5 ms·

The fundamental feature that makes such an example easy is first-class functions. You are simply using objects to encode this and not making any use of their id

The fundamental feature that makes such an example easy is first-class functions. You are simply using objects to encode this and not making any use of their identify and mutable state.

jcelerier 9y ago
> You are simply using objects to encode this and not making any use of their identify and mutable state. Most software with undo-redo will show you some metadata related to the action (for instance "Undo 'Drag stuff'", "Redo 'Set text in bold'"...) so you have to have a way to associate metadata with the "undo / redo" functions; the class is a good tool for this. Also, a common optimization is to alter the head of the stack instead of deleting / creating new commands when for instance you are moving an object in the GUI. e.g. say you have a command that moves a box at a specific position. If command objects are immutable, every time the mouse moves, you have to pop the head of your stack and push a new command instead (which may be a waste of resources); instead you can just have an "update" method which will change the position stored inside the command which is microscopic in comparison of malloc / free.
- icebraining 9y ago
  You can do that optimization in FP languages if you have unique types, ie, types that can only have one reference to them. You use the list as if it's immutable, by creating a new list with the new command and the rest of the previous list. But since the compiler can prove that the old list is no longer referenced, it can reuse it and simply make the update in-place.
  - jcelerier 9y ago
    could you show an actual example where commands are polymorphic types (e.g. they come from a separate DLL) and this optimization actually happens ? (the actual "updating" of the command doesn't need to cross DLL boundaries, eg a DLL both provides a command, instantiates it, and updates it. The command queue is instantiated by another DLL though).
    - tome 9y ago
      I don't know what icebraining is talking about becaue AFAIK there aren't widely used languages with unique types, except perhaps Rust.
- tome 9y ago
  > If command objects are immutable, every time the mouse moves, you have to pop the head of your stack and push a new command instead (which may be a waste of resources) Have you profiled that?! :)
  - jcelerier 9y ago
    yes, actually, it used to eat 30% of the CPU on a software I worked on because of malloc-fest when moving the mouse quickly across the screen and dragging multiple objects.

The fundamental feature that makes such an example easy is first-class functions. You are simply using objects to encode this and not making any use of their identify and mutable state.

jcelerier 9y ago

> You are simply using objects to encode this and not making any use of their identify and mutable state. Most software with undo-redo will show you some metadata related to the action (for instance "Undo 'Drag stuff'", "Redo 'Set text in bold'"...) so you have to have a way to associate metadata with the "undo / redo" functions; the class is a good tool for this. Also, a common optimization is to alter the head of the stack instead of deleting / creating new commands when for instance you are moving an object in the GUI. e.g. say you have a command that moves a box at a specific position. If command objects are immutable, every time the mouse moves, you have to pop the head of your stack and push a new command instead (which may be a waste of resources); instead you can just have an "update" method which will change the position stored inside the command which is microscopic in comparison of malloc / free.

icebraining 9y ago
You can do that optimization in FP languages if you have unique types, ie, types that can only have one reference to them. You use the list as if it's immutable, by creating a new list with the new command and the rest of the previous list. But since the compiler can prove that the old list is no longer referenced, it can reuse it and simply make the update in-place.
- jcelerier 9y ago
  could you show an actual example where commands are polymorphic types (e.g. they come from a separate DLL) and this optimization actually happens ? (the actual "updating" of the command doesn't need to cross DLL boundaries, eg a DLL both provides a command, instantiates it, and updates it. The command queue is instantiated by another DLL though).
  - tome 9y ago
    I don't know what icebraining is talking about becaue AFAIK there aren't widely used languages with unique types, except perhaps Rust.
tome 9y ago
> If command objects are immutable, every time the mouse moves, you have to pop the head of your stack and push a new command instead (which may be a waste of resources) Have you profiled that?! :)
- jcelerier 9y ago
  yes, actually, it used to eat 30% of the CPU on a software I worked on because of malloc-fest when moving the mouse quickly across the screen and dragging multiple objects.