Wingman: Using the completion system for case analysis

[kalhauge]

Is your feature request related to a problem? Please describe.

When I write code I often write it like this:

foldMaybe :: Monoid m => Maybe m -> m
foldMaybe m = case m of 
   -- hmm.. what was the cases for Maybe again...

Right now I have to,

1. stop what I'm doing,
2. click on the code lens hint,
3. and apply the change,
4. navigate to the first _ and start writing.

Describe the solution you'd like

It would be really nice if we could somehow "TAB" (pun intended) into the lsp completion/snippet system,
and when I have written the of I can tap complete and be presented with a case expansion,
(and possibly a 'hylo'-morphic case expansion and a monadic case expansion):

foldMaybe :: Monoid m => Maybe m -> m
foldMaybe m = case m of<TAB>(choose case)
   Just a -> _
   Nothing -> _

mapMaybe :: (a -> b) -> Maybe a -> Maybe b
mapMaybe  fn m  = case m of<TAB>(choose hylo)
   Just a -> Just _
   Nothing -> Nothing

mapMaybeM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
mapMaybeM m = case m of<TAB>(choose "do" hylo)
   Just a -> do 
    pure $ Just _
   Nothing -> do 
    pure $ Nothing

This suggestion is only for the case analysis part, it might however also work for refinements. If you
are on a "_" and press tab, it could suggest a refined solution if it finds one in less than 0.x seconds, suggest it, otherwise it might suggest one of the other transformations.

Describe alternatives you've considered

Wingman works great as is, but this would really be a "killer" feature.

Additional context

There seem to be some kind of support for snippets in the LSP protocol, but I don't know how well it is integrated in the different editors (and in the haskell-language-server).

[pepeiborra]

You could create key bindings to trigger code lenses, depending on your editor, if that's what bothers you.

But I agree that having a Wingman powered completion provider would be awesome.

[pepeiborra]

Would you be interested in contributing a PR? I'm sure @isovector could provide some starting tips

[kalhauge]

@pepeiborra I do have some time over the Christmas break, but I have no Idea about where to start. I'm pretty familiar to Haskell, but new to both the HLS code-base and to LSP.

[isovector]

This sounds like a great idea. It looks like we can use the command? property in CompletionItem to connect to. Might be as easy as running the code lens action?

[kalhauge]

It seems like there is an example setup (sadly, not using the _comand option) use in plugins/hls-pragmas-plugin/src/Ide/Plugin/Pragmas.hs:L212 which might be usefull?

[michaelpj]

The ideal way would be to actually return the text to be inserted as part of the completion item edit, since then you could indeed send it as a snippet, which would potentially give a nicer UX. I guess computing the edit is expensive, but since we'd presumably only do this completion after an of token, it shouldn't be too hard to mostly avoid doing the work?

[kalhauge]

The ideal way would be to actually return the text to be inserted as part of the completion item edit, since then you could indeed send it as a snippet, which would potentially give a nicer UX. I guess computing the edit is expensive, but since we'd presumably only do this completion after an of token, it shouldn't be too hard to mostly avoid doing the work?

This is exactly something I think would be really useful. Also I think we could expand it to also work for _. Seeing the snippet before applying it, could make the user more willing to experiment with different tactics?