Mastering Ruby Closures

Chapter 1

1. A closure is a function whose body references some variable that is declared in a parent scope.

2. amount is the free variable. This is because amount is not defined in the lambda, but rather, as is_larger_than’s argument.

3. Here’s one possible implementation:

   new_db = lambda do
     db = {}
   
     insert = lambda do |artiste, album|
       db[artist] = album
     end
   
     dump = lambda do
       db
     end
   
     delete = lambda do |artiste|
       old = db[artiste]
       db[artiste] = nil
       old
     end
   
     {:insert => insert, :dump => dump, :delete => delete}
   end

4. complement = lambda do |pred|
     lambda do |args|
       not pred.call(args)
     end
   end

5. The main trick here is that the starting value can be anything you want it to be, including ...