Problem:
POSIX-compatible Ex-mode requires special-cases all over the codebase to
match various quirks that don't actually matter to users.
- The main utility of *interactive* Ex-mode is its REPL behavior, and
that can be achieved with `cmdwin`, which also gains extra UX
benefits.
- The main utility of *non-interactive* `nvim -es` is for shell
scripting, where Ex-mode quirks are mostly unhelpful (e.g. the
"Entering Ex mode" message).
Solution:
- Reimplement *interactive* Ex-mode as a "persistent, insert-mode
cmdwin" in Lua.
- "nvim -e/-E" is simply an alias to "gQ".
- Reframe *non-interactive* Ex-mode (`nvim -es`) as "script mode".
- Drop POSIX Ex-mode quirks.
Improvements:
- "nvim -V1 -es" output ends with a final newline!
- "nvim -V1 -es" no longer shows the "Entering Ex mode" msg. (This was
pointless noise, unwanted for scripting purposes.)
- stdin is no longer typeahead. Scripts (":lua io.read()") can read
stdin as data.
- Empty line is a no-op: a stray blank line no longer moves the cursor
(deviates from POSIX ex "+1"), no longer exits 1 at EOF (E501).
Preserved behavior:
- cursor starts at "$"
- mode()=="cv" (for non-interactive)
- multiline commands (:append/:function/heredoc pull continuation lines)
- bare-range print
- :print=>stdout
- -V1=>stderr
- CRLF input
- continue-after-error and exit codes
Dropped (regressed) POSIX behavior (non-interactive):
- Event loop only ticks while/between commands, not while blocked
waiting for a stdin line.
- ":g/pat/visual...Q"
- input()/getchar()/":s/x/y/c" no longer consume stdin lines as
answers: Nvim stops at end-of-input, skipping the rest of the script,
exit 0. Use ":lua io.read()" instead.
- If users care about this they should use interactive Ex-mode (`gQ`).
- ":@r" stops at end of the register instead of continuing to read
cmdline input from stdin.
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