This document summarizes the new task feature in NetLogo 5.0 which allows chunks of code to be passed around and stored in variables. Tasks are implemented using closures and provide two new data types: command tasks and reporter tasks. Several primitives were added or modified to work with tasks including map, filter, and foreach. Tasks close over variables from their enclosing context and provide a more efficient way to run code than using strings. Possible future enhancements mentioned include named inputs for tasks and automatic updating of tasks.

1. “Tasks” in NetLogo 5.0beta1March 21, 2011

2. What are tasks?chunks of code to be run once,run repeatedly, passed around, stored in variables, etc.

3. Known elsewhere asclosuresfirst-class functionslambda

4. Tasks in NetLogo 5.02 new data types3 new primitives8 primitives altered

5. 2 new data typesCommand tasksReporter tasks

6. 3 new primitivestaskis-command-task?is-reporter-task?

7. 6 altered primitivesmap, filter, reduce,n-values, sort-byforeach

8. 2 more altered primitivesrunrunresult

9. NOT altered (for now)ifelse, while, repeat, loop, ...

10. NOT altered (for now)ask, of, with, all?, ...

11. Backward compatible syntaxsquare bracketsinputs: ?1 (aka ?), ?2, ?3, ...

12. Implicit task syntaxmap [? * ?] [1 2 3] => [1 4 9]filter [? mod 2 = 0] [1 2 3 4] => [2 4]foreach sort turtles [ print ? ]

13. Explicit task syntaxset next-task task [ fd 1 ]set coloring task [ ifelse-value (wealth > 100) [ red ] [ blue ] ]

14. Disambiguation of task typecommand or reporter?look at first token after the opening bracket (skipping left parens)

15. No tasks “escape”unless you use the explicit syntax

16. Running tasksrun task [ print 5 ]=> 5print runresult task [5]=> 5

17. Running tasks (variadic)let printer task [ print ?1 + ?2 ](run print 2 3)=> 5let square task [? * ?]print (runresult square 5)=> 25

18. Concise task syntaxtask die short for task [ die ]task fd short for task [ fd ? ]task + short for task [?1 + ?2]

19. Concise + implicit syntaxmap abs [1 -2 3 -4] => [1 2 3 4]reduce + [1 2 3 4] => 10filter is-number? [1 “x” 3] => [1 3]

20. Extra inputs are ignored(runresult task [? * 2] 1 10 100)=> 2

21. Tasks are closuresclose over procedure inputsclose over local variables

22. Closing over procedure inputsto-report printer [x] report task [ print x ]endrun printer 5=> 5

23. Closing over local variableslet x 5let y task [ print x ]run y=> 5

24. Close over bindings, not valueslet x 5let y task [x]set x 10print runresult y=> 10

25. Current agent not closed overglobals [g]ask turtle 0 [ set g task [ print who ] ]ask turtle 1 [ run g ]=> 1

26. Nonlocal exitsstop and report refer to dynamically enclosing procedure, not enclosing task(backwards compatible)

27. It’s fast100x faster than compiling a string

28. Models using explicit tasksState Machine ExampleTermites 3D(afraid that’s all at the moment)

29. Use tasks, not strings!but, still need strings with run/runresult for running code entered by end user

30. Possible future work:named inputs[x => x * x](or something like that?not sure yet)

31. Possible future work:complex reporter taskstask [ let x ... ... report ...]

32. Wouldn’t it be cool if?:automatic updatingset label task [energy]set color task [ ifelse-value (wealth < 100) [red] [blue] ]

33. Wouldn’t it be cool if?:schedulingin-ticks 5 task [ fd 1 ]

34. Questions?http://groups.yahoo.com/group/netlogo-users