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

2023-07-13

Categories: Programming

2.5.1 Generic Arithmetic Operations

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))

However, upon running the code, instead of the expected error from apply-generic, a different error occurred:

$ racket 2.77-apply-generic-magnitude.rkt
'done
>(make-complex-from-real-imag 3 4)
application: not a procedure;
 expected a procedure that can be applied to arguments
  given: #f

I wondered why it returned #f when 'make-from-real-imag 'complex had already been put into the dispatch table:

$ racket --repl
Welcome to Racket v8.9 [cs].
> (require "2.77-apply-generic-magnitude.rkt")
> (get 'make-from-real-imag 'complex)
#<procedure:...neric-magnitude.rkt:75:16>

To debug the issue, I utilized the racket/trace library:

$ racket --repl
Welcome to Racket v8.9 [cs].
> (require "2.77-apply-generic-magnitude.rkt")
>(install-complex-package)
> (put 'make-from-real-imag 'complex #<procedure:...neric-magnitude.rkt:75:16>)
< #<void>
<'done
'done
> (make-complex-from-real-imag 3 4)
>(make-complex-from-real-imag 3 4)
> (get 'make-from-real-imag 'complex)
< #<procedure:...neric-magnitude.rkt:75:16>
> (make-from-real-imag 3 4)
> >(get 'make-from-real-imag 'rectangular)
< <#f
application: not a procedure;
 expected a procedure that can be applied to arguments
  given: #f
 [,bt for context]

As a result, it became evident that when we called (make-complex-from-real-imag 3 4), it was actually invoking make-from-real-imag 3 4, which, in turn, called (get 'make-from-real-imag 'rectangular) and this returned #f.

To address this issue, we need to add the install-rectangular-package procedure mentioned in 2.4.3 Data-Directed Programming and Additivity to get the expected error:

>(make-complex-from-real-imag 3 4)
> (get 'make-from-real-imag 'complex)
< #<procedure:...neric-magnitude.rkt:75:16>
> (make-from-real-imag 3 4)
> >(get 'make-from-real-imag 'rectangular)
< <#<procedure:...neric-magnitude.rkt:65:16>
> (attach-tag 'rectangular '(3 . 4))
< '(rectangular 3 . 4)
>(tag '(rectangular 3 . 4))
>(attach-tag 'complex '(rectangular 3 . 4))
<'(complex rectangular 3 . 4)
>(get 'magnitude '(complex))
<#f
No method for these types: APPLY-GENERIC '(magnitude (complex))

Tags: sicp racket scheme

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