Looking for generic text transformation utility library

[Phil Eaton]

I can't seem to find a way to do simple programmatic transformations
without writing a good portion of code (that I imagine must have been
written before). I want to be able to make a text selection and run
`(map-selection #'prettify)` and have my `prettify` function called with
the selection and the result of `prettify` written back in place of the
selection.

Same thing goes for a hypothetical `map-buffer`, `map-line`, etc.

Is this a (builtin) thing? Or does every package re-implement this
functionality manually? Is there a library that provides these basic
transformation utilities?

Thank you!

-- 
Phil Eaton