Improve shellFor selectComponents performance (#1146)

* factor out isSelectedComponent

* Optimise `flatLibDepends`

We no longer force the `outPath` attribute to avoid instantiating the
derivation, which is expensive

Author: TeofilC

builder/shell-for.nix

@@ -55,19 +55,20 @@ let
       (builtins.map (x: lib.nameValuePair (x.name) x)
         (haskellLib.flatLibDepends {depends = directlySelectedComponents;}));
 
+  isSelectedComponent =
+    comp: selectedComponentsBitmap."${((haskellLib.dependToLib comp).name or null)}" or false;
   selectedComponentsBitmap =
     lib.mapAttrs
-      (_: x: (builtins.any
-        (dep: selectedComponentsBitmap."${(haskellLib.dependToLib dep).name}") x.config.depends))
+      (_: x: (builtins.any isSelectedComponent x.config.depends))
       transitiveDependenciesComponents
     // builtins.listToAttrs (map (x: lib.nameValuePair x.name true) directlySelectedComponents); # base case
 
   selectedComponents =
-    lib.filter (x: selectedComponentsBitmap."${x.name}") (lib.attrValues transitiveDependenciesComponents);
+    lib.filter isSelectedComponent  (lib.attrValues transitiveDependenciesComponents);
 
   # Given a list of `depends`, removes those which are selected components
   removeSelectedInputs =
-    lib.filter (input: !(selectedComponentsBitmap."${((haskellLib.dependToLib input).name or null)}" or false));
+    lib.filter (input: !(isSelectedComponent input));
 
   # The configs of all the selected components
   selectedConfigs = map (c: c.config) selectedComponents

lib/default.nix

@@ -132,14 +132,7 @@ in {
   ## flatLibDepends :: Component -> [Package]
   flatLibDepends = component:
     let
-      # this is a minor improvement over the "cannot coerce set to string"
-      # error.  It will now say:
-      #
-      # > The option `packages.Win32.package.identifier.name' is used but not defined.
-      #
-      # which indicates that the package.Win32 is missing and not defined.
-      getKey = x: if x ? "outPath" then "${x}" else (throw x.identifier.name);
-      makePairs = map (p: rec { key=getKey val; val=(p.components.library or p); });
+      makePairs = map (p: rec { key=val.name; val=(p.components.library or p); });
       closure = builtins.genericClosure {
         startSet = makePairs component.depends;
         operator = {val,...}: makePairs val.config.depends;