feature: <expr> mappings

[tmillr]

Did you check the docs?

• [x] I have read all the which-key.nvim docs

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

Regular vim/nvim supports expression mappings. For one, they can be useful with conditional mappings (e.g. noremap back to the default keys/action depending upon some runtime condition, otherwise do a different action/keys). Providing this ability would make it so that you don't have to use :normal or feedkeys() within the function (i.e. the fn passed as part of the mapping definition) as a workaround.

Describe the solution you'd like

Allow passing expr = true in mapping definition, in which case the string returned by the fn at runtime becomes the result of the mapping.

Describe alternatives you've considered

using :normal or nvim_feedkeys() inside the lua fn

I suppose you could also resort to a vimscript command string mapping (using <Cmd>{logic}<CR>), but it'd be nicer to be able to write the all of the logic within an/the actual lua function instead (where syntax hl and lsp works, formatting works, you can write multiple lines easily, etc.), returning only the string/keys as the result. This probably still requires the use of :normal or feedkeys as well. Plus, v:lua.* within the mapping string can only be used to call global or required functions.

TLDR: providing this feature would provide a cleaner, nicer and more straightforward solution, vim.keymap.set() allows passing expr = true

Additional context

No response

[tmillr]

I'm not sure if which-key already allows passing expr = true? (which seems to be the case)

If so, it doesn't appear to be documented along with the other options.

[folke]

That's already supported

[tmillr]

Oh ok, I guess it's just not documented then

[thomastthai]

For anyone coming from the future, expr = true support is unofficially documented in issue #67.