A language for humans and computers.

nil

x = nil

x = nil

x = "Hello BorderlessEngConf!"

x = nil
# Here 'x' is a Nil

x = "Hello BorderlessEngConf!"
# 'x' is now a String

if rand > 0.5
  x = nil
else
  x = "Hello BorderlessEngConf!"
end

x
# ?

if rand > 0.5
  x = nil
else
  x = "Hello BorderlessEngConf!"
end

x
# Here 'x' is a `String?`. This is
# shorthand for `String | Nil`.

"foo".upcase # => "FOO"

"foo".does_no_exist

(Saturday, 2am)

"foo".does_not_exist

error in line 1
Error: undefined method 'does_not_exist' for String

if rand > 0.5
  x = "foo"
else
  x = 42
end

x.upcase

if rand > 0.5
  x = "foo"
else
  x = 42
end

x.upcase

Error: undefined method 'upcase' for Int32 (compile-time type is (Int32 | String))

if rand > 0.5
  x = "foo"
  # All good!
  x.upcase
else
  x = 42
  # Also fine
  x * 1729
end

class Foo
  
end

class Foo < Bar
  
end

class Foo
  include Bar
end

struct Foo
  
end

class Foo
  # Accessible to the local instance only
  @instance_var : String

  # Shared between all `Foo`s
  @@class_var : Bool
end

class Foo
  def hello
    #...
  end
end

class Foo
  def hello : String
    return "Hello world!"
  end
end

class Foo
  def hello(name : String) : String
    return "Hello #{name}!"
  end
end

class Foo
  def hello(name : String = "world") : String
    return "Hello #{name}!"
  end
end

class Foo
  def hello(name : String = "world", shout : Bool = false) : String
    if shout
      name = name.upcase
    end
    return "Hello #{name}!"
  end
end

class Foo
  def hello(name : String = "world", shout : Bool = false)
    if shout
      name = name.upcase
    end
    return "Hello #{name}!"
  end
end

class Foo
  def hello(name : String = "world", shout : Bool = false)
    if shout
      name = name.upcase
    end
    "Hello #{name}!"
  end
end

class Foo
  def hello(name : String = "world", shout : Bool = false)
    name = name.upcase if shout
    "Hello #{name}!"
  end
end

class Foo
  def hello(name = "world", shout = false)
    name = name.upcase if shout
    "Hello #{name}!"
  end
end

class Foo
  def hello(name, shout = false)
    name = name.upcase if shout
    hello name
  end
  
  def hello(name = "world")
    "Hello #{name}!"
  end
end

def twice
  yield
  yield
end

twice do
  puts "Hello!"
end

3.times do |i|
  puts i
end

3.times do |i|
  puts i
end

struct Int
  def times
    i = 0
    while i < self
      yield i
      i += 1
    end
  end
end

3.times do |i|
  puts i
end

struct Int
  def times
    i = 0
    while i < self
      yield i
      i += 1
    end
  end
end

i = 0
while i < 3
  puts i
  i += 1
end

3.times do |i|
  puts i
end

i = 0
while i < 3
  puts i
  i += 1
end

Crystal’s job is to generate LLVM code

—Ary

Macros for meta-programming
Human friendly concurrency via CSP
An extensive, modern standard library

Doc generation
Test framework
Common code formatting
Dependency management

All in the compiler and standard tools

A welcoming, and friendly community.

WTF is this thing?

Why?

WTF is this thing?

Why Crystal?

crystal-lang.org

PlaceOS.com