The Crystal Programming Language

# A very basic HTTP server require "http/server" server = HTTP::Server.new do |context| context.response.content_type = "text/plain" context.response.print "Hello world, got #{context.request.path}!" end address = server.bind_tcp(8080) puts "Listening on http://#{address}" # This call blocks until the process is terminated server.listen

def shout(x) # Notice that both Int32 and String respond_to `to_s` x.to_s.upcase end # If `ENV["FOO"]` is defined, use that, else `10` foo = ENV["FOO"]? || 10 puts typeof(foo) # => (Int32 | String) puts typeof(shout(foo)) # => String

foo = [nil, "hello world"].sample # The type of `foo` is a union of `String` and `Nil`` puts typeof(foo) # => String | Nil # This would be a type error: # puts foo.upcase # Error: undefined method 'upcase' for Nil # The condition excludes `Nil` and inside the branch `foo`'s type is `String`. if foo puts typeof(foo) # => String puts foo.upcase end

class String def longest_repetition? max = chars .chunk(&.itself) .map(&.last) .max_by?(&.size) {max[0], max.size} if max end end puts "aaabb".longest_repetition? # => {'a', 3}

channel = Channel(Int32).new 3.times do |i| spawn do 3.times do |j| sleep rand(100).milliseconds # add non-determinism for fun channel.send 10 * (i + 1) + j end end end 9.times do puts channel.receive end

# Define the lib bindings and link info: @[Link("m")] lib LibM fun pow(x : LibC::Double, y : LibC::Double) : LibC::Double end # Call a C function like a Crystal method: puts LibM.pow(2.0, 4.0) # => 16.0 # This example intentionally uses a simple standard C function to be succinct. # Of course you could do *this specific* calculation in native Crystal as well: # 2.0 ** 4.0 # => 16.0

class Object def has_instance_var?(name) : Bool {{ @type.instance_vars.map &.name.stringify }}.includes? name end end class Person property name : String def initialize(@name) end end person = Person.new "John" p! person.has_instance_var?("name") # => true p! person.has_instance_var?("birthday") # => false

name: my-first-crystal-app version: 1.0.0 license: Apache-2.0 authors: - Crys <crystal@manas.tech> dependencies: mysql: github: crystal-lang/crystal-mysql version: ~>0.16.0 development_dependencies: ameba: github: crystal-ameba/ameba