{
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  "path": "/t/haskells-missing-mutable-reference-type/14248#post_6",
  "publishedAt": "2026-06-09T12:39:49.000Z",
  "site": "https://discourse.haskell.org",
  "textContent": "Oops, i did misread the example slightly sorry ^^ You’re using the `IOScopedRef` to implement `modifySeverity` not to modify the `Logger` value directly.\n\nMy point still stands though. Everything that (indirectly) uses this `IOScopedRef` needs to go through the `logger` variable anyway so you might as well use it to carry the state in a lexically scoped way.\n\n> OK, then tell me how I could obtain the same behaviour using a lexically-scoped variable but without `IOScopedRef`.\n\n\n    data Logger = Logger {\n        logImpl :: Severity -> String -> IO (),\n        baseSeverity :: Severity\n    }\n\n    logMsg :: Logger -> Severity -> String -> IO ()\n    logMsg logger severity msg = logger.logImpl (logger.baseSeverity + severity) msg\n\n    newLogger :: Severity -> Logger\n    newLogger baseSeverity = Logger { baseSeverity, logImpl = \\severity message -> putStrLn ... }\n\n    modifySeverity :: Logger -> (Severity -> Severity) -> Logger\n    modifySeverity logger f = logger { baseSeverity = f logger.baseSeverity }\n\n    loggerExample :: IO ()\n    loggerExample = do\n      let logger = newLogger 0\n      logMsg logger 1 \"Getting user\"\n      user <- getUser\n      logMsg logger 1 (\"Is VIP: \" <> show (isVip user))\n      let modification = if isVip user then (+ 10) else id\n      d <- do\n        logger <- pure (modifySeverity logger modification)\n        logMsg logger 0 \"Getting data\"\n        getData user\n      writeData d\n      logMsg logger 0 \"Done\"\n",
  "title": "Haskell's missing mutable reference type"
}