Syntax

# ===== Variables (sigils signal scope) =====
local_var = 'hi'
@instance_var = 'state'
@@class_var = 'shared'
$global_var = 'avoid'
CONSTANT = 'caps'

# ===== Strings =====
'single quoted'              # literal, no interpolation
"double \#{1 + 1}"            # interpolated
'multi
line'

# Heredoc:
text = <<~HEREDOC
  indented and stripped
  by the squiggly heredoc operator
HEREDOC

# ===== Numbers =====
42
1_000_000      # underscores for readability
3.14
1/2            # 0 (integer division)
1.0/2          # 0.5

# ===== Control flow =====
puts 'big' if age >= 18           # postfix
puts 'small' unless age >= 18      # negated if

if age < 18 then puts 'minor'
elsif age < 65 then puts 'adult'
else puts 'senior'
end

# case (pattern-matching since 3.0):
case order
in { kind: 'pizza', size: }
  puts "pizza, \#{size}"
in [first, *rest]
  puts "list start \#{first}"
in Integer => n if n > 0
  puts "positive int \#{n}"
else
  puts 'other'
end

# ===== Iteration =====
[1, 2, 3].each { |x| puts x }
(1..10).each do |x|
  puts x
end

# Times:
5.times { |i| puts i }

# Map + filter:
[1, 2, 3, 4].map { |x| x * x }
[1, 2, 3, 4].select(&:even?)

# Reduce:
[1, 2, 3].sum
[1, 2, 3].reduce(0) { |s, x| s + x }

# ===== Methods + blocks =====
def greet(name)
  "Hi \#{name}!"
end

def with_db
  conn = open_db
  yield conn
ensure
  conn&.close
end

with_db { |db| db.query('SELECT 1') }

# ===== Classes + modules =====
class User
  attr_accessor :name, :email
  def initialize(name:, email:)
    @name = name; @email = email
  end
  def to_s = "\#{@name} <\#{@email}>"
end

module Greetable
  def greet = "Hi \#{@name}"
end

class Employee < User
  include Greetable
end

# ===== Symbols =====
:status                # immutable interned symbol
%i[a b c]              # array of symbols

# ===== Hashes =====
{ name: 'Alex', age: 30 }
"Alex".tap { |s| puts s.upcase }   # tap returns the receiver

# ===== Patterns to internalise =====
# - Default to symbols for hash keys + identifiers
# - Blocks over explicit loops; everything maps + selects
# - .tap and .then for inline debugging + chaining
# - Pattern matching in case/in for structured destructuring

# ===== Pitfalls =====
# - Single quotes do not interpolate; common surprise
# - nil and false are the ONLY falsy values; 0 and '' are truthy
# - method_missing magic that no one can grep for
# - Symbol GC was added; older code worried about leaks

name = 'Ada'
puts "Hello, #{name}!"   # double-quoted string interpolation