Refactor getLine to return TextLines/Rope

[jhrcek]

As @michaelpj pointed out, for many use-cases it might be preferable if getLine stays within the world of Ropes, to allow for further processing (an example of this is in lsp and similar, currently incorrect usage of getLine-like functionality mentioned in this comment in hls: haskell/haskell-language-server#4303 (comment)

The initial impl. ini this PR might not be ideal, because its complexity is linear in the length of line being wrapped. But let me open a PR to get initial feedback.

[Bodigrim]

It feels a bit strange when lines gives [Text], but getLine returns TextLines / Rope. Does it save much on HLS side? One can apply fromText there, probably to the same effect.

[michaelpj]

That would be fine I guess. Bascially we need the mixed-encoding indexing that we get from Text.Rope.Mixed! It feels a little strange to be converting back to Text and then back to Rope, but maybe that's fine?

[Bodigrim]

If we want to return Rope, can we avoid firstLineRope and stick to firstLineRope? Otherwise we are doing a double conversion.

[michaelpj]

I guess the neatest alternative would be to provide some utilities for mixed-encoding indexing of Text itself, on the reasoning that it should be okay on the Texts we get from getLine, since they are likely to be short. WDYT @Bodigrim ?

[Bodigrim]

After thinking about it, I'm largely fine with both options:

• We keep functions as they were before this PR, TextLines -> Text, Rope -> Text, and HLS wraps results in fromText to get access to encoding-based indexing. The only downside I see is slight inefficiency: e. g., a hypothetical getLine :: Int -> TextLines -> TextLines knows that the result contains no newlines, while with fromText . (getLine :: Int -> TextLines -> Text) we would have scan Text once again. Not a big deal for the sort of strings HLS usually operates on.
• We change function signatures as in this PR. I'm fine to merge it, if HLS finds it useful. The only requirement is to be efficient and avoid double conversion. E. g., getLine :: Int -> Rope -> Rope should not convert intermediate Rope to Text only to execute fromText later.

[jhrcek]

I'll get to it over weekend. Will probably go with option 2 (keep Rope/TextLines).
Regarding avoiding double conversion I'll just have to figure out how to drop the last \n from the Rope without converting it to Text (or alternatively throw away/change the nice looking property that Rope.getLine i r == Text.lines (Rope.toText r) !! i. Suggestions are welcome 😄