nixos-config/home-manager/roles/battery.nix

{
  lib,
  pkgs,
  config,
  ...
}: let
  battery-watch = pkgs.writeHaskellScript
  {
    name = "battery-watch";
    bins = [pkgs.acpi];
    imports = [
      "DBus.Notify"
      "Control.Concurrent"
      "Text.Megaparsec"
      "Text.Megaparsec.Char"
      "Text.Megaparsec.Char.Lexer"
      "Replace.Megaparsec"
      "Data.Maybe"
    ];
  } ''
    moderateLevel = 50 -- percent
    lowLevel = 20 -- percent
    criticalLevel = 8 -- percent
    minute = 60 * 1000 * 1000 -- threadDelay takes microseconds

    main = do
     client <- connectSession
     let loop = \lastState handleMay -> do
          newState <- getState
          let noteMay = chooseAction lastState newState
          handle <- if | Just note <- noteMay -> Just <$> maybe (notify client note) (flip (replace client) note) handleMay
                       | otherwise -> pure handleMay
          threadDelay $ minute `div` 4
          loop newState handle
     loop (BatState True 100) Nothing

    data BatState = BatState { charging :: Bool, level :: Int }

    getState = do
      batteryStateText <- decodeUtf8 <$> (acpi "-a" |> captureTrim)
      batteryLevelText <- decodeUtf8 <$> (acpi "-b" |> captureTrim)
      chargerOnline <- maybe (fail "Couldn‘t get charging state") pure $ parseMaybe onlineParser batteryStateText
      batteryLevel <- maybe (fail "Couldn‘t get battery level") pure $ parseMaybe levelParser batteryLevelText
      pure $ BatState chargerOnline batteryLevel

    type Parser = Parsec Text LText

    onlineParser :: Parser Bool
    onlineParser = not . null . rights <$> sepCap (string "on-line")

    levelParser :: Parser Int
    levelParser = (maybe (fail "No Number found") pure . listToMaybe . rights) =<< sepCap (decimal <* "%")

    chooseAction :: BatState -> BatState -> Maybe Note
    chooseAction (BatState wasCharging lastLevel) (BatState isCharging currentLevel)
      | wasCharging && isCharging = Nothing
      | wasCharging && not isCharging = Just $ myNote{summary = "Charger disconnected." }
      | not wasCharging && isCharging = Just $ myNote{summary = "Charger connected.", expiry = Milliseconds 5000 }
      | currentLevel <= criticalLevel = Just $ myNote{summary = "Battery is very low!" }
      | currentLevel <= lowLevel && currentLevel < lastLevel = Just $ myNote{summary = "Battery is low!"}
      | ((currentLevel `mod` 5 == 0 && currentLevel <= moderateLevel) || (currentLevel `mod` 10 == 0)) && currentLevel < lastLevel = Just $ myNote{summary = "Battery is discharging."}
      | otherwise = Nothing
     where
      myNote = blankNote { body = Just $ Text [i|#{currentLevel}% remaining.|]}
  '';
in {
  systemd.user = {
    services.battery = {
      Unit.Description = "Watch battery state and warn user";
      Service = {
        ExecStart = "${battery-watch}/bin/battery-watch";
        Restart = "always";
        RestartSec = 60;
      };
      Install.WantedBy = ["default.target"];
    };
  };
}