feat(users/Profpatsch): init haskell arglib-netencode

Mirrors the rust exec-helpers

Change-Id: I96f211d6545e228b8ae0c1dce4e60e6f6867690c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6117
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/arglib/ArglibNetencode.hs

@@ -0,0 +1,25 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE QuasiQuotes #-}
+
+module ArglibNetencode where
+
+import qualified Data.Attoparsec.ByteString as Atto
+import Data.Maybe (fromMaybe)
+import ExecHelpers
+import MyPrelude
+import Netencode
+import qualified System.Environment as Env
+import qualified System.Posix.Env.ByteString as ByteEnv
+
+arglibNetencode :: CurrentProgramName -> Maybe Text -> IO T
+arglibNetencode progName mEnvvar = do
+  let envvar = mEnvvar & fromMaybe "ARGLIB_NETENCODE" & textToBytesUtf8
+  ByteEnv.getEnv envvar >>= \case
+    Nothing -> dieUserError progName [fmt|could not read args, envvar {envvar} not set|]
+    Just bytes ->
+      case Atto.parseOnly (Netencode.netencodeParser <* Atto.endOfInput) bytes of
+        Left err -> dieEnvironmentProblem progName [fmt|arglib parsing error: {err}|]
+        Right t -> do
+          ByteEnv.unsetEnv envvar
+          pure t