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 an array. In this case, the lambda becomes a mapping function:

   ​	reducer.call([], lambda { |acc, x| acc << x * 2 }, [1, 2, 3, 4, 5])