Why foldRight is beautiful

Explains why "foldRight" is beautiful.

Transcript

1. Why foldRight is beautiful Jun Tomioka (M3, inc.)
2. M3, Inc. Jun Tomioka Twitter: @jooohn1234 Github: jooohn Love: Our very first baby Had great paternity leave!
4. foldLeft / foldRight
5. trait List[+A]
6. foldLeft[B](z: B)(op: (B, A) => B): B A A A A A B A A A A B B OP ...
7. foldRight[B](z: B)(op: (A, B) => B): B A A A A A B A A A A B B OP ...
8. So what’s the difference between them?
9. Suppose you define your own Linked List
10. foldLeft would be like this
11. foldRight would be like this
12. This naive foldRight can cause stack overflow
13. If so, why can foldRight be beautiful?
14. Let’s implement “map” with foldLeft
15. A A A A A List[B] A A A A OP ... List[B] List[B]
16. Let’s implement “map” with foldLeft
17. foldRight
18. Let’s implement “map” with foldRight
19. A A A A A List[B] A A A A List[B] List[B] OP ...
20. Why is it that natural to write “map” with foldRight?
21. Immutable recursive data structure
22. Bigger part holds smaller part
23. We must create them from smaller part to bigger part
24. This is the folding order of foldRight
25. foldRight folds items by its creation order
26. Why foldRight is beautiful • Immutable recursive data structure is built from smaller part to bigger part • In this sense, foldRight folds items from smaller part to bigger part rather than from right to left ◦ Is quite natural to treat immutable recursive data structure
27. Thanks!