nix: New installables syntax

The general syntax for an installable is now
<flakeref>:<attrpath>. The attrpath is relative to the flake's
'provides.packages' or 'provides' if the former doesn't yield a
result. E.g.

  $ nix build nixpkgs:hello

is equivalent to

  $ nix build nixpkgs:packages.hello

Also, '<flakeref>:' can be omitted, in which case it defaults to
'nixpkgs', e.g.

  $ nix build hello

src/libexpr/primops/flake.cc

@@ -57,8 +57,9 @@ Path getUserRegistryPath()
 
 std::shared_ptr<FlakeRegistry> getGlobalRegistry()
 {
-    // TODO: Make a global registry, and return it here.
+    // FIXME: get from nixos.org.
-    return std::make_shared<FlakeRegistry>();
+    Path registryFile = settings.nixDataDir + "/nix/flake-registry.json";
+    return readRegistry(registryFile);
 }
 
 std::shared_ptr<FlakeRegistry> getUserRegistry()
@@ -66,33 +67,17 @@ std::shared_ptr<FlakeRegistry> getUserRegistry()
     return readRegistry(getUserRegistryPath());
 }
 
-// Project-specific registry saved in flake-registry.json.
-std::shared_ptr<FlakeRegistry> getLocalRegistry()
-{
-    Path registryFile = settings.nixDataDir + "/nix/flake-registry.json";
-    return readRegistry(registryFile);
-}
-
 std::shared_ptr<FlakeRegistry> getFlagRegistry()
 {
     return std::make_shared<FlakeRegistry>();
     // TODO: Implement this once the right flags are implemented.
 }
 
-// This always returns a vector with globalReg, userReg, localReg, flakeReg.
-// If one of them doesn't exist, the registry is left empty but does exist.
 const std::vector<std::shared_ptr<FlakeRegistry>> EvalState::getFlakeRegistries()
 {
     std::vector<std::shared_ptr<FlakeRegistry>> registries;
-    if (!evalSettings.pureEval) {
+    registries.push_back(getGlobalRegistry());
-        registries.push_back(std::make_shared<FlakeRegistry>()); // global
+    registries.push_back(getUserRegistry());
-        registries.push_back(std::make_shared<FlakeRegistry>()); // user
-        registries.push_back(std::make_shared<FlakeRegistry>()); // local
-    } else {
-        registries.push_back(getGlobalRegistry());
-        registries.push_back(getUserRegistry());
-        registries.push_back(getLocalRegistry());
-    }
     registries.push_back(getFlagRegistry());
     return registries;
 }
@@ -149,8 +134,7 @@ struct FlakeSourceInfo
 static FlakeSourceInfo fetchFlake(EvalState & state, const FlakeRef & flakeRef)
 {
     FlakeRef directFlakeRef = FlakeRef(flakeRef);
-    if (!flakeRef.isDirect())
+    if (!flakeRef.isDirect()) {
-    {
         directFlakeRef = lookupFlake(state, flakeRef, state.getFlakeRegistries());
     }
     assert(directFlakeRef.isDirect());
@@ -274,8 +258,8 @@ static std::tuple<FlakeId, std::map<FlakeId, Flake>> resolveFlake(EvalState & st
     std::optional<FlakeId> topFlakeId; /// FIXME: ambiguous
     todo.push({topRef, true});
 
-    std::vector<std::shared_ptr<FlakeRegistry>> registries = state.getFlakeRegistries();
+    auto registries = state.getFlakeRegistries();
-    std::shared_ptr<FlakeRegistry> localRegistry = registries.at(2);
+    //std::shared_ptr<FlakeRegistry> localRegistry = registries.at(2);
 
     while (!todo.empty()) {
         auto [flakeRef, toplevel] = todo.front();
@@ -283,12 +267,15 @@ static std::tuple<FlakeId, std::map<FlakeId, Flake>> resolveFlake(EvalState & st
 
         if (auto refData = std::get_if<FlakeRef::IsFlakeId>(&flakeRef.data)) {
             if (done.count(refData->id)) continue; // optimization
-            flakeRef = lookupFlake(state, flakeRef, registries);
+            flakeRef = lookupFlake(state, flakeRef,
+                !evalSettings.pureEval || (toplevel && impureTopRef) ? registries : std::vector<std::shared_ptr<FlakeRegistry>>());
             // This is why we need the `registries`.
         }
 
+#if 0
         if (evalSettings.pureEval && !flakeRef.isImmutable() && (!toplevel || !impureTopRef))
             throw Error("mutable flake '%s' is not allowed in pure mode; use --no-pure-eval to disable", flakeRef.to_string());
+#endif
 
         auto flake = getFlake(state, flakeRef);
 
@@ -299,6 +286,7 @@ static std::tuple<FlakeId, std::map<FlakeId, Flake>> resolveFlake(EvalState & st
         for (auto & require : flake.requires)
             todo.push({require, false});
 
+#if 0
         // The following piece of code basically adds the FlakeRefs from
         // the lockfiles of dependencies to the localRegistry. This is used
         // to resolve future `FlakeId`s, in `lookupFlake` a bit above this.
@@ -307,6 +295,7 @@ static std::tuple<FlakeId, std::map<FlakeId, Flake>> resolveFlake(EvalState & st
                 if (localRegistry->entries.count(entry.first)) continue;
                 localRegistry->entries.emplace(entry.first, entry.second);
             }
+#endif
 
         done.emplace(flake.id, std::move(flake));
     }
@@ -344,19 +333,9 @@ void updateLockFile(EvalState & state, std::string path)
     }
 }
 
-Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v)
+Value * makeFlakeValue(EvalState & state, const FlakeRef & flakeRef, bool impureTopRef, Value & v)
 {
-    // FIXME: temporary hack to make the default installation source
+    auto [topFlakeId, flakes] = resolveFlake(state, flakeRef, impureTopRef);
-    // work.
-    bool impure = false;
-    if (hasPrefix(flakeUri, "impure:")) {
-        flakeUri = std::string(flakeUri, 7);
-        impure = true;
-    }
-
-    auto flakeRef = FlakeRef(flakeUri);
-
-    auto [topFlakeId, flakes] = resolveFlake(state, flakeUri, impure);
 
     // FIXME: we should call each flake with only its dependencies
     // (rather than the closure of the top-level flake).
@@ -387,7 +366,7 @@ Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v)
 
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    makeFlakeValue(state, state.forceStringNoCtx(*args[0], pos), v);
+    makeFlakeValue(state, state.forceStringNoCtx(*args[0], pos), false, v);
 }
 
 static RegisterPrimOp r2("getFlake", 1, prim_getFlake);

src/libexpr/primops/flake.hh

@@ -23,7 +23,7 @@ Path getUserRegistryPath();
 
 Value * makeFlakeRegistryValue(EvalState & state);
 
-Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v);
+Value * makeFlakeValue(EvalState & state, const FlakeRef & flakeRef, bool impureTopRef, Value & v);
 
 std::shared_ptr<FlakeRegistry> readRegistry(const Path &);
 

src/nix/installables.cc

@@ -21,13 +21,6 @@ SourceExprCommand::SourceExprCommand()
         .label("file")
         .description("evaluate FILE rather than use the default installation source")
         .dest(&file);
-
-    mkFlag()
-        .shortName('F')
-        .longName("flake")
-        .label("flake")
-        .description("evaluate FLAKE rather than use the default installation source")
-        .dest(&flakeUri);
 }
 
 Value * SourceExprCommand::getSourceExpr(EvalState & state)
@@ -36,17 +29,9 @@ Value * SourceExprCommand::getSourceExpr(EvalState & state)
 
     vSourceExpr = state.allocValue();
 
-    if (file && flakeUri)
-        throw Error("cannot use both --file and --flake");
-
     if (file)
         state.evalFile(lookupFileArg(state, *file), *vSourceExpr);
-    else if (flakeUri) {
+    else {
-        // FIXME: handle flakeUri being a relative path
-        auto vTemp = state.allocValue();
-        auto vFlake = *makeFlakeValue(state, "impure:" + *flakeUri, *vTemp);
-        *vSourceExpr = *((*vFlake.attrs->get(state.symbols.create("provides")))->value);
-    } else {
         // FIXME: remove "impure" hack, call some non-user-accessible
         // variant of getFlake instead.
         auto fun = state.parseExprFromString(
@@ -176,6 +161,43 @@ struct InstallableAttrPath : InstallableValue
     }
 };
 
+struct InstallableFlake : InstallableValue
+{
+    FlakeRef flakeRef;
+    std::string attrPath;
+
+    InstallableFlake(SourceExprCommand & cmd, FlakeRef && flakeRef, const std::string & attrPath)
+        : InstallableValue(cmd), flakeRef(flakeRef), attrPath(attrPath)
+    { }
+
+    std::string what() override { return flakeRef.to_string() + ":" + attrPath; }
+
+    Value * toValue(EvalState & state) override
+    {
+        auto vTemp = state.allocValue();
+        auto vFlake = *makeFlakeValue(state, flakeRef, true, *vTemp);
+
+        auto vProvides = (*vFlake.attrs->get(state.symbols.create("provides")))->value;
+
+        state.forceValue(*vProvides);
+
+        auto emptyArgs = state.allocBindings(0);
+
+        if (auto aPackages = *vProvides->attrs->get(state.symbols.create("packages"))) {
+            try {
+                auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *aPackages->value);
+                state.forceValue(*v);
+                return v;
+            } catch (AttrPathNotFound & e) {
+            }
+        }
+
+        auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *vProvides);
+        state.forceValue(*v);
+        return v;
+    }
+};
+
 // FIXME: extend
 std::string attrRegex = R"([A-Za-z_][A-Za-z0-9-_+]*)";
 static std::regex attrPathRegex(fmt(R"(%1%(\.%1%)*)", attrRegex));
@@ -196,19 +218,34 @@ static std::vector<std::shared_ptr<Installable>> parseInstallables(
         if (s.compare(0, 1, "(") == 0)
             result.push_back(std::make_shared<InstallableExpr>(cmd, s));
 
-        else if (s.find("/") != std::string::npos) {
+        /*
+        else if (s.find('/') != std::string::npos) {
 
             auto path = store->toStorePath(store->followLinksToStore(s));
 
             if (store->isStorePath(path))
                 result.push_back(std::make_shared<InstallableStorePath>(path));
         }
+        */
 
+        else {
+            auto colon = s.rfind(':');
+            if (colon != std::string::npos) {
+                auto flakeRef = std::string(s, 0, colon);
+                auto attrPath = std::string(s, colon + 1);
+                result.push_back(std::make_shared<InstallableFlake>(cmd, FlakeRef(flakeRef), attrPath));
+            } else {
+                result.push_back(std::make_shared<InstallableFlake>(cmd, FlakeRef("nixpkgs"), s));
+            }
+        }
+
+        /*
         else if (s == "" || std::regex_match(s, attrPathRegex))
             result.push_back(std::make_shared<InstallableAttrPath>(cmd, s));
 
         else
             throw UsageError("don't know what to do with argument '%s'", s);
+        */
     }
 
     return result;