fix: add `polars` to `requirements.txt` for `derive_expression` example

[cmdlineluser]

I was just experimenting with the new Plugins API example and needed to make a few edits.

make run was failing as polars was missing from requirements.txt so wasn't being installed into the venv

make clean failed due to a typo in the directory name.

The URL for the derive_example in the README.md needed the branch name adjusted. (currently 404)

[ritchie46]

Thank you. Other than than, were you successful?

[cmdlineluser]

Yes, thanks @ritchie46

It was for a recent "vertical cumfold" question. (similar to https://github.com/pola-rs/polars/issues/7658)

https://stackoverflow.com/questions/77196845/how-to-compute-a-new-polars-column-based-on-its-previous-value

I thought it may be a good opportunity to showcase this new behaviour, so I basically just modified the example you provided: https://github.com/cmdlineluser/polars-plugin-example

It can probably be improved upon with better Rust/Polars-internals knowledge.

Not sure if there is some way to generalize this Iterator::scan() approach for any type.

[ritchie46]

Not sure if there is some way to generalize this Iterator::scan() approach for any type.

No, you need to specialize it. That's why we don't have a good cumulative story in polars. It is hard to do properly in an interpreted way without paying a huge dynamic dispatch and recompute cost.

Deriving specialized expressions for this is indeed perfect.