Autocompletion in REPL in multiline mode hangs Emacs

[mathlover2]

I like to work in multiline mode with auto-complete, and have noticed the following bug: when I press TAB to complete a word, the entire Emacs session hangs. I have not found anything that mitigates the issue, short of restarting Emacs.

[gracjan]

What is multiline mode?

[mathlover2]

Multiline mode is a GHCi mode in which one can insert statements over multiple lines. It is activated by using ":set +m" in the GHCi prompt.

[gracjan]

Oh boy. I do not think anybody thought about mixing this one... Note that interactive-haskell-mode interacts with GHCi as a subprocess when inferior-haskell-mode interacts wih GHCi as a subshell (with terminal capabilities). I guess this might be the reason multiline does not work as it should.

[geraldus]

I did some research.

• start a new session for some haskell project
• now REPL completions is available when editing Haskell files
• go to REPL buffer
• enter multi-line editing mode: type :{ hit RET
• go back to Haskell file you edited before
• now completions is not available

Likely, all this is because when REPL in multi-line mode doesn't evaluate received commands and doesn't produce output, that is it's waiting :} to leave multi-line mode. And this is a pitfall.

I see few ways for now:

1. do not complete things when REPL is in multi-line edit mode
2. implement timeout argument for async REPL calls
3. split REPL somehow into several process threads, one for direct user interaction and another for internal purposes
4. implement another way for multi-line editing.

Options 1-3 do not seem good to me. As for last option, first I want to mention few things about multi-line edit in GHCi itself:

• when you entered a single line it is not possible to change it, this is pain if you tend to write 10+ lines of code and made typo somewhere (in this case you have to re-type everything again)
• TAB-completions work in multi-line mode in GHCi

Here is my thoughts: when user hits RET examine input to determine that user is tended to enter multi-line editing mode. Open special temporal buffer where user can enter the code, allowing to easily navigate across lines and changing them; bind C-c C-c to finish editing, C-c C-k to cancel, and optionally something else to test input (e.g. send buffer contents to REPL and notify is it OK or not). Optionally enable syntax highlight and completions in this buffer.
Another less comfortable way would be: change prompt to indicate multi-line editing mode but do not send :{ to REPL, rather start collecting lines that user type in some session variable. When user enters newline do not send it to REPL immediately, rather collect it. When user finished (e.g. entered :}) send everything that was collected to REPL.

[gracjan]

Hmm, so basically haskell-mode would need to track if GHCi is currently in mulitline mode by checking when it enters multiline mode and when it exits multiline mode.

This is too much to ask for.

Solution for now is:

• do not use multiline mode with GHCi
• start using haskell-ide-engine when it is ready

I'm sorry but we have too much work in other places of haskell-mode to support multiline mode.

[geraldus]

@gracjan let's keep this open as a reminder. I have many thoughts about interactive Haskell improvements (and going to spend some time on it).

[geraldus]

Just add some labels such as milestone infinity or won't be fixed in near future.

[gracjan]

We can reopen anytime anybody has something useful to add. Resolution for now is that we aren't going to implement this, sadly it is too much.