elmPackages: Refactor lib and builds of packages

pkgs/development/compilers/elm/packages/generate-node-packages.sh

@@ -6,7 +6,7 @@ set -eu -o pipefail
 
 rm -f node-env.nix
 $(nix-build $ROOT -A nodePackages.node2nix --no-out-link)/bin/node2nix \
-    --nodejs-10 \
+    --nodejs-12 \
     -i node-packages.json \
     -o node-packages.nix \
     -c node-composition.nix \

pkgs/development/compilers/elm/packages/lib.nix

@@ -0,0 +1,43 @@
+{ writeScriptBin, stdenv, lib, elm }:
+let
+  patchBinwrap =
+    let
+      # Patching binwrap by NoOp script
+      binwrap = writeScriptBin "binwrap" ''
+        #! ${stdenv.shell}
+        echo "binwrap called: Returning 0"
+        return 0
+      '';
+      binwrap-install = writeScriptBin "binwrap-install" ''
+        #! ${stdenv.shell}
+        echo "binwrap-install called: Doing nothing"
+      '';
+    in targets: pkg:
+      pkg.override (old: {
+        nativeBuildInputs = (old.nativeBuildInputs or []) ++ [ binwrap binwrap-install ];
+
+        # Manually install targets
+        # by symlinking binaries into `node_modules`
+        postInstall = let
+          binFile = module: lib.strings.removeSuffix ("-" + module.version) module.name;
+        in (old.postInstall or "") + ''
+          ${lib.concatStrings (map (module: ''
+              echo "linking ${binFile module}"
+              ln -sf ${module}/bin/${binFile module} \
+                  node_modules/${binFile module}/bin/${binFile module}
+          '') targets)}
+        '';
+      });
+
+  patchNpmElm = pkg:
+    pkg.override (old: {
+      preRebuild = (old.preRebuild or "") + ''
+        rm node_modules/elm/install.js
+        echo "console.log('Nixpkgs\' version of Elm will be used');" > node_modules/elm/install.js
+      '';
+      postInstall = (old.postInstall or "") + ''
+        ln -sf ${elm}/bin/elm node_modules/elm/bin/elm
+      '';
+    });
+in
+{ inherit patchBinwrap patchNpmElm; }

pkgs/development/compilers/elm/packages/node-composition.nix

@@ -2,7 +2,7 @@
 
 {pkgs ? import <nixpkgs> {
     inherit system;
-  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-10_x"}:
+  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-12_x"}:
 
 let
   nodeEnv = import ../../../node-packages/node-env.nix {

pkgs/development/compilers/elm/packages/patch-binwrap.nix

@@ -1,30 +0,0 @@
-{ writeScriptBin, stdenv, lib }:
-let
-  # Patching binwrap by NoOp script
-  binwrap = writeScriptBin "binwrap" ''
-    #! ${stdenv.shell}
-    echo "binwrap called: Returning 0"
-    return 0
-  '';
-  binwrap-install = writeScriptBin "binwrap-install" ''
-    #! ${stdenv.shell}
-    echo "binwrap-install called: Doing nothing"
-  '';
-in
-targets:
-pkg:
-pkg.override {
-  nativeBuildInputs = pkg.nativeBuildInputs ++ [ binwrap binwrap-install ];
-
-  # Manually install targets
-  # by symlinking binaries into `node_modules`
-  postInstall = let
-    binFile = module: lib.strings.removeSuffix ("-" + module.version) module.name;
-  in ''
-    ${lib.concatStrings (map (module: ''
-        echo "linking ${binFile module}"
-        ln -sf ${module}/bin/${binFile module} \
-            node_modules/${binFile module}/bin/${binFile module}
-    '') targets)}
-  '';
-}