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SICP Exercise 2.77: expected a procedure that can be applied to arguments, given #f

2023-07-13

Louis Reasoner tries to evaluate the expression (magnitude z) where z is the object shown in figure 2.24. To his surprise, instead of the answer 5 he gets an error message from apply-generic, saying there is no method for the operation magnitude on the types (complex).

To simplify the install-complex-package procedure, I made the following changes:

(define (install-complex-package)
  (define (make-from-real-imag x y)
		((get 'make-from-real-imag 'rectangular) x y))
	(define (tag z) (attach-tag 'complex z))
	(put 'make-from-real-imag 'complex
		(lambda (x y) (tag (make-from-real-imag x y))))

(install-complex-package)

(trace-define (make-complex-from-real-imag x y)
	((get 'make-from-real-imag 'complex) x y))

(magnitude (make-complex-from-real-imag 3 4))