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2.3 Representation Strategies for Data Types

2.3 Representation Strategies for Data Types. Environment : Data type associating symbols with values { a : 4 s : 'foo f : l x.x } Interface: (empty-env) = [ Ø ] (apply-env [ f ] s ) = f ( s ) (extend-env '( s 1 ... s k ) '( v 1 ... v k ) [ f ] ) = [ g ]

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2.3 Representation Strategies for Data Types

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  1. 2.3 Representation Strategies for Data Types • Environment: Data type associating symbols with values • { a: 4 s: 'foo f: lx.x } • Interface: • (empty-env) = [Ø] • (apply-env [f] s) = f(s) • (extend-env '(s1 ... sk) '(v1 ... vk) [f]) = [g] • where g(s') = vi if s' = si for some i, 1 ≤ i ≤ k • = f (s') otherwise

  2. Example: • > (define dxy-env • (extend-env '(d x) '(6 7) • (extend-env '(y) '(8) • (empty-env)))) • > (apply-env dxy-env 'x) • 7

  3. A constructor builds a datatype: empty-env, extend-env • An observer extracts info from a datatype: apply-env • We will consider three representations for environments: • Procedural • Abstract Syntax Tree • “Alternative” (list-based)

  4. Procedural representation of environments • In Scheme, procedures (functions) are first-class objects: can be passed to / returned from other procedures, and stored in data structures • E.g., we can represent numbers as procedures: • [0] = ls.lz.z • [1] = ls.lz.(s z) • [2] = ls.lz. (s (s z)) • [succ] = ln.ls.lz. (s (n s z)) • Now compute (succ 0) ....

  5. Environment as procedure: (define empty-env (lambda () (lambda (sym) (eopl:error 'apply-env “No binding for ~s” sym)))) • An environment is a function that takes a symbol and returns its assoicated value. • Empty environment defines no symbols. • Therefore empty environment returns an error if you try to apply it to a symbol

  6. Environment as procedure: (define apply-env (lambda (env sym) (env sym))) So apply-env is really just “syntactic sugar”

  7. Environment as procedure: (define extend-env (lambda (syms vals env) (lambda (sym) ; get position of sym in syms (let ((pos (list-find-pos sym syms))) (if (number? pos) ; we found it! ; return value at that pos (list-ref vals pos) ; not found; use current env (apply-env env sym))))))

  8. Examples: > (define xy-env (extend-env '(x y) '(3 4) (empty-env))) > xy-env #<procedure:13:7> > (apply-env xy-env 'y) 4 > (apply-env xy-env 'z) apply-env: No binding for z

  9. Environment as Abstract Syntax Tree: <env-rep> ::= (empty-env) empty-env-record ::= (extend-env ({<symbol>}*) ({<value>}*) <env-rep>) extended-env-record (syms vals env

  10. Environment as AST: extended-env-record syms env vals (x y) (3 4) empty-env-record

  11. Environment as AST: (define-datatype environment environment? (empty-env-record) (extended-env-record (syms (list-of symbol?)) (vals (list-of scheme-value?)) (env environment?))) (define scheme-value? (lambda (v) #t) > ((list-of number?) '(1 2 3)) #t

  12. Environment as AST: (define empty-env (lambda () (empty-env-record))) (define extend-env (lambda (syms vals env) (extended-env-record syms vals env)))

  13. Environment as AST: (define apply-env (lambda (env sym) (cases environment env (empty-env-record () (eopl:error ...)) (extended-env-record (syms vals env) (let ((pos (list-find-pos sym syms))) (if (number? pos) (list-ref vals pos) (apply-env env sym)))))))

  14. Alternative (List) Representation Actually the most intuitive: <env-rep> ::= () ::= (({<symbol>}*) ({<value>}*) <env-rep>) (define empty-env (lambda () '())) (define extend-env (lambda (syms vals env) (cons (list syms vals) env)))

  15. Alternative Representation (define apply-env (lambda (env sym) (if (null? env) (eopl:error ...) (let (syms (car (car env))) (vals (cadr (car env))) (env (cdr env))) (let ((pos (rib-find-pos sym syms))) (if (number? pos) (list-ref vals pos) (apply-env env sym))))))) (define rib-find-pos list-find-pos)

  16. Alternative Representation: Ribcage (define dxy-env (extend-env '(d x) '(6 7) (extend-env '(y) '(8) (empty-env)))) d x 6 7 y 8

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