default.nix

 let
   sources = import ./nix/sources.nix;
-  nixpkgs = import sources.nixpkgs {};
-in with nixpkgs;
+in
+
+{ 
+  pkgs ? import sources.nixpkgs {},
+  ghc-master ? sources.ghc-master
+}:
+
+with pkgs;
 
 let
   artefacts = import "${sources.ghc-artefact-nix.outPath}/releases.nix" { baseNixpkgs = pkgs; };
@@ -612,7 +618,12 @@ let
 
 in
   {
-    inherit nixpkgs join cabal-install ghcs ghcs_make ghcs_hadrian sources;
+    inherit pkgs join cabal-install ghcs ghcs_make ghcs_hadrian sources;
+    
+    # Things needed by flake.nix
+    lib = {
+      inherit mapNames underscoredVersion;
+    };
 
     # derivations to be built by CI
     ciDrvs = attrs;

flake.lock

+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1678901627,
+        "narHash": "sha256-U02riOqrKKzwjsxc/400XnElV+UtPUQWpANPlyazjH0=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "93a2b84fc4b70d9e089d029deacc3583435c2ed6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "ghc-master": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1679411953,
+        "narHash": "sha256-dQxVYqgtUKulEU75WTFfopivmaGz+3Cvn9JOI8DHIAM=",
+        "ref": "refs/heads/master",
+        "rev": "be1d4be8d09072091b77cb68ccf234434754af00",
+        "revCount": 60956,
+        "type": "git",
+        "url": "https://gitlab.haskell.org/ghc/ghc.git"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://gitlab.haskell.org/ghc/ghc.git"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1659446231,
+        "narHash": "sha256-hekabNdTdgR/iLsgce5TGWmfIDZ86qjPhxDg/8TlzhE=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "eabc38219184cc3e04a974fe31857d8e0eac098d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-21.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "ghc-master": "ghc-master",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

+{
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-21.11";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+  inputs.ghc-master = {
+    url = "git+https://gitlab.haskell.org/ghc/ghc.git";
+    flake = false;
+  };
+
+  outputs = inputs@{ self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        x = import ./default.nix {
+          inherit pkgs;
+          inherit (inputs) ghc-master;
+        };
+      in {
+        packages = rec {
+          ghc = x.lib.mapNames x.lib.underscoredVersion x.ghcs;
+          ghc-make = x.ghcs_make;
+          ghc-hadrian = x.ghcs_hadrian;
+          cabal-install = x.cabals;
+        };
+
+        apps =
+          let
+            inherit (pkgs) lib;
+            inherit (flake-utils.lib) mkApp;
+
+            ghcAttrs = lib.map (drv: lib.nameValuePair "ghc-${drv.version}" (mkApp {
+              inherit drv;
+              exePath = "bin/ghc";
+            })) self.${system}.packages.ghc;
+
+            cabalAttrs = lib.map (drv: lib.nameValuePair "cabal-install-${drv.version}" (mkApp {
+              inherit drv;
+              exePath = "bin/cabal";
+            })) self.${system}.packages.cabal-install;
+
+          in lib.listToAttrs (
+            ghcAttrs ++ cabalAttrs
+          );
+      }
+    );
+}