Pure Functions

pure.rb

# lambda is impure: mutates shared state (the fruits hash)
fruits = {}
hashify = lambda do |fruit|
  fruits[fruit] = fruit
end
["apple", "pear"].each &hashify

# lambda is pure: operates only on its arguments
to_nv_pair = lambda do |fruit|
  [fruit, fruit]
end
fruits = Hash[["apple", "pear"].map &to_nv_pair]

# lambda is impure: writes to STDOUT as side-effect
to_nv_pair = lambda do |fruit|
  puts "building a pair..."
  [fruit, fruit]
end
fruits = Hash[["apple", "pear"].map &to_nv_pair]