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Environments and the Contour Model

Environments and the Contour Model. Environments. Consist of: frames chained together. Frames are:. Global Environment. Primitives. Global user declarations. Function Evaluation. > (Y 2). Function Evaluation. So, X is bound in Y What about +? Unbound When unbound lookup ….

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Environments and the Contour Model

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  1. Environments and the Contour Model

  2. Environments Consist of: frames chained together Frames are:

  3. Global Environment Primitives Global user declarations

  4. Function Evaluation > (Y 2)

  5. Function Evaluation • So, X is bound in Y • What about +? • Unbound • When unbound lookup …

  6. Traverse Frames

  7. Y 3 What about multiple bindings? • (define x 3) • (define y 2) • (define z (lambda (x) (let ((y 3)) (+ x y)))) • (z 4) X and Y multiply defined. Always use first binding!

  8. Passing Mechanism • By value: • Copy of argument is value for formal argument • Pointer in the case of lists • Copy of string or integer for primitive types

  9. Primitive-Environment User-Environment Points at line where  defined b 5 square (2) Points at environment in which  defined • line 1:    (define b 5) line 2:    (define (square num) line 3:        (* num num)) line 4:    (define result (square 5)) P = 4 Points at line where execution snapshot created

  10. Primitive-Environment User-Environment b 5 square (2) square’ • line 1:    (define b 5) line 2:    (define (square num) line 3:        (* num num)) line 4:    (define result (square 5)) num 5 P = 3 P = 4 Points at line where function execution ends Return pointer

  11. Primitive-Environment User-Environment b 5 square 2 result 25 • line 1:    (define b 5) line 2:    (define (square num) line 3:        (* num num)) line 4:    (define result (square 5)) P = 5

  12. Primitive-Environment User-Environment (1) factorial line 1:    (define (factorial n) line 2:            (if (< n 2) line 3:                    1 line 4:                    (* n (factorial (- n 1))))) line 5:    (define result (factorial 3)) P = 5

  13. Primitive-Environment User-Environment (1) factorial factorial’ n 3 line 1:    (define (factorial n) line 2:            (if (< n 2) line 3:                    1 line 4:                    (* n (factorial (- n 1))))) line 5:    (define result (factorial 3)) P = 5 P = 4

  14. Primitive-Environment User-Environment (1) factorial factorial’ n 3 P = 5 P = 4 factorial’’ line 1:    (define (factorial n) line 2:            (if (< n 2) line 3:                    1 line 4:                    (* n (factorial (- n 1))))) line 5:    (define result (factorial 3)) n 2 P = 4

  15. Primitive-Environment User-Environment (1) factorial factorial’ n 3 P = 5 P = 4 factorial’’ line 1:    (define (factorial n) line 2:            (if (< n 2) line 3:                    1 line 4:                    (* n (factorial(- n 1))))) line 5:    (define result (factorial 3)) n 2 P = 4 factorial’’’ n 1 P = 4

  16. Primitive-Environment User-Environment (1) factorial 6 result P = 6

  17. Warning Simplified Diagrams! • Next slides are simplified: • Lambdas for a, b, etc not present Extra environment frame is for the let inside of main. Remember: (let ((a 0) (b 0)) <body>) is equivalent to ((lambda (a b) <body>) 0 0) Main’ 5 a 6 b (3) f g (8)

  18. Code Line 1:        (define (main) Line 2:             (let (( a 0) (b 0)) Line 3:             (define (f) Line 4:                (let ((a 0)) Line 5:                    (set! a b) Line 6:                     (writeln "in f: A and B are - " a b) Line 7:                    (g))) Line 8:             (define (g) Line 9:                (let ((b 0)) Line10:                    (set! b (+ a 2)) Line11:                    (writeln "in g: A and B are - " a b) Line12:                     (f))) Line13:            (set! a 5) Line14:            (set! b 6) Line15:            (f))) Line16:     (define a 10) Line17:     (define b 20) Line18:     (main)

  19. Primitive-Environment User-Environment P = 18 10 a 20 b main (1) Main’ 5 a P = 15 6 b (3) f g (8) F’ a 6 P = 7

  20. Primitive-Environment User-Environment P = 18 10 a 20 b main (1) Main’ 5 a P = 15 6 b (3) f g (8) G’ F’ a 6 b 7 P = 7 P = 12

  21. Primitive-Environment User-Environment P = 18 10 a 20 b main (1) Main’ 5 a P = 15 6 b (3) f g (8) F’ G’ F’’ a 6 b 7 a 6 P = 7 P = 12 P = 7

  22. Primitive-Environment User-Environment P = 18 10 a 20 b main (1) Main’ 5 a P = 15 6 b (3) f g (8) F’ G’ F’’ G’’ a 6 b 7 a 6 b 7 P = 7 P = 12 P = 7 P = 12

  23. Primitive-Environment User-Environment 10 P = 18 a 20 b main (1) Main’ P = 15 F’ 5 a a 6 P = 7 6 b (3) f g (8)

  24. Primitive-Environment User-Environment 10 P = 18 a 20 b main (1) Main’ P = 15 F’ 5 a a 6 P = 7 6 b G’ (3) f b 8 g (8) P = 12

  25. Primitive-Environment User-Environment 10 P = 18 a 20 b main (1) Main’ F’ P = 15 5 a a 6 P = 7 6 b G’ (3) f b 8 F’’ g (8) a 8 P = 12 P = 7

  26. Primitive-Environment User-Environment 10 P = 18 a 20 b main (1) Main’ F’ P = 15 5 a a 6 P = 7 6 b G’ (3) f b 8 F’’ g (8) G’’ a 8 b 10 P = 12 P = 7 P = 12

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