Use nixpkgs upstream expression

nix/arion.nix

@@ -1,42 +1,12 @@
+# Like the upstreamable expression but wired up for the local arion.
 { pkgs ? import ./. {}
 , lib ? pkgs.lib
+, haskell ? pkgs.haskell
 , haskellPackages ? pkgs.haskellPackages
 , arion-compose ? import ./haskell-arion-compose.nix { inherit pkgs haskellPackages; }
+, runCommand ? pkgs.runCommand
 }:
-
-let
-  inherit (pkgs.haskell.lib) justStaticExecutables overrideCabal;
-
-  srcDir = ../src;
-  eval = import (srcDir + "/nix/eval-composition.nix");
-  build = args@{...}:
-    let composition = eval args;
-    in composition.config.out.dockerComposeYaml;
-
-in
-  justStaticExecutables (overrideCabal arion-compose (o: {
-    buildTools = o.buildTools ++ [pkgs.makeWrapper];
-    passthru = o.passthru // {
-      inherit eval build;
-    };
-    pname = "arion"; # Cover up the needlessly long Haskell package name
-
-    # PYTHONPATH
-    #
-    # We close off the python module search path!
-    #
-    # Accepting directories from the environment into the search path
-    # tends to break things. Docker Compose does not have a plugin
-    # system as far as I can tell, so I don't expect this to break a
-    # feature, but rather to make the program more robustly self-
-    # contained.
-
-    postInstall = ''${o.postInstall or ""}
-      mkdir -p $out/libexec
-      mv $out/bin/arion $out/libexec
-      makeWrapper $out/libexec/arion $out/bin/arion \
-        --unset PYTHONPATH \
-        --prefix PATH : ${lib.makeBinPath [ pkgs.docker-compose ]} \
-        ;
-    '';
-  }))
+import ./upstreamable/default.nix {
+  inherit pkgs lib haskell runCommand;
+  haskellPackages = haskellPackages // { inherit arion-compose; };
+}

nix/upstreamable/default.nix

@@ -0,0 +1,89 @@
+{ pkgs
+, lib
+, haskellPackages
+, haskell
+, runCommand
+}:
+
+let
+
+  /* This derivation builds the arion tool.
+
+     It is based on the arion-compose Haskell package, but adapted and extended to
+       - have the correct name
+       - have a smaller closure size
+       - have functions to use Arion from inside Nix: arion.eval and arion.build
+       - make it self-contained by including docker-compose
+   */
+  arion =
+    justStaticExecutables (
+      overrideCabal
+        arion-compose
+        cabalOverrides
+      );
+
+  inherit (haskell.lib) justStaticExecutables overrideCabal;
+
+  inherit (haskellPackages) arion-compose;
+
+  cabalOverrides = o: {
+    buildTools = (o.buildTools or []) ++ [pkgs.makeWrapper];
+    passthru = (o.passthru or {}) // {
+      inherit eval build;
+    };
+    # Patch away the arion-compose name. Unlike the Haskell library, the program
+    # is called arion (arion was already taken on hackage).
+    pname = "arion";
+    src = arion-compose.src;
+
+    # PYTHONPATH
+    #
+    # We close off the python module search path!
+    #
+    # Accepting directories from the environment into the search path
+    # tends to break things. Docker Compose does not have a plugin
+    # system as far as I can tell, so I don't expect this to break a
+    # feature, but rather to make the program more robustly self-
+    # contained.
+
+    postInstall = ''${o.postInstall or ""}
+      mkdir -p $out/libexec
+      mv $out/bin/arion $out/libexec
+      makeWrapper $out/libexec/arion $out/bin/arion \
+        --unset PYTHONPATH \
+        --prefix PATH : ${lib.makeBinPath [ pkgs.docker-compose ]} \
+        ;
+    '';
+  };
+
+  # Unpacked sources for evaluation by `eval`
+  srcUnpacked = pkgs.stdenv.mkDerivation {
+    name = "arion-src";
+    inherit (arion-compose) src;
+    buildPhase = ''
+      cp -r $src $out
+    '';
+    installPhase = "";
+  };
+
+  /* Function for evaluating a composition
+
+     Re-uses this Nixpkgs evaluation instead of `arion-pkgs.nix`.
+
+     Returns the module system's `config` and `options` variables.
+   */
+  eval = args@{...}:
+    import (srcUnpacked + "/src/nix/eval-composition.nix")
+      ({ inherit pkgs; } // args);
+
+  /* Function to derivation of the docker compose yaml file
+     NOTE: The output will change: https://github.com/hercules-ci/arion/issues/82
+
+    This function is particularly useful on CI, although the references
+    to image tarballs may not always be desirable.
+   */
+  build = args@{...}:
+    let composition = eval args;
+    in composition.config.out.dockerComposeYaml;
+
+in arion