2.23 Flattening Syntactic Sequences: nest

(require scheme/nest)

The bindings documented in this section are provided by the scheme/nest and scheme libraries, but not scheme/base.

(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 ...))]))