Write stack safe non-tailrec recursive functions

Recrusive function is cool, but it sometimes cause stack overflow in naive implementation. I'll explain how to write any recursive functions in stack safe way.

Transcript

1. Write stack safe non-tailrec recursive functions @jooohn1234
2. Jun Tomioka @jooohn1234 Software Engineer at M3, Inc.
3. Recursion is cool
4. Recursive data structures
5. Recursive functions
6. Recursive functions • Describe WHAT it is rather than HOW it’d be done
7. Recursive functions • Describe WHAT it is rather than HOW it’d be done Cool! FP Programmers
8. Stack Overflow
9. Recursive functions • Tend to cause StackOverflowException
10. Tail recursion • Make functions tailrec to avoid StackOverflow
11. Tail recursion • Make functions tailrec to avoid StackOverflow Cool! FP Programmers
12. Tail recursion • Make functions tailrec to avoid StackOverflow
13. Tail recursion • Make functions tailrec to avoid StackOverflow It’s… cool! FP Programmers
14. Tail recursion? • Make functions tailrec to avoid StackOverflow … but can we?
15. Tail recursion? • Make functions tailrec to avoid StackOverflow … but can we?
16. Tail recursion? • Make functions tailrec to avoid StackOverflow … but can we? It’s… still cool I believe! FP Programmers
17. Tail recursion? • How many unique paths from S to G are there? S G
18. Tail recursion? • Can you make this recursive function tailrec?
19. Tail recursion? • Can you make this recursive function tailrec? ・・・ FP Programmers
20. Give up
21. Tail-recursion’s limitation • It tends to be difficult to convert recursive functions that depend on results of multiple recursive calls.
22. Tail-recursion’s limitation • It tends to be difficult to convert recursive functions that depend on results of multiple recursive calls.
23. Tail-recursion’s limitation • It tends to be difficult to convert recursive functions that depend on results of multiple recursive calls. Monad! FP Programmers
24. The Monad version of recursion • This function still describes the problem’s structure intuitively, doesn’t it?
25. The Monad version of recursion • This function still describes the problem’s structure intuitively, doesn’t it? Cool! FP Programmers
26. The Monad version of recursion • Of course just rewriting the recursive function with Monad doesn’t mean it is stack safe.
27. Trampoline
28. Trampoline • > a trampoline is a loop that iteratively invokes thunk-returning functions ◦ https://en.wikipedia.org/wiki/Trampoline_(computing) #High-level_programming ◦ “Trampoline” in computer science has a lot of meanings. This is just one of them.
29. Trampoline • A trampoline loop evaluates values one by one until it returns final result. • We can write this evaluation loop as tailrec! Eval evaluate Eval Result OR loop
30. Eval * We typically implement “Defer”, “Suspend” or something like that to avoid stack overflow when building the Eval.
31. Eval * We typically implement “Defer”, “Suspend” or something like that to avoid stack overflow when building the Eval. Represents single value
32. Eval * We typically implement “Defer”, “Suspend” or something like that to avoid stack overflow when building the Eval. Has following evaluation
33. Eval: trampoline
34. Eval: trampoline
35. [Optional] Memoization • This is not the essence, but memoization to reduce running time • In many cases, we can make such “What it is” style of recursive functions faster by using memoization
38. Beautiful FP Programmers
39. Run with Id
40. Run with Eval
41. Run with Eval Cool! FP Programmers
42. Let’s implement stack safe foldRight with Eval
43. Let’s implement stack safe foldRight with Eval
44. Let’s implement stack safe foldRight with Eval !!! FP Programmers
45. Let’s implement stack safe foldRight with Eval This recursive call of loop causes stack overflow before stack safe trampoline evaluation
46. Use Defer to defer Eval instance construction
47. Use Defer to defer building Eval instance
48. Use Defer to defer building Eval instance Stack safe! FP Programmers
49. Recap • Recursive functions is cool especially when it describes ”what” rather than ”how” • We can apply the trampoline technique to avoid stack overflow when tail-recursion is not effective
50. Thank you!