hammerspoon_remapper.lua

local Remapper = {}

function Remapper:new()
  local remapper = {}

  setmetatable(remapper, self)
  self.__index = self

  remapper.keyMaps = {}
  remapper.eventTap = remapper:_createEventTap()

  return remapper
end

function Remapper:map(mapModifiers, mapKey)
  local remapper = self

  return {
    to = function(destinationModifiers, destinationKey)
      if not remapper.keyMaps[mapKey] then
        remapper.keyMaps[mapKey] = {}
      end

      local modifiersMatchEvent = function(event)
        return event:getFlags():containExactly(mapModifiers)
      end

      table.insert(
        remapper.keyMaps[mapKey],
        {
          modifiersMatchEvent = modifiersMatchEvent,
          destination = {
            key = destinationKey,
            modifiers = destinationModifiers
          }
        }
      )
    end
  }
end

function Remapper:_findRemapByEvent(event)
  local keyCode = event:getKeyCode()
  local keyPressed = hs.keycodes.map[keyCode]
  local maps = self.keyMaps[keyPressed]

  if not maps then return nil end

  for _, remap in ipairs(maps) do
    if remap.modifiersMatchEvent(event) then
      return remap
    end
  end

  return nil
end

function Remapper:_createEventTap()
  local tap = hs.eventtap.new(
    { hs.eventtap.event.types.keyDown },
    self:_getKeyEventHandler()
  )

  tap:start()

  return tap
end

function Remapper:_getKeyEventHandler()
  return function(event)
    local remap = self:_findRemapByEvent(event)

    if remap then
      local destinationKey = remap.destination.key
      local destinationModifiers = remap.destination.modifiers

      return true, {
        hs.eventtap.event.newKeyEvent(destinationModifiers, destinationKey, true), -- key down
        hs.eventtap.event.newKeyEvent(destinationModifiers, destinationKey, false) -- key up
      }
    end

    return false
  end
end

local remapper = Remapper:new()
remapper:map({'alt'}, 'r').to({'cmd'}, 'space')