Revised(5) Report on the Algorithmic Language Scheme

5.2.2 Internal definitions

Definitions may occur at the beginning of a <body> (that is, the body of a lambda, let, let*, letrec, let-syntax, or letrec-syntax expression or that of a definition of an appropriate form). Such definitions are known as internal definitions as opposed to the top level definitions described above. The variable defined by an internal definition is 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

A <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 must be possible to evaluate each <expression> of every internal definition in a <body> without assigning or referring to the value of any <variable> being defined.

Wherever an internal definition may occur (begin <definition1> ...) is equivalent to the sequence of definitions that form the body of the begin.