How can I edit pure text file stored in iCloud Drive from iPhone?
I use Textastic for basic coding on my iPad but it is also available for iPhone. However, it is $8.99 so you will have to judge if it is worth it. Most coding apps will work with straight text files without altering them with formatting. I also see one called Fast Keyboard on the iTunes store that is free if you just need to edit a text file once in a while and re-sync it with Dropbox or something.