How can I show that the Ruby `for` loop is in fact implemented using the `each` method?

Question

In the book Eloquent Ruby (page 21, first edition, sixth printing), the author (Russ Olsen) advocates using the each method instead of

Answer 1

The semantics of the for expression are defined in the ISO Ruby Language Specification like this:

§11.4.1.2.3 The for expression

Syntax

• for-expression → for for-variable in expression do-clause end
• for-variable → left-hand-side | multiple-left-hand-side

The expression of a for-expression shall not be a jump-expression.

Semantics

A for-expression is evaluated as follows:

1. Evaluate the expression. Let O be the resulting value.

2. Let E be the primary-method-invocation of the form primary-expression [no line-terminator here].each do | block-formal-argument-list | block-body end, where the value of the primary-expression is O,the block-formal-argument-list is the for-variable, the block-body is the compound-statement of the do-clause.

   Evaluate E, but skip Step c of §11.2.2.

3. The value of the for-expression is the resulting value of the invocation.

Okay, so basically this means that

for for_variable in expression
  do_clause
end

is evaluated the same as

O = expression
O.each do |for_variable|
  do_clause
end

Aha! But we forgot something! There's this ominous "skip Step c of §11.2.2." thing! So, what does Step c of §11.2.2. say?

• Push an empty set of local variable bindings onto ⟦local-variable-bindings⟧.

Note that Step b

• Set the execution context to Eb.

is not skipped.

So, a for loop gets its own execution context, which starts out as a copy of the current execution context, but it does not get its own set of local variable bindings. IOW: it gets its own dynamic execution context, but not its own lexical scope.