Cache cabal2nix expr to avoid IFD

nix/build-haskell-package.nix

@@ -27,17 +27,37 @@ let
       name = "${name}";
       inherit src;
     };
+
+  callPackageKeepDeriver = src: args:
+    pkgs.haskell.lib.compose.overrideCabal
+      (orig: {
+        passthru = orig.passthru or { } // {
+          # When using callCabal2nix or callHackage, it is often useful
+          # to debug a failure by inspecting the Nix expression
+          # generated by cabal2nix. This can be accessed via this
+          # cabal2nixDeriver field.
+          cabal2nixDeriver = src;
+        };
+      })
+      (self.callPackage src args);
+
+  callCabal2nixWithOptions = name: src: expr: args:
+    pkgs.haskell.lib.compose.overrideCabal
+      (orig: {
+        inherit src;
+      })
+      (callPackageKeepDeriver expr args);
 in
 
-name: root:
+name: root: expr:
 lib.pipe root
   [
     # Avoid rebuilding because of changes in parent directories
     (mkNewStorePath "source-${name}")
     (x: log.traceDebug "${name}.mkNewStorePath ${x.outPath}" x)
 
-    (root: self.callCabal2nix name root { })
-    (x: log.traceDebug "${name}.cabal2nixDeriver ${x.cabal2nixDeriver.outPath}" x)
+    (root: callCabal2nixWithOptions name root expr { })
+    # (x: log.traceDebug "${name}.cabal2nixDeriver ${x.cabal2nixDeriver.outPath}" x)
 
     # Make sure all files we use are included in the sdist, as a check
     # for release-worthiness.

nix/modules/project/outputs.nix

@@ -104,6 +104,24 @@ in
             );
       };
 
+      gen-pkgset =
+        let
+          drv = pkgs.runCommandNoCC "cabal2nix-generated" { } (
+            "mkdir -p $out\n" +
+            builtins.concatStringsSep "\n" (
+              lib.mapAttrsToList (name: expr: "cp -a ${expr} $out/${name}.nix") config.packagesNix
+            )
+          );
+        in
+        pkgs.writeShellApplication {
+          name = "gen-pkgset";
+          runtimeInputs = [ pkgs.rsync ];
+          text = ''
+            set -x
+            mkdir -p .haskellSrc2nix
+            rsync -a --delete --chmod=u+rw ${drv}/ .haskellSrc2nix
+          '';
+        };
     in
     {
       outputs = {
@@ -113,8 +131,11 @@ in
 
         apps =
           lib.mkMerge
-            (lib.mapAttrsToList (_: packageInfo: packageInfo.exes) config.outputs.packages);
+            (lib.mapAttrsToList (_: packageInfo: packageInfo.exes) config.outputs.packages ++ [{
+              gen-pkgset.program = gen-pkgset;
+            }]);
       };
+
     };
 }
 

nix/modules/project/packages/default.nix

@@ -58,16 +58,64 @@ in
           build-haskell-package = import ../../../build-haskell-package.nix {
             inherit pkgs lib self log;
           };
+          callPackageKeepDeriver = src: args:
+            pkgs.haskell.lib.compose.overrideCabal
+              (orig: {
+                passthru = orig.passthru or { } // {
+                  # When using callCabal2nix or callHackage, it is often useful
+                  # to debug a failure by inspecting the Nix expression
+                  # generated by cabal2nix. This can be accessed via this
+                  # cabal2nixDeriver field.
+                  cabal2nixDeriver = src;
+                };
+              })
+              (self.callPackage src args);
           getOrMkPackage = name: cfg:
             if lib.types.path.check cfg.source
             then
-              log.traceDebug "${name}.callCabal2nix ${cfg.source}"
-                (build-haskell-package name cfg.source)
+              log.traceDebug "${name}.callCabal2nix[cached] ${cfg.source}"
+                (build-haskell-package name cfg.source "${project.config.projectRoot}/.haskellSrc2nix/${name}.nix")
             else
-              log.traceDebug "${name}.callHackage ${cfg.source}"
-                (self.callHackage name cfg.source { });
+              log.traceDebug "${name}.callHackage[cached] ${cfg.source}"
+                (
+                  # (self.callHackage name cfg.source { })
+                  # callPackageKeepDeriver (self.hackage2nix name cfg.source) {}
+                  callPackageKeepDeriver "${project.config.projectRoot}/.haskellSrc2nix/${name}.nix" { }
+                );
         in
         lib.mapAttrs getOrMkPackage project.config.packages;
     };
+
+    # cabal2nix generated Nix expressions for packages
+    # uses `hackage2nix`
+    packagesNix = lib.mkOption {
+      default =
+        lib.mapAttrs
+          (name: cfg:
+            let
+              self = project.config.basePackages;
+              callCabal2nixWithOptionsExr = name: src: extraCabal2nixOptions:
+                let
+                  filter = path: type:
+                    pkgs.lib.hasSuffix ".cabal" path ||
+                    baseNameOf path == "package.yaml";
+                in
+                self.haskellSrc2nix {
+                  inherit name extraCabal2nixOptions;
+                  src =
+                    if pkgs.lib.canCleanSource src
+                    then pkgs.lib.cleanSourceWith { inherit src filter; }
+                    else src;
+                };
+              readCabal2NixExpr = drv:
+                "${builtins.trace drv.outPath drv}/default.nix";
+            in
+            readCabal2NixExpr (if lib.types.path.check cfg.source
+            # FIXME: Patch `src` to not point to nix store.
+            then callCabal2nixWithOptionsExr name cfg.source ""
+            else self.hackage2nix name cfg.source)
+          )
+          project.config.packages;
+    };
   };
 }