Make sure the hackageTarball's store path doesn't change

previously the fetchurl would produce a different store path each
and every time as hackage's index is a moving target.  With this
impurity setup, we can ignore this.

Author: angerman

default.nix

@@ -127,11 +127,21 @@ let
     # Make this handy overridable fetch function available.
     inherit fetchExternal;
 
+    # Produce a fixed output derivation from a moving target (hackage index tarball)
     hackageTarball = { index-state, sha256 }:
-      let index = builtins.fetchurl https://hackage.haskell.org/01-index.tar.gz;
-      in pkgs.runCommand "01-index.tar.gz-at-${builtins.replaceStrings [":"] [""] index-state}" { outputHashAlgo = "sha256"; outputHash = sha256; }
+      pkgs.runCommand "01-index.tar.gz-at-${builtins.replaceStrings [":"] [""] index-state}" {
+        nativeBuildInputs = [ pkgs.curl ];
+        outputHashAlgo = "sha256";
+        outputHash = sha256;
+        # We'll use fetchurl's result in an env var ...
+        HACKAGE_INDEX = builtins.fetchurl "https://hackage.haskell.org/01-index.tar.gz";
+        # ... and mark that impure.  That way we can
+        # ensure the sore path stays the same and doesn't
+        # depend on the fetchurl result path.
+        impureEnvVars = [ "HACKAGE_INDEX" ];
+      }
       ''
-      ${self.nix-tools}/bin/truncate-index -o $out -i ${index} -s ${index-state}
+      ${self.nix-tools}/bin/truncate-index -o $out -i $HACKAGE_INDEX -s ${index-state}
       '';
 
     mkLocalHackageRepo = import ./mk-local-hackage-repo { inherit (self) hackageTarball; inherit pkgs; };