Change battery watcher to Haskell

home/battery.nix

@@ -1,32 +1,70 @@
 { lib, pkgs, config, ... }:
 let
-  battery-watch = pkgs.writeScript "battery-watch" ''
-    #!${pkgs.stdenv.shell}
+  inherit (import ../lib/default.nix) writeHaskellScript;
+  battery-watch = writeHaskellScript {
+    name = "battery-watch";
+    libraries = [
+      pkgs.haskellPackages.fdo-notify
+      pkgs.haskellPackages.megaparsec
+      pkgs.haskellPackages.replace-megaparsec
+    ];
+    bins = [ pkgs.acpi ];
+    imports = [
+      "DBus.Notify"
+      "Control.Concurrent"
+      "Text.Megaparsec"
+      "Text.Megaparsec.Char"
+      "Text.Megaparsec.Char.Lexer"
+      "Replace.Megaparsec"
+      "Data.Maybe"
+    ];
+  } ''
+    criticalLevel = 20 -- percent
+    minutes = 60 * 1000 * 1000 -- threadDelay takes microseconds
 
-    critical_level=20    #percent
+    main = do
+     client <- connectSession
+     let loop = \handleMay -> 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
+          let NextAction { note = noteMay , delay = delay } = chooseAction chargerOnline batteryLevel
+          handle <- if | Just note <- noteMay -> Just <$> maybe (notify client note) (flip (replace client) note) handleMay
+                       | otherwise -> pure handleMay
+          echo ([i|Waiting for #{delay} minutes until next message.|] :: String)
+          threadDelay $ delay * minutes
+          loop handle
+     loop Nothing
 
-    export PATH=$PATH:${pkgs.coreutils}/bin:${pkgs.gnugrep}/bin:${pkgs.gnused}/bin
+    data NextAction = NextAction { note :: Maybe Note, delay :: Int }
 
-    while true
-    do
-        if [ "$(${pkgs.acpi}/bin/acpi -a | grep -o off)" == "off" ]; then
-            battery_level=`${pkgs.acpi}/bin/acpi -b | sed 's/.*[dg], //g;s/\%,.*//g'`
-            if [ $battery_level -le $critical_level ]; then
-              ${pkgs.libnotify}/bin/notify-send 'Battery level is low!' "Only $battery_level% of the charge remains."
-            else
-              ${pkgs.libnotify}/bin/notify-send 'Battery level is discharging!' "Only $battery_level% of the charge remains."
-              sleep 18m
-            fi
-        fi
-        sleep 2m
-    done
+    type Parser = Parsec Text LT.Text
+
+    onlineParser :: Parser Bool
+    onlineParser = not . null . rights <$> sepCap (string "on-line")
+
+    levelParser :: Parser Int
+    levelParser = (maybe (fail "No Number found") pure . listToMaybe . rights) =<< sepCap (decimal <* "%")
+
+    chooseAction :: Bool -> Int -> NextAction
+    chooseAction chargerOnline batteryLevel
+      | chargerOnline = NextAction Nothing 1
+      | batteryLevel <= criticalLevel = mkMsg $ myNote {summary = "Battery is low!"}
+      | otherwise = mkMsg $ myNote {summary = "Battery is discharging!"}
+     where
+      mkMsg = flip NextAction (max 1 (batteryLevel `div` 5)) . Just
+      myNote = blankNote { body = Just $ Text [i|Only #{batteryLevel}% remaining.|]}
   '';
 in {
 
   systemd.user = {
     services.battery = {
       Unit = { Description = "Watch battery state and warn user"; };
-      Service = { ExecStart = toString battery-watch; };
+      Service = {
+        ExecStart = "${battery-watch}/bin/battery-watch";
+        Restart = "always";
+      };
       Install = { WantedBy = [ "graphical-session.target" ]; };
     };
   };

lib/default.nix

@@ -55,6 +55,7 @@ rec {
       {-# LANGUAGE QuasiQuotes #-}
       {-# LANGUAGE OverloadedStrings #-}
       {-# LANGUAGE ExtendedDefaultRules #-}
+      {-# LANGUAGE MultiWayIf #-}
 
       import Shh
       import qualified Prelude