5.3.2 Internal definitions

Definitions can occur at the beginning of a ⟨body⟩ (that is, the body of a lambda, let, let*, letrec, letrec*, let-values, let*-values, let-syntax, letrec-syntax, parameterize, guard, or case-lambda). Note that such a body might not be apparent until after expansion of other syntax. Such definitions are known as internal definitions as opposed to the global definitions described above. The variables defined by internal definitions are local to the ⟨body⟩. That is, ⟨variable⟩ is bound rather than assigned, and the region of the binding is the entire ⟨body⟩. For example,

(let ((x 5))
  (define foo (lambda (y) (bar x y)))
  (define bar (lambda (a b) (+ (* a b) a)))
  (foo (+ x 3))) ⇒ 45

An expanded ⟨body⟩ containing internal definitions can always be converted into a completely equivalent letrec* expression. For example, the let expression in the above example is equivalent to

(let ((x 5))
  (letrec* ((foo (lambda (y) (bar x y)))
            (bar (lambda (a b) (+ (* a b) a))))
    (foo (+ x 3))))

Just as for the equivalent letrec* expression, it is an error if it is not possible to evaluate each ⟨expression⟩ of every internal definition in a ⟨body⟩ without assigning or referring to the value of the corresponding ⟨variable⟩ or the ⟨variable⟩ of any of the definitions that follow it in ⟨body⟩.

It is an error to define the same identifier more than once in the same ⟨body⟩.

Wherever an internal definition can occur, (begin ⟨definition₁⟩ …) is equivalent to the sequence of definitions that form the body of the begin.