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  "path": "/t/haskells-missing-mutable-reference-type/14248#post_20",
  "publishedAt": "2026-06-10T08:14:07.000Z",
  "site": "https://discourse.haskell.org",
  "textContent": "tomjaguarpaw:\n\n> Well, I explained how I can get my API from yours.\n\nYes, and I explained that I have no idea what “reading from the `vault`” would look like. I’m essentially getting\n\n\n    contextual vault :: Contextual Vault\n\n    foo :: IO ()\n    foo = do\n      a <- _ vault\n      print a\n\n    ---\n\n    main =\n      withScopedIORef vault 5 $ \\x ->\n        overriding x (+2) $\n          foo\n\n\nwith the questions:\n\n  * How is `foo` expected to access `x` without breaking type safety? Passing a key is not an answer, that’s still data.\n\n  * What happens if `main = foo`?\n\n\n\n\nI guess you could solve both of these if `IO` is an effect that carries `x` at the type level, but that would require proper type-level programming implemented first.",
  "title": "Haskell's missing mutable reference type"
}