Minimalistic example on how to use the Undo Monad Transformer

Question

I was searching the web on tutorials / examples on how to use the Undo Monad Transformer but haven't found any.

Could someone please give a minimalistic example that shows how to use it?

Or pointing out a book where it is explained would be helpful too.

I would like to use it to implement undo/redo operations in a simple editor.

Answer 1

I can't find any tutorials either, but I think I understand this well enough to explain.

We first need to cobble together a type to represent our editors current state, for now will just do

 type EditorState = Int

Then we can give this to our Undo monad

 type Editor = Undo EditorState

Now we can do something like

 fakeUser = do
   put 1 -- Current state is 1
   put 2 -- Current state is 2, 1 is saved in undo buffer
   put 3 -- Current state is 3, 1 and 2 are saved
   undo  -- Move 3 to redo buffer, pop 2 off of undo buffer, current state is 2
   redo  -- reverse of the above, current state is 3

So you can think of this as a cursor in a list*, and we can move back and forth in this list where going left means we "undo" things and going right means we "redo" things. Using put sticks an element into the list at the current point and moves the old element to the undo buffer.

I would recommend not relying on the MonadState instance to do insertions because it hints a bit too much at the internals of Undo and it complicates things if we wanted to track other state through our editor.

However, this is a pretty simple piece of code that you could update to not use MonadState, get rid of the datatype contexts, remove the dependency on -f-glasgow-extensions which is frowned upon and then upload to hackage as a simple library if you have a bit of time.

*The name of such a structure is a "zipper"