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31 scheme/nest

 (require scheme/nest)

(nest ([datum ...+] ...) body ...+)
Combines nested expressions that syntactically drift to the right into a more linear textual format, much in the same way that let* linearizes a sequence of nested let expressions.

For example,

  (nest ([let ([x 10]
               [y 6])]
         [with-handlers ([exn:fail? (lambda (x) 15)])]
         [parameterize ([current-output-port (current-error-port)])]
         [let-values ([(d r) (quotient/remainder x y)])])
    (display (+ d r)))

is equivalent to

  (let ([x 10]
        [y 6])
    (with-handlers ([exn:fail? (lambda (x) 15)])
      (parameterize ([current-output-port (current-error-port)])
        (let-values ([(d r) (quotient/remainder x y)])
          (display (+ d r))))))

The nest form is unusual in that it has no semantics apart from its expansion, and its implementation is easier to understand than a precise prose description:

  (define-syntax nest
    (syntax-rules ()
      [(nest () body0 body ...)
       (let () body0 body ...)]
      [(nest ([form forms ...]) body0 body ...)
       (form forms ... (let () body0 body ...))]
      [(nest ([form forms ...] . more) body0 body ...)
       (form forms ... (nest more body0 body ...))]))