Proc composition in ruby 2.6

proc_composition-2.rb

require "bigdecimal"

# List of our individual pricing rules
TAX           = ->(val) { val + val*0.05 }
FEE           = ->(val) { val + 1 }
PREMIUM       = ->(val) { val + 10 }
DISCOUNT      = ->(val) { val * 0.90 }
ROUND_TO_CENT = ->(val) { val.round(2) }
# One presenter
PRESENT       = ->(val) { val.to_f }

# Pre-define some rule set for some pricing scenarios
REGULAR_SET    = [FEE, TAX, ROUND_TO_CENT, PRESENT]
PREMIUM_SET    = [FEE, PREMIUM, TAX, ROUND_TO_CENT, PRESENT]
DISCOUNTED_SET = [FEE, DISCOUNT, TAX, ROUND_TO_CENT, PRESENT]

def apply_rules(rules:, base_price:)
  rules.inject(:>>).call(base_price)
end

amount = BigDecimal(100)

puts "regular:    #{apply_rules(rules: REGULAR_SET, base_price: amount)}"    # => 106.05
puts "premium:    #{apply_rules(rules: PREMIUM_SET, base_price: amount)}"    # => 116.55
puts "discounted: #{apply_rules(rules: DISCOUNTED_SET, base_price: amount)}" # => 95.45

proc_composition_1.rb

# This lambda take one argument and return the same prefixed by "hello "
greet = ->(val) { "hello #{val}" }
# This lambda take one argument and return the upcased version
upper = ->(val) { val.upcase }

# So you can do
puts greet["world"]    # => hello world
puts upper["world"]    # => WORLD

present = greet >> upper

puts present["world"]  # => HELLO WORLD

present = greet << upper

puts present["world"]  # => hello WORLD