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25.3 Internal definitions

A define form which appears inside the body of a lambda, let, let*, letrec or equivalent expression is called an internal definition. An internal definition differs from a top level definition (see Top Level), because the definition is only visible inside the complete body of the enclosing form. Let us examine the following example.

(let ((frumble "froz"))
   (define banana (lambda () (apple 'peach)))
   (define apple (lambda (x) x))
   (banana))
=>
peach

Here the enclosing form is a let, so the defines in the let-body are internal definitions. Because the scope of the internal definitions is the complete body of the let-expression, the lambda-expression which gets bound to the variable banana may refer to the variable apple, even though it's definition appears lexically after the definition of banana. This is because a sequence of internal definition acts as if it were a letrec expression.

(let ()
  (define a 1)
  (define b 2)
  (+ a b))

is equivalent to

(let ()
  (letrec ((a 1) (b 2))
    (+ a b)))

Another noteworthy difference to top level definitions is that within one group of internal definitions all variable names must be distinct. That means where on the top level a second define for a given variable acts like a set!, an exception is thrown for internal definitions with duplicate bindings.