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3.7 Variable Assignment

3.7 Variable Assignment. Recall instance variables in Python: class BankAccount: def __init__(self): self.balance = 0 # instance var def deposit(self, amount): self.balance += amount return self.balance def withdraw(self, amount): self.balance -= amount

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3.7 Variable Assignment

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  1. 3.7 Variable Assignment Recall instance variables in Python: class BankAccount: def __init__(self): self.balance = 0 # instance var def deposit(self, amount): self.balance += amount return self.balance def withdraw(self, amount): self.balance -= amount return self.balance

  2. Want to allow procedures to “share” data this way: --> let bal = 0 in letrec deposit(amt) = let d = set bal = +(bal, amt) in bal withdraw(amt) = let d = set bal = -(bal, amt) in bal in (deposit 100)

  3. Add to grammar: • <expression> ::= set <identifier> = <expression> • varassign-exp (id rhs-exp) • Add to eval-expression: Must alter current env, • not just add to it. • So need a new datatype: reference, with operations setref and deref

  4. Use Scheme vector (more efficient than list): (define-datatype reference reference? (a-ref ; just one variant (position integer?) (vec vector?))) a-ref 2 7 0 1 2 3

  5. Now we can define setref and deref: (define setref! (lambda (ref val) (cases reference ref (a-ref (pos vec) ; only variant (vector-set! vec pos val))))) (define deref (lambda (ref) (cases reference ref (a-ref (pos vec) ; only variant (vector-ref vec pos)))))

  6. Use setref in eval-expression: (define eval-expression (lambda (exp env) (cases expression exp ... (varassign-exp (id rhs-exp) (begin (setref! (apply-env-ref env id) (eval-expression rhs-exp env)) 1)) ; huh?

  7. So what does apply-env-ref do? (define apply-env-ref (lambda (env sym) (cases environment env (empty-env-record () (eopl:error 'apply-env-ref “No binding...”)) (extended-env-record (syms vals env) (let ((pos (list-find-position sym syms))) (if (number? pos) ; found (a-ref pos vals) (apply-env-ref env sym)))))))

  8. Can redefine apply-env in terms of apply-env-ref: (define apply-env (lambda (env sym) (deref (apply-env-ref env sym)))) • Why deref ? • Because apply-env-ref creates a reference structure.

  9. Recall original example: • --> let bal = 0 • in letrec • deposit(amt) = • let d = set bal = +(bal, amt) • in bal • withdraw(amt) = • let d = set bal = -(bal, amt) • in bal • in (deposit 100) • Why is this uninteresting? What do we need?

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