SICPゼミ第34回
2章に帰ってきました。
put, get など
(define (key-compare a b) 0) (define (make-table) (let (( local-table (list '*table* ))) (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 )) (define operation-table (make-table )) (define get (operation-table 'lookup-proc )) (define put (operation-table 'insert-proc! )) (define (attach-tag type-tag contents) (cons type-tag contents )) (define (type-tag datum) (if (pair? datum) (car datum) (error "Bad tagged datum: TYPE-TAG" datum ))) (define (contents datum) (if (pair? datum) (cdr datum) (error "Bad tagged datum: CONTENTS" datum ))) (define (apply-generic op . args) (let (( type-tags (map type-tag args ))) (let ((proc (get op type-tags ))) (if proc (apply proc (map contents args)) (error "No method for these types: APPLY-GENERIC" (list op type-tags )))))) (define (add x y) (apply-generic 'add x y)) (define (sub x y) (apply-generic 'sub x y)) (define (mul x y) (apply-generic 'mul x y)) (define (div x y) (apply-generic 'div x y)) (define (install-scheme-number-package) (define (tag x) (attach-tag 'scheme-number x)) (put 'add '(scheme-number scheme-number) (lambda (x y) (tag (+ x y)))) (put 'sub '(scheme-number scheme-number) (lambda (x y) (tag (- x y)))) (put 'mul '(scheme-number scheme-number) (lambda (x y) (tag (* x y)))) (put 'div '(scheme-number scheme-number) (lambda (x y) (tag (/ x y)))) (put 'make 'scheme-number (lambda (x) (tag x))) 'done) (define (install-rational-package) ;; 内部手続き (define (numer x) (car x)) (define (denom x) (cdr x)) (define (make-rat n d) (let ((g (gcd n d))) (cons (/ n g) (/ d g)))) (define (add-rat x y) (make-rat (+ (* (numer x) (denom y)) (* (numer y) (denom x))) (* (denom x) (denom y)))) (define (sub-rat x y) (make-rat (- (* (numer x) (denom y)) (* (numer y) (denom x))) (* (denom x) (denom y)))) (define (mul-rat x y) (make-rat (* (numer x) (numer y)) (* (denom x) (denom y)))) (define (div-rat x y) (make-rat (* (numer x) (denom y)) (* (denom x) (numer y)))) ;; システムのほかの部分とのインターフェイス (define (tag x) (attach-tag 'rational x)) (put 'add '(rational rational) (lambda (x y) (tag (add-rat x y)))) (put 'sub '(rational rational) (lambda (x y) (tag (sub-rat x y)))) (put 'mul '(rational rational) (lambda (x y) (tag (mul-rat x y)))) (put 'div '(rational rational) (lambda (x y) (tag (div-rat x y)))) (put 'make 'rational (lambda (n d) (tag (make-rat n d)))) 'done) (define (make-rational n d) ((get 'make 'rational) n d)) (define (install-complex-package) ;; 直交形式パッケージと極形式パッケージからインポートした手続き (define (make-from-real-imag x y) 204 ((get 'make-from-real-imag 'rectangular) x y)) (define (make-from-mag-ang r a) ((get 'make-from-mag-ang 'polar) r a)) ;; 内部手続き (define (add-complex z1 z2) (make-from-real-imag (+ (real-part z1) (real-part z2)) (+ (imag-part z1) (imag-part z2)))) (define (sub-complex z1 z2) (make-from-real-imag (- (real-part z1) (real-part z2)) (- (imag-part z1) (imag-part z2)))) (define (mul-complex z1 z2) (make-from-mag-ang (* (magnitude z1) (magnitude z2)) (+ (angle z1) (angle z2)))) (define (div-complex z1 z2) (make-from-mag-ang (/ (magnitude z1) (magnitude z2)) (- (angle z1) (angle z2)))) ;; システムのほかの部分とのインターフェイス (define (tag z) (attach-tag 'complex z)) (put 'add '(complex complex) (lambda (z1 z2) (tag (add-complex z1 z2)))) (put 'sub '(complex complex) (lambda (z1 z2) (tag (sub-complex z1 z2)))) (put 'mul '(complex complex) (lambda (z1 z2) (tag (mul-complex z1 z2)))) (put 'div '(complex complex) (lambda (z1 z2) (tag (div-complex z1 z2)))) (put 'make-from-real-imag 'complex (lambda (x y) (tag (make-from-real-imag x y)))) (put 'make-from-mag-ang 'complex (lambda (r a) (tag (make-from-mag-ang r a)))) 'done)
by tube
練習問題2.81
ちなみに put-coercion, get-coercion は新しいテーブルを持てばいいので
(define coercion-table (make-table )) (define get-coercion (coercion-table 'lookup-proc )) (define put-coercion (coercion-table 'insert-proc! ))
a.
型変換メソッドが存在し続ける、つまり apply-generic 内にある cond 条件文の else 節に入らないため、無限ループがおきる。
b.
そのままで動く。
c.
(define (apply-generic op . args) (let (( type-tags (map type-tag args ))) (let ((proc (get op type-tags ))) (if proc (apply proc (map contents args)) (if (= (length args) 2) (let (( type1 (car type-tags )) (type2 (cadr type-tags )) (a1 (car args )) (a2 (cadr args ))) (if (equal? type1 type2) (error "No method for these types") (let (( t1->t2 (get-coercion type1 type2 )) (t2->t1 (get-coercion type2 type1 ))) (cond (t1->t2 (apply-generic op (t1->t2 a1) a2)) (t2->t1 (apply-generic op a1 (t2->t1 a2))) (else (error "No method for these types" (list op type-tags ))))))) (error "No method for these types" (list op type-tags )))))))