From: Ramakrishnan Muthukrishnan Date: Fri, 17 Jun 2011 15:05:43 +0000 (+0530) Subject: corrected solution to 3.45 X-Git-Url: https://git.rkrishnan.org/about.html?a=commitdiff_plain;h=1b0af65b90a33df862b0722af8d356940222f720;p=sicp.git corrected solution to 3.45 --- diff --git a/src/sicp/ex3_45.rkt b/src/sicp/ex3_45.rkt index f3dd618..8176e70 100644 --- a/src/sicp/ex3_45.rkt +++ b/src/sicp/ex3_45.rkt @@ -2,13 +2,45 @@ #| -Deposits and Withdrawals are fine. But when it comes to exchange, we need a -"mutually serialized" procedure with something like this: +(define (exchange account1 account2) + (let ((difference (- (account1 'balance) + (account2 'balance)))) + ((account1 'withdraw) difference) + ((account2 'deposit) difference))) - (serializer-1 (serializer-2 exchange)) +(define (serialized-exchange account1 account2) + (let ((serializer1 (account1 'serializer)) + (serializer2 (account2 'serializer))) + ((serializer1 (serializer2 exchange)) + account1 + account2))) -Otherwise, we are creating a "time hole" when another execution process can -get hold of one of the account and deposit/withdraw from it which will make -the end result of exchange erroneous. +(define (make-account-and-serializer balance) + (define (withdraw amount) + (if (>= balance amount) + (begin (set! balance (- balance amount)) + balance) + "Insufficient funds")) + (define (deposit amount) + (set! balance (+ balance amount)) + balance) + (let ((balance-serializer (make-serializer))) + (define (dispatch m) + (cond ((eq? m 'withdraw) (balance-serializer withdraw)) + ((eq? m 'deposit) (balance-serializer deposit)) + ((eq? m 'balance) balance) + ((eq? m 'serializer) balance-serializer) + (else (error "Unknown request -- MAKE-ACCOUNT" + m)))) + dispatch)) + +Now let us see what happens when we call (serialized-exchange a1 a2) +We invoke the procedure exchange serialized by the two serializers + + ((s1 (s2 exchange)) a1 a2) + +Now, exchange will in turn, call serialized withdraw and deposit but +it cannot do that because we are already calling a serialized procedure +and so it will lock up waiting for the serializer. |# \ No newline at end of file