The revision of Nixpkgs you pinned your project at is fairly old and doesn’t actually include GHC 8.6.5, only 8.6.4. Also IIRC all-hies uses a Nixpkgs pin per compiler version so perhaps you picked a wrong revision?
The revision of Nixpkgs you pinned your project at is fairly old and doesn’t actually include GHC 8.6.5, only 8.6.4. Also IIRC all-hies uses a Nixpkgs pin per compiler version so perhaps you picked a wrong revision?