x = {1, 2, 3}
# to get the value use
x[0]

x[0] = 10

undefined method '[]=' for Tuple(Int32, Int32)

(1..100).minmax

Note: Since we just mentioned minmax, have a look at minmax_by method as well. It would let you apply a block of code over your range and then return the minimum and maximum based on the returned collection.

["1234", "12", "123"].minmax_by { |i| i.size }
# => {"12", "1234"}

Tuple(Int32, Int32).from([1, 2])

argument to splat must be a tuple, not Array(Int32)

Named Tuple

data = {x: 1, y: 2}
# to get the value
data[:x]
# raises errors when we try to change it
data[:x] = 1

def print_date(year = nil, month = nil, day = nil)
  puts "#{year}/#{month}/#{day}"
end
birth_day = { year: 1990, month: 4, day: 3}
print_date(**birth_day)
card_expiry = { year: 2020, month: 1}
print_date(**card_expiry)

NamedTuple(name: String, val: String).from({"name" => "number", "val" => "Harisankar P S"}

k = 42.as(Int32 | String)
NamedTuple(name: String, val: String).from({"name" => "number", "val" => K}

cast from Int32 to String failed, at /usr/local/Cellar/crystal-lang/0.23.1_1/src/class.cr:41:5:41 (TypeCastError)
0x10e8f1085: *CallStack::unwind:Array(Pointer(Void)) at ??
0x10e8f1021: *CallStack#initialize:Array(Pointer(Void)) at ??
0x10e8f0ff8: *CallStack::new:CallStack at ??
0x10e8ec295: *raise<TypeCastError>:NoReturn at ??
0x10e90feb8: *String@Object::cast<(Int32 | String)>:String at ??
0x10e95faa3: *NamedTuple(name: String:Class, val: String:Class)@NamedTuple(T)#from<Hash(String, Int32 | String)>:NamedTuple(name: String, val: String) at ??
0x10e95f787: *NamedTuple(name: String, val: String)@NamedTuple(T)::from<Hash(String, Int32 | String)>:NamedTuple(name: String, val: String) at ??
0x10e8ef8a6: *__icr_exec__:NamedTuple(name: String, val: String) at ??
0x10e8db130: __crystal_main at ??
0x10e8ee578: main at ??

Extra Note:

def a_method(*data)
  puts data
end
a_method(1,2,3)
#=> {1,2,3}

def a_method(**data)
  puts data
end
a_method(x: 1, y: 10)
#=> {x: 1, y: 10}

To summarize:

• Tuples are immutable data structure
• Regular tuple is like a frozen array
• You can use splat only with a tuple
• NamedTuple is like a frozen hash
• Double splat can only be used with NamedTuple

• Native WebSocket support
• Compiles to efficient native code
• Statically typed => if any errors it will let you know at compile time.
• Less memory consumption

Web frameworks in Crystal

crystal init app sample-app

Kemal

Install Kemal

shards install

require "kemal"
# Matches GET ""
get "/" do
"Hello World!"
end
Kemal.run

crystal run src/sample-app.cr

Amethyst

Amber

git clone https://github.com/amber-crystal/amber.git

cd amber

shards install

make

dependencies:
  granite_orm:
    github: Amber-Crystal/granite-orm

Kemalyst

Raze

• Two matching routes that both having a block is not possible

get "/hello*" do |ctx|
  do_something
end
get "/hello/raze" do |ctx|
  "hello, raze"
end

get "/hello*", DoSomething.new

get "/hello*", DoSomething.new
get "/hello/raze" do |ctx|
  "hello, raze"
end

• Route ambiguity order

Limitations

Conclusion

Reference

• https://github.com/veelenga/awesome-crystal#web-frameworks

Getting started

dependencies:
  schedule:
    github: hugoabonizio/schedule.cr

Examples

Periodic execution

require "schedule"
Schedule.every(3.seconds) do
  puts "I run every 3 seconds"
end

require "schedule"
Schedule.every(1.hour) do
  ClassName.hourly_job
end

Scheduled execution

require "schedule"
# Execute a task after X interval
Schedule.after(10.seconds) do
  puts "I am going to run after 10 seconds"
end

Stop and Retry jobs

MAX_COUNT = 3
Schedule.every(10.seconds) do
  count = get_count
  Schedule.retry if count == 0
  Schedule.stop if count >= MAX_COUNT
end

 Exception handlers

require "schedule"
Schedule.exception_handler do |ex|
  puts "Exception recued! #{ex.message}"
end
Schedule.every(100.milliseconds) do
  raise "I'm an Exception"
end

handler = ->(ex : Exception) { puts "Exception recued! #{ex.message}" }
Schedule.exception_handler = handler

Create a sample application

$ crystal init app sample-app

$ crystal init app sample-app
      create  sample-app/.gitignore
      create  sample-app/.editorconfig
      create  sample-app/LICENSE
      create  sample-app/README.md
      create  sample-app/.travis.yml
      create  sample-app/shard.yml
      create  sample-app/src/sample-app.cr
      create  sample-app/src/sample-app/version.cr
      create  sample-app/spec/spec_helper.cr
      create  sample-app/spec/sample-app_spec.cr
Initialized empty Git repository in /Users/yourname/Desktop/sample-app/.git/

name: sample-app
version: 0.1.0
authors:
  - John Doe <email@example.co>
dependencies:
  kemal:
    github: kemalcr/kemal
    branch: master
targets:
  sample-app:
    main: src/sample-app.cr
crystal: 0.23.1
license: MIT

Add a “/” route

require "kemal"
get "/" do
"Hello World"
end
Kemal.run

Deploy to Heroku

$ heroku create your-custom-name --buildpack https://github.com/crystal-lang/heroku-buildpack-crystal.git
Creating ⬢ your-custom-name... done
Setting buildpack to https://github.com/crystal-lang/heroku-buildpack-crystal.git... done
https://your-custom-name.herokuapp.com/ | https://git.heroku.com/your-custom-name.git

$ git status
$ git add -A
$ git commit -m "commit message"
$ git push heroku master