1. CSE305 – Programming Languages Daniel R. Schlegel April 25, 2011 “Should array indices start at 0 or 1? My compromise of 0.5 was rejected without, I thought, proper consideration.” (Stan Kelly-Bootle)

2. JavaScript Syntax Is this valid? int fact(int n) { if (n === 0) { return 1; } else return n * fact(n - 1); }

3. JavaScript Syntax Is this valid? int fact(int n) { if (n === 0) { return 1; } else return n * fact(n - 1); } NO! function fact(n) { if (n === 0) { return 1; } else return n * fact(n - 1); }

4. JavaScript Syntax What does this return? function array(f, g) { //f and g are functions return [f(), g()]; }

5. JavaScript Syntax What does this return? What about this? function array(f, g) { //f and g are functions return [f(), g()]; } function array(f, g) { //f and g are functions return [f, g]; }

6. Assignment 5, Part 1 iterator ireverse(array a) { int n = a.length -1; for (int i=0; i<=n; i++) { yield a[n-i]; } void main { foreach x in ireverse(a) { foreach y in ireverse (b) { sum = sum + x*y; } Q: When you call ireverse(a) does it give you all the answers at once?

7. Assignment 5, Part 1 iterator ireverse(array a) { int n = a.length -1; for (int i=0; i<=n; i++) { yield a[n-i]; } void main { foreach x in ireverse(a) { foreach y in ireverse (b) { sum = sum + x*y; } Q: When you call ireverse(a) does it give you all the answers at once? NO! What does this do?

8. Assignment 5, Part 1 iterator ireverse(array a) { int n = a.length -1; for (int i=0; i<=n; i++) { yield a[n-i]; } void main { foreach x in ireverse(a) { foreach y in ireverse (b) { sum = sum + x*y; } Q: When you call ireverse(a) does it give you all the answers at once? NO! What does this do? 1.Get the next value of ireverse(a). 2.Set x to it.

9. We need to simulate this behavior! void main(){ List l1, l2; void thk(int x) { l2 := cons(x, l2); } l2 := nil; elements(l1, thk); print l2; } void elements(List l, void thk(int)){ while (l != nil) do { thk(l.val); l := l.next; } void main(){ List l1, l2; … l2 := nil; foreach x in elements(l1) do{ l2 := cons(x, l2); } print l2; } iterator int elements(List l){ while(l != nil) { yield l.val; l := l.next; }

10. Lazy Evaluation What’s the advantage?

11. Lazy Evaluation What’s the advantage? – Doesn’t find a result until that result is needed! – We can use functions that generate infinite lists. Does JavaScript have it built in?

12. Lazy Evaluation What’s the advantage? – Doesn’t find a result until that result is needed! – We can use functions that generate infinite lists. Does JavaScript have it built in? – No! Can we simulate it? If yes, how?

13. Lazy Evaluation What’s the advantage? – Doesn’t find a result until that result is needed! – We can use functions that generate infinite lists. Does JavaScript have it built in? – No! Can we simulate it? If yes, how? – Yes, using higher order functions.

14. How we usually do things… function f(x) { return [x+1, x*2, x*x]; } f(100); //[101, 200, 10000] Short running tasks Okay, so these are short running tasks… but what if we didn’t need all of this?

15. What about this… function f(x) { return [longtask(x), longertask(x), reallylongtask(x)]; } function g(n) { print(f(100)[n]); //Some value } Is there a problem with this? We never use f(100)[0] or f(100)[2], but they are still evaluated!

16. A lazy version function f(x) { thk1() = longtask(x); thk2() = longertask(x); thk3() = reallylongtask(x); return [thk1, thk2, thk3]; } function g(n) { print(f(100)[n]()); //Some value }

17. So now extend this to infinite lists… numsfrom = function(n){ return cons(n, numsfrom(n+1)); }; What happens when we run this?

18. So now extend this to infinite lists… numsfrom = function(n){ var thk = function(){return numsfrom(n+1);}; return [n,thk]; };

19. Final Notes JIVE Survey Today is the last recitation  Good luck on the assignment and final! Also, I’m teaching CSE111 this summer… If you know anyone who needs to take it, let them know!