Simple Vi mode with Karabiner-elements and Hammerspoon

init.lua

local module = {}

module.debugging = false -- whether to print status updates

local eventtap = require "hs.eventtap"
local event    = eventtap.event
local inspect  = require "hs.inspect"

local keyHandler = function(e)
    local watchFor = {
            h = "left",
            j = "down",
            k = "up",
            l = "right",
            u = "delete",
            i = "forwarddelete"
        }
    local actualKey = e:getCharacters(true)
    local replacement = watchFor[actualKey:lower()]
    if replacement then
        local isDown = e:getType() == event.types.keyDown
        local flags  = {}
        for k, v in pairs(e:getFlags()) do
            if v and k ~= "fn" then -- fn will be down because that's our "wrapper", so ignore it
                table.insert(flags, k)
            end
        end
        if module.debugging then print("viKeys: " .. replacement, inspect(flags), isDown) end
        local replacementEvent = event.newKeyEvent(flags, replacement, isDown)
        if isDown then
            -- allow for auto-repeat
            replacementEvent:setProperty(event.properties.keyboardEventAutorepeat, e:getProperty(event.properties.keyboardEventAutorepeat))
        end
        return true, { replacementEvent }
    else
        return false -- do nothing to the event, just pass it along
    end
end

local modifierHandler = function(e)
    local flags = e:getFlags()
    local onlyControlPressed = false
    for k, v in pairs(flags) do
        onlyControlPressed = v and k == "fn"
        if not onlyControlPressed then break end
    end
    -- you must tap and hold fn by itself to turn this on
    if onlyControlPressed and not module.keyListener then
        if module.debugging then print("viKeys: keyhandler on") end
        module.keyListener = eventtap.new({ event.types.keyDown, event.types.keyUp }, keyHandler):start()
    -- however, adding additional modifiers afterwards is ok... its only when fn isn't down that we switch back off
    elseif not flags.fn and module.keyListener then
        if module.debugging then print("viKeys: keyhandler off") end
        module.keyListener:stop()
        module.keyListener = nil
    end
    return false
end

module.modifierListener = eventtap.new({ event.types.flagsChanged }, modifierHandler)

module.start = function()
    module.modifierListener:start()
end

module.stop = function()
    if module.keyListener then
        module.keyListener:stop()
        module.keyListener = nil
    end
    module.modifierListener:stop()
end

module.start() -- autostart

return module

karabiner.json

{
    "profiles": [{
        "devices": [],
        "name": "Default profile",
        "selected": true,
        "simple_modifications": {
            "caps_lock": "fn"
        },
        "standalone_keys": {
            "caps_lock": "caps_lock"
        },
        "one_to_many_mappings": {},
        "virtual_hid_keyboard": {
            "caps_lock_delay_milliseconds": 0,
            "keyboard_type": "ansi",
            "standalone_keys_delay_milliseconds": 100
        }
    }]
}