Eval plugin: output difff when there's trailing newline

[jakio6]

Your environment

Which OS do you use?
Linux

Which version of GHC do you use and how did you install it?
9.8.2, ghcup

How is your project built (alternative: link to the project)?
single file

Which LSP client (editor/plugin) do you use?
neovim builtin

Which version of HLS do you use and how did you install it?
2.7.0.0, ghcup

Have you configured HLS in any way (especially: a hie.yaml file)?
no

Steps to reproduce

Evaluate twice

-- |
-- >>> import Prettyprinter
--
-- >>> pretty "foo"
--
-- >>> pretty "foo\n"
--
-- >>> pretty "foo\n\n"
--

Expected behaviour

No diff for the last two test.

Actual behaviour

First evaluate:

-- |
-- >>> import Prettyprinter
--
-- >>> pretty "foo"
-- foo
--
-- >>> pretty "foo\n"
-- foo
--
-- >>> pretty "foo\n\n"
-- foo
-- <BLANKLINE>
--

Second evaluate:

-- |
-- >>> import Prettyprinter
--
-- >>> pretty "foo"
-- foo
--
-- >>> pretty "foo\n"
-- WAS foo
-- NOW foo
--
-- >>> pretty "foo\n\n"
-- WAS foo
-- WAS <BLANKLINE>
-- NOW foo
-- <BLANKLINE>
--

Debug information

[fendor]

Hi, thank you for the bug report!

I feel like the reason why this is difficult to do correctly is that it is impossible to tell when the expected output ends... We would need a terminating character, or another identifier for multi line outputs. Or, we might want to forbid multi-line outputs altogether.

Maybe we can take inspiration from cabal-docspec and see how it behaves in this situation?

[jakio6]

Append an extra <BLANKLINE> to the last two case would be enough to pass doctest.