The document discusses implementing undo and redo functionality in web applications using promises and the command pattern. It presents a use case of a user wanting to undo price changes without remembering previous prices. It proposes using a double stack and promises to handle asynchronous actions and allow any functions to be undone or redone by providing inverse functions. This approach aims to provide reusable and composable undo/redo capabilities for web apps.

1. Temporal Composability
Undo / Redo in angular
with promises
and the command pattern
Image by Gossins: Flow of Time

2. Undo / Redo
 Native apps (including browser) support undo
Web apps and Mobile apps do not
 Power users produce more errors than consumers
 Power users on a computer with native app
 What happens when a power user uses a web app?

3. Power User

4. Use Case
 I want to be able to build a collection
 I want to be able to undo a price change without
having to remember which price I destroyed.

5. Sets all items to 222% margin

6. function
x f(x)

7. invertable
x f(x)
f-1(f(x))

8. State Machine
Dual Transitions

9. Command Pattern

10. Double Stack

11. Double
Stack
 asdf

12. Discontinuity
 Creating a collection
 ID is returned asynchronously as a promise
 ID not specified until after creation but needed for
deletion

13. Promises and double closure
*Each function has the result of its inverse as input
msg
Results Promise
Context
Arguments
Function
Results
Promise
inverse
msg

14. Promises

15. Reusablility & Composiblity
 F -> $q(f) - Function that generates a promise
(reuse allows for unlimited undo/redo)
 Asynchronous actions are handled (composition)
 Anything can undo/redo if providing 2 functions
 User messaging for current state of undo/redo

16. Answers & Questions