TSS set!

The Seasoned Schemerの15章になって初めて出てきたset!。set!ってのはこういうのだよーという章。

	;; chapter.15
	(define x
	(cons (quote chicago)
	(cons (quote pizza)
	(quote ()))))
	; -> x
	x
	; -> (chicago pizza)
	(set! x (quote gone))
	; -> gone
	x
	; -> gone
	(set! x (quote skins))
	; -> skins
	x
	; -> skins
	(define gourmet
	(lambda (food)
	(cons food
	(cons x (quote ())))))
	x
	; -> skins
	(gourmet (quote onion))
	; -> (onion skins)
	(set! x (quote rings))
	(gourmet (quote onion))
	; -> (onion rings)
	(define gourmand
	(lambda (food)
	(set! x food)
	(cons food
	(cons x
	(quote ())))))
	(gourmand (quote potato))
	; -> (potato potato)
	(define dinner
	(lambda (food)
	(cons (quote milkshake)
	(cons food
	(quote ())))))
	(define dinnerR
	(lambda (food)
	(set! x food)
	(cons (quote milkshake)
	(cons food
	(quote ())))))
	(dinnerR (quote onion))
	; -> (milkshake onion)
	(dinnerR (quote pecanpie))
	; -> (milkshake pecanpie)
	x
	; -> pecanpie
	(gourmand (quote onion))
	; -> (onion onion)
	x
	; -> onion
	(define omnivore
	(let ((x (quote minestrone)))
	(lambda (food)
	(set! x food)
	(cons food
	(cons x
	(quote ()))))))
	(omnivore (quote bouillabaisse))
	; -> (bouillabaisse bouillabaisse)
	(define gobbler
	(let ((x (quote minestrone)))
	(lambda (food)
	(set! x food)
	(cons food
	(cons x
	(quote ()))))))
	(gobbler (quote gumbo))
	; -> (gumbo gumbo)
	(define nibbler
	(lambda (food)
	(let ((x (quote donut)))
	(set! x food)
	(cons food
	(cons x
	(quote ()))))))
	(nibbler (quote cheerio))
	; -> (cheerio cheerio)
	;; Isn't (let ...) like (letrec ...)
	(define food (quote none))
	(define glutton
	(lambda (x)
	(set! food x)
	(cons (quote more)
	(cons x
	(cons (quote more)
	(cons x
	(quote ())))))))
	(glutton (quote garlic))
	; -> (more garlic more garlic)
	food
	; -> garlic
	(define chez-nous
	(lambda ()
	(set! food x)
	(set! x food)))
	food
	; -> garlic
	x
	; -> onion
	(chez-nous)
	food
	; -> onion
	x
	; -> onion
	(define chez-nous
	(lambda ()
	(let ((a food))
	(set! food x)
	(set! x a))))
	(glutton (quote garlic))
	; -> (more garlic more garlic)
	food
	; -> garlic
	(gourmand (quote potato))
	; -> (potato potato)
	x
	; -> potato
	(chez-nous)
	food
	; -> potato
	x
	; -> garlic

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