive or exclusive
+(3..5).to_a  #=> [3, 4, 5]
+(3...5).to_a #=> [3, 4]
+
+# Check whether range includes the given value or not
+(1..8).includes? 2 #=> true
+
+# Tuples are a fixed-size, immutable, stack-allocated sequence of values of
+# possibly different types.
+{1, "hello", 'x'}.class #=> Tuple(Int32, String, Char)
+
+# Acces tuple's value by its index
+tuple = {:key1, :key2}
+tuple[1] #=> :key2
+tuple[2] #=> syntax error : Index out of bound
+
+# Can be expanded into multiple variables
+a, b, c = {:a, 'b', "c"}
+a #=> :a
+b #=> 'b'
+c #=> "c"
+
+# Procs represent a function pointer with an optional context (the closure data)
+# It is typically created with a proc litteral
+proc = ->(x : Int32) { x.to_s }
+proc.class # Proc(Int32, String)
+# Or using the new method
+Proc(Int32, String).new { |x| x.to_s }
+
+# Invoke proc with call method
+proc.call 10 #=> "10"
+
+# Control statements
+
+if true
+  "if statement"
+elsif false
+  "else-if, optional"
+else
+  "else, also optional"
+end
+
+puts "if as a suffix" if true
+
+# If as an expression
+a = if 2 > 1
+      3
+    else
+      4
+    end
+
+a #=> 3
+
+# Ternary if
+a = 1 > 2 ? 3 : 4 #=> 4
+
+# Case statement
+cmd = "move"
+
+action = case cmd
+  when "create"
+    "Creating..."
+  when "copy"
+    "Copying..."
+  when "move"
+    "Moving..."
+  when "delete"
+    "Deleting..."
+end
+
+action #=> "Moving..."
+
+# Loops
+index = 0
+while index <= 3
+  puts "Index: #{index}"
+  index += 1
+end
+# Index: 0
+# Index: 1
+# Index: 2
+# Index: 3
+
+index = 0
+until index > 3
+  puts "Index: #{index}"
+  index += 1
+end
+# Index: 0
+# Index: 1
+# Index: 2
+# Index: 3
+
+# But the preferable way is to use each
+(1..3).each do |index|
+  puts "Index: #{index}"
+end
+# Index: 0
+# Index: 1
+# Index: 2
+# Index: 3
+
+# Variable's type depends on the type of the expression
+# in control statements
+if a < 3
+  a = "hello"
+else
+  a = true
+end
+typeof a #=> (Bool | String)
+
+if a && b
+  # here both a and b are guaranteed not to be Nil
+end
+
+if a.is_a? String
+  a.class #=> String
+end
+
+# Functions
+
+def double(x)
+  x * 2
+end
+
+# Functions (and all blocks) implicitly return the value of the last statement
+double(2) #=> 4
+
+# Parentheses are optional where the call is unambiguous
+double 3 #=> 6
+
+double double 3 #=> 12
+
+def sum(x, y)
+  x + y
+end
+
+# Method arguments are separated by a comma
+sum 3, 4 #=> 7
+
+sum sum(3, 4), 5 #=> 12
+
+# yield
+# All methods have an implicit, optional block parameter
+# it can be called with the 'yield' keyword
+
+def surround
+  puts '{'
+  yield
+  puts '}'
+end
+
+surround { puts "hello world" }
+
+# {
+# hello world
+# }
+
+
+# You can pass a block to a function
+# "&" marks a reference to a passed block
+def guests(&block)
+  block.call "some_argument"
+end
+
+# You can pass a list of arguments, which will be converted into an array
+# That's what splat operator ("*") is for
+def guests(*array)
+  array.each { |guest| puts guest }
+end
+
+# If a method returns an array, you can use destructuring assignment
+def foods
+    ["pancake", "sandwich", "quesadilla"]
+end
+breakfast, lunch, dinner = foods
+breakfast #=> "pancake"
+dinner    #=> "quesadilla"
+
+# By convention, all methods that return booleans end with a question mark
+5.even? # false
+5.odd?  # true
+
+# And if a method ends with an exclamation mark, it does something destructive
+# like mutate the receiver. Some methods have a ! version to make a change, and
+# a non-! version to just return a new changed version
+company_name = "Dunder Mifflin"
+company_name.gsub "Dunder", "Donald"  #=> "Donald Mifflin"
+company_name  #=> "Dunder Mifflin"
+company_name.gsub! "Dunder", "Donald"
+company_name  #=> "Donald Mifflin"
+
+
+# Define a class with the class keyword
+class Human
+
+  # A class variable. It is shared by all instances of this class.
+  @@species = "H. sapiens"
+
+  # type of name is String
+  @name : String
+
+  # Basic initializer
+  # Assign the argument to the "name" instance variable for the instance
+  # If no age given, we will fall back to the default in the arguments list.
+  def initialize(@name, @age = 0)
+  end
+
+  # Basic setter method
+  def name=(name)
+    @name = name
+  end
+
+  # Basic getter method
+  def name
+    @name
+  end
+
+  # The above functionality can be encapsulated using the attr_accessor method as follows
+  property :name
+
+  # Getter/setter methods can also be created individually like this
+  getter :name
+  setter :name
+
+  # A class method uses self to distinguish from instance methods.
+  # It can only be called on the class, not an instance.
+  def self.say(msg)
+    puts msg
+  end
+
+  def species
+    @@species
+  end
+end
+
+
+# Instantiate a class
+jim = Human.new("Jim Halpert")
+
+dwight = Human.new("Dwight K. Schrute")
+
+# Let's call a couple of methods
+jim.species #=> "H. sapiens"
+jim.name #=> "Jim Halpert"
+jim.name = "Jim Halpert II" #=> "Jim Halpert II"
+jim.name #=> "Jim Halpert II"
+dwight.species #=> "H. sapiens"
+dwight.name #=> "Dwight K. Schrute"
+
+# Call the class method
+Human.say("Hi") #=> print Hi and returns nil
+
+# Variables that start with @ have instance scope
+class TestClass
+  @var = "I'm an instance var"
+end
+
+# Variables that start with @@ have class scope
+class TestClass
+  @@var = "I'm a class var"
+end
+# Variables that start with a capital letter are constants
+Var = "I'm a constant"
+Var = "can't be updated" # Already initialized constant Var
+
+# Class is also an object in crystal. So class can have instance variables.
+# Class variable is shared among the class and all of its descendants.
+
+# base class
+class Human
+  @@foo = 0
+
+  def self.foo
+    @@foo
+  end
+
+  def self.foo=(value)
+    @@foo = value
+  end
+end
+
+# derived class
+class Worker < Human
+end
+
+Human.foo   #=> 0
+Worker.foo  #=> 0
+
+Human.foo = 2 #=> 2
+Worker.foo    #=> 0
+
+Worker.foo = 3 #=> 3
+Human.foo   #=> 2
+Worker.foo  #=> 3
+
+module ModuleExample
+  def foo
+    "foo"
+  end
+end
+
+# Including modules binds their methods to the class instances
+# Extending modules binds their methods to the class itself
+
+class Person
+  include ModuleExample
+end
+
+class Book
+  extend ModuleExample
+end
+
+Person.foo     # => undefined method 'foo' for Person:Class
+Person.new.foo # => 'foo'
+Book.foo       # => 'foo'
+Book.new.foo   # => undefined method 'foo' for Book
+
+
+# Exception handling
+
+# Define new exception
+class MyException < Exception
+end
+
+# Define another exception
+class MyAnotherException < Exception; end
+
+ex = begin
+   raise MyException.new
+rescue ex1 : IndexError
+  "ex1"
+rescue ex2 : MyException | MyAnotherException
+  "ex2"
+rescue ex3 : Exception
+  "ex3"
+rescue ex4 # catch any kind of exception
+  "ex4"
+end
+
+ex #=> "ex2"
+
+```
+
+## Additional resources
+
+- [Official Documentation](http://crystal-lang.org/)