The Environment Model an extension of the substitution model more "operational" fully explains static scoping and the process by which variable names are resolved in Dylan An expression only has a value with respect to an environment
Binding identifier : value Frame unordered set of bindings Environment ordered list of frames
x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff) global environment (top level frame)
The Lookup Rule the value of an identifier x is the value associated with x in the first (lowest) frame containing a binding for x undefined if there aren't any
Define rule To evaluate (define ident expr): evaluate expr in the current environment add a binding to the top-level frame (the global environment) Set! rule To evaluate (set! ident expr): evaluate expr in the current environment look up ident using lookup rule change that binding to value of expr
(define (d ) (+ 5 7)) x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff)
(define (d ) (+ 5 7)) x:10 +:{add} y:20 d:12 z:30 x:() w:foobar x:oh y:(list of stuff)
x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff) (set! x 'my)
x:10 +:{add} y:20 z:30 x:() w:foobar x:my y:(list of stuff) (set! x 'my)
Method rule To evaluate (method (params) body) in environment env : create a function object consisting of params body (unevaluated) env Example: value of (method ((x ) (y )) (sqrt (+ (* x x) (* y y)))) in environment env is params: ((x ) (y )) body: (sqrt (+ (* x x) (* y y))) env: env
Eval rule To evaluate (f a1... an) in environment e : evaluate f,a1,...,an in e apply f to arguments a1,...,an Apply rule To apply f to arguments a1,...,an : create a new frame with parameters of f bound to arguments a1,...,an append that frame to front of environment e’ in the closure of f to get e’’ evaluate body of f in environment e’’
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 x:100 +:{add} y:200 z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 x:100 +:{add} y:200 z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 x:10 +:{add} y:200 z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 x:10 +:{add} y:200 z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300 x: 15
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300 x: 15
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300 x: 15
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300 x: 15y: 20
(define x 10) (define (foo ) (method (y) x)) (bind ((x 15)) (foo 20)) => 10 params: (y) body: x env: x:10 +:{add} y:200 foo: z:300 x: 15y: 20