{
  "$type": "site.standard.document",
  "bskyPostRef": {
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    "uri": "at://did:plc:pi6woz4d47bkuws673w2il2r/app.bsky.feed.post/3mosvnm6253e2"
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  "path": "/t/error-ghc-91510-illegal-polymorphic-type/14304#post_3",
  "publishedAt": "2026-06-21T17:26:03.000Z",
  "site": "https://discourse.haskell.org",
  "tags": [
    "GHC Users’ guide"
  ],
  "textContent": "Also reading the GHC Users’ guide, I think a quantified constraint can only have a concrete class or a constraint variable as the head (not the one that is involved in that same quantified constraint though). In your case the head is `Objects` which is a type family which is not allowed.",
  "title": "Error GHC-91510: illegal polymorphic type"
}