nix flake show: Ignore empty attrsets

src/nix/flake.cc

@@ -1002,6 +1002,61 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
  auto flake = std::make_shared<LockedFlake>(lockFlake());
  auto localSystem = std::string(settings.thisSystem.get());
 
+ std::function<bool(
+ eval_cache::AttrCursor & visitor,
+ const std::vector<Symbol> &attrPath,
+ const Symbol &attr)> hasContent;
+
+ // For frameworks it's important that structures are as lazy as possible
+ // to prevent infinite recursions, performance issues and errors that
+ // aren't related to the thing to evaluate. As a consequence, they have
+ // to emit more attributes than strictly (sic) necessary.
+ // However, these attributes with empty values are not useful to the user
+ // so we omit them.
+ hasContent = [&](
+ eval_cache::AttrCursor & visitor,
+ const std::vector<Symbol> &attrPath,
+ const Symbol &attr) -> bool
+ {
+ auto attrPath2(attrPath);
+ attrPath2.push_back(attr);
+ auto attrPathS = state->symbols.resolve(attrPath2);
+ const auto & attrName = state->symbols[attr];
+
+ auto visitor2 = visitor.getAttr(attrName);
+
+ if ((attrPathS[0] == "apps"
+ || attrPathS[0] == "checks"
+ || attrPathS[0] == "devShells"
+ || attrPathS[0] == "legacyPackages"
+ || attrPathS[0] == "packages")
+ && (attrPathS.size() == 1 || attrPathS.size() == 2)) {
+ for (const auto &subAttr : visitor2->getAttrs()) {
+ if (hasContent(*visitor2, attrPath2, subAttr)) {
+ return true;
+ }
+ }
+ return false;
+ }
+
+ if ((attrPathS.size() == 1)
+ && (attrPathS[0] == "formatter"
+ || attrPathS[0] == "nixosConfigurations"
+ || attrPathS[0] == "nixosModules"
+ || attrPathS[0] == "overlays"
+ )) {
+ for (const auto &subAttr : visitor2->getAttrs()) {
+ if (hasContent(*visitor2, attrPath2, subAttr)) {
+ return true;
+ }
+ }
+ return false;
+ }
+
+ // If we don't recognize it, it's probably content
+ return true;
+ };
+
  std::function<nlohmann::json(
  eval_cache::AttrCursor & visitor,
  const std::vector<Symbol> & attrPath,
@@ -1027,7 +1082,12 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
  {
  if (!json)
  logger->cout("%s", headerPrefix);
- auto attrs = visitor.getAttrs();
+ std::vector<Symbol> attrs;
+ for (const auto &attr : visitor.getAttrs()) {
+ if (hasContent(visitor, attrPath, attr))
+ attrs.push_back(attr);
+ }
+
  for (const auto & [i, attr] : enumerate(attrs)) {
  const auto & attrName = state->symbols[attr];
  bool last = i + 1 == attrs.size();

tests/flakes/show.sh

@@ -37,3 +37,30 @@ in
 assert show_output.legacyPackages.${builtins.currentSystem}.hello.name == "simple";
 true
 '
+
+# Test that attributes are only reported when they have actual content
+cat >flake.nix <<EOF
+{
+ description = "Bla bla";
+
+ outputs = inputs: rec {
+ apps.$system = { };
+ checks.$system = { };
+ devShells.$system = { };
+ legacyPackages.$system = { };
+ packages.$system = { };
+ packages.someOtherSystem = { };
+
+ formatter = { };
+ nixosConfigurations = { };
+ nixosModules = { };
+ };
+}
+EOF
+nix flake show --json --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output == { };
+true
+'