Chapter 3, Modularity, Objects, and State
Exercise 3.24
It only require a small change, just make assoc part of the procedure object i.e. declare assoc inside procedure object make-table and except an argument same-key? and use this inplace of equal inside assoc.
The code with the changes required:
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#lang sicp
(define (make-table same-key?)
(let ((local-table (list '*table*)))
(define (assoc key records)
(cond ((null? records) false)
((same-key? key (caar records)) (car records))
(else (assoc key (cdr records)))))
(define (lookup key-1 key-2)
(let ((subtable (assoc key-1 (cdr local-table))))
(if subtable
(let ((record (assoc key-2 (cdr subtable))))
(if record
(cdr record)
false))
false)))
(define (insert! key-1 key-2 value)
(let ((subtable (assoc key-1 (cdr local-table))))
(if subtable
(let ((record (assoc key-2 (cdr subtable))))
(if record
(set-cdr! record value)
(set-cdr! subtable
(cons (cons key-2 value)
(cdr subtable)))))
(set-cdr! local-table
(cons (list key-1
(cons key-2 value))
(cdr local-table)))))
'ok)
(define (dispatch m)
(cond ((eq? m 'lookup-proc) lookup)
((eq? m 'insert-proc!) insert!)
(else (error "Unknown operation -- TABLE" m))))
dispatch))
Example/Output:
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> (define tbl (make-table equal?))
> ((tbl 'insert-proc!) 'math '+ 43)
'ok
> ((tbl 'insert-proc!) 'math '- 45)
'ok
> ((tbl 'insert-proc!) 'math '* 42)
'ok
> ((tbl 'insert-proc!) 'letters 'a 97)
'ok
> ((tbl 'insert-proc!) 'letters 'b 98)
'ok
> ((tbl 'lookup-proc) 'math '+)
43
> ((tbl 'lookup-proc) 'letters 'b)
98
> ((tbl 'lookup-proc) 'math '/)
#f
> ((tbl 'lookup-proc) 'random '+)
#f
>