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  "path": "/t/error-ghc-91510-illegal-polymorphic-type/14304#post_11",
  "publishedAt": "2026-06-23T12:16:39.000Z",
  "site": "https://discourse.haskell.org",
  "textContent": "Mmm, indeed—the change to `type Objects c :: k -> Constraint` seems to be necessary for GHC 9.14 to handle the quantified constraint, though I don’t really know why.\n\nUnrelatedly, you lost the precondition on `type Objects (Transform c d)`; `CCompose (Objects d)` is too strong. Call me crazy, but how about:\n\n\n    class (Category c, Category d, Objects c ==> ObjectClass d . f)\n       => Functor c d f where\n      map :: a `c` b -> f a `d` f b\n\n    instance Category (Transform c d) where\n      type Objects (Transform c d) = Functor c d\n      ...\n",
  "title": "Error GHC-91510: illegal polymorphic type"
}