nix flake init: Add a '--template' flag

The initial contents of the flake is specified by the
'templates.<name>' or 'defaultTemplate' output of another flake. E.g.

  outputs = { self }: {

    templates = {

      nixos-container = {
        path = ./nixos-container;
        description = "An example of a NixOS container";
      };

    };

  };

allows

  $ nix flake init -t templates#nixos-container

Also add a command 'nix flake new', which is identical to 'nix flake
init' except that it initializes a specified directory rather than the
current directory.

8 files changed, 258 insertions, 41 deletions

diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 1b8edf9c1..a47a539f5 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -320,6 +320,8 @@ Value & AttrCursor::forceValue()
     if (root->db && (!cachedValue || std::get_if<placeholder_t>(&cachedValue->second))) {
         if (v.type == tString)
             cachedValue = {root->db->setString(getKey(), v.string.s), v.string.s};
+        else if (v.type == tPath)
+            cachedValue = {root->db->setString(getKey(), v.path), v.path};
         else if (v.type == tBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type == tAttrs)
@@ -434,10 +436,10 @@ std::string AttrCursor::getString()
 
     auto & v = forceValue();
 
-    if (v.type != tString)
-        throw TypeError("'%s' is not a string", getAttrPathStr());
+    if (v.type != tString && v.type != tPath)
+        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type));
 
-    return v.string.s;
+    return v.type == tString ? v.string.s : v.path;
 }
 
 bool AttrCursor::getBool()
diff --git a/src/nix/flake-template.nix b/src/nix/flake-template.nix
deleted file mode 100644
index 195aef2cc..000000000
--- a/src/nix/flake-template.nix
+++ /dev/null
@@ -1,11 +0,0 @@
-{
-  description = "A flake for building Hello World";
-
-  outputs = { self, nixpkgs }: {
-
-    packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hello;
-
-    defaultPackage.x86_64-linux = self.packages.x86_64-linux.hello;
-
-  };
-}
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 15a3e261a..439003908 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -207,6 +207,8 @@ struct CmdFlakeCheck : FlakeCommand
         auto state = getEvalState();
         auto flake = lockFlake();
 
+        // FIXME: rewrite to use EvalCache.
+
         auto checkSystemName = [&](const std::string & system, const Pos & pos) {
             // FIXME: what's the format of "system"?
             if (system.find('-') == std::string::npos)
@@ -320,6 +322,40 @@ struct CmdFlakeCheck : FlakeCommand
             }
         };
 
+        auto checkTemplate = [&](const std::string & attrPath, Value & v, const Pos & pos) {
+            try {
+                Activity act(*logger, lvlChatty, actUnknown,
+                    fmt("checking template '%s'", attrPath));
+
+                state->forceAttrs(v, pos);
+
+                if (auto attr = v.attrs->get(state->symbols.create("path"))) {
+                    if (attr->name == state->symbols.create("path")) {
+                        PathSet context;
+                        auto path = state->coerceToPath(*attr->pos, *attr->value, context);
+                        if (!store->isInStore(path))
+                            throw Error("template '%s' has a bad 'path' attribute");
+                        // TODO: recursively check the flake in 'path'.
+                    }
+                } else
+                    throw Error("template '%s' lacks attribute 'path'", attrPath);
+
+                if (auto attr = v.attrs->get(state->symbols.create("description")))
+                    state->forceStringNoCtx(*attr->value, *attr->pos);
+                else
+                    throw Error("template '%s' lacks attribute 'description'", attrPath);
+
+                for (auto & attr : *v.attrs) {
+                    std::string name(attr.name);
+                    if (name != "path" && name != "description")
+                        throw Error("template '%s' has unsupported attribute '%s'", attrPath, name);
+                }
+            } catch (Error & e) {
+                e.addPrefix(fmt("while checking the template '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
+                throw;
+            }
+        };
+
         {
             Activity act(*logger, lvlInfo, actUnknown, "evaluating flake");
 
@@ -432,6 +468,16 @@ struct CmdFlakeCheck : FlakeCommand
                         else if (name == "hydraJobs")
                             checkHydraJobs(name, vOutput, pos);
 
+                        else if (name == "defaultTemplate")
+                            checkTemplate(name, vOutput, pos);
+
+                        else if (name == "templates") {
+                            state->forceAttrs(vOutput, pos);
+                            for (auto & attr : *vOutput.attrs)
+                                checkTemplate(fmt("%s.%s", name, attr.name),
+                                    *attr.value, *attr.pos);
+                        }
+
                         else
                             warn("unknown flake output '%s'", name);
 
@@ -449,29 +495,135 @@ struct CmdFlakeCheck : FlakeCommand
     }
 };
 
-struct CmdFlakeInit : virtual Args, Command
+struct CmdFlakeInitCommon : virtual Args, EvalCommand
 {
-    std::string description() override
+    std::string templateUrl = "templates";
+    Path destDir;
+
+    CmdFlakeInitCommon()
     {
-        return "create a skeleton 'flake.nix' file in the current directory";
+        addFlag({
+            .longName = "template",
+            .shortName = 't',
+            .description = "the template to use",
+            .labels = {"template"},
+            .handler = {&templateUrl},
+            .completer = {[&](size_t, std::string_view prefix) {
+                completeFlakeRef(prefix);
+            }}
+        });
     }
 
-    void run() override
+    void run(nix::ref<nix::Store> store) override
+    {
+        auto flakeDir = absPath(destDir);
+
+        auto evalState = getEvalState();
+
+        auto [templateFlakeRef, templateName] = parseFlakeRefWithFragment(templateUrl, absPath("."));
+
+        auto installable = InstallableFlake(
+            evalState, std::move(templateFlakeRef),
+            Strings{templateName == "" ? "defaultTemplate" : templateName},
+            Strings{"templates."}, { .writeLockFile = false });
+
+        auto cursor = installable.getCursor(*evalState, true);
+
+        auto templateDir = cursor.first->getAttr("path")->getString();
+
+        assert(store->isInStore(templateDir));
+
+        std::vector<Path> files;
+
+        std::function<void(const Path & from, const Path & to)> copyDir;
+        copyDir = [&](const Path & from, const Path & to)
+        {
+            createDirs(to);
+
+            for (auto & entry : readDirectory(from)) {
+                auto from2 = from + "/" + entry.name;
+                auto to2 = to + "/" + entry.name;
+                auto st = lstat(from2);
+                if (S_ISDIR(st.st_mode))
+                    copyDir(from2, to2);
+                else if (S_ISREG(st.st_mode)) {
+                    auto contents = readFile(from2);
+                    if (pathExists(to2)) {
+                        auto contents2 = readFile(to2);
+                        if (contents != contents2)
+                            throw Error("refusing to overwrite existing file '%s'", to2);
+                    } else
+                        writeFile(to2, contents);
+                }
+                else if (S_ISLNK(st.st_mode)) {
+                    auto target = readLink(from2);
+                    if (pathExists(to2)) {
+                        if (readLink(to2) != target)
+                            throw Error("refusing to overwrite existing symlink '%s'", to2);
+                    } else
+                          createSymlink(target, to2);
+                }
+                else
+                    throw Error("file '%s' has unsupported type", from2);
+                files.push_back(to2);
+            }
+        };
+
+        copyDir(templateDir, flakeDir);
+
+        if (pathExists(flakeDir + "/.git")) {
+            Strings args = { "-C", flakeDir, "add", "--intent-to-add", "--" };
+            for (auto & s : files) args.push_back(s);
+            runProgram("git", true, args);
+        }
+    }
+};
+
+struct CmdFlakeInit : CmdFlakeInitCommon
+{
+    std::string description() override
     {
-        Path flakeDir = absPath(".");
+        return "create a flake in the current directory from a template";
+    }
 
-        Path flakePath = flakeDir + "/flake.nix";
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To create a flake using the default template:",
+                "nix flake init"
+            },
+            Example{
+                "To see available templates:",
+                "nix flake show templates"
+            },
+            Example{
+                "To create a flake from a specific template:",
+                "nix flake init -t templates#nixos-container"
+            },
+        };
+    }
 
-        if (pathExists(flakePath))
-            throw Error("file '%s' already exists", flakePath);
+    CmdFlakeInit()
+    {
+        destDir = ".";
+    }
+};
 
-        writeFile(flakePath,
-          #include "flake-template.nix.gen.hh"
-        );
+struct CmdFlakeNew : CmdFlakeInitCommon
+{
+    std::string description() override
+    {
+        return "create a flake in the specified directory from a template";
+    }
 
-        if (pathExists(flakeDir + "/.git"))
-            runProgram("git", true,
-                { "-C", flakeDir, "add", "--intent-to-add", "flake.nix" });
+    CmdFlakeNew()
+    {
+        expectArgs({
+            .label = "dest-dir",
+            .handler = {&destDir},
+            .completer = completePath
+        });
     }
 };
 
@@ -662,7 +814,8 @@ struct CmdFlakeShow : FlakeCommand
                             || attrPath[0] == "devShell"
                             || attrPath[0] == "nixosConfigurations"
                             || attrPath[0] == "nixosModules"
-                            || attrPath[0] == "defaultApp"))
+                            || attrPath[0] == "defaultApp"
+                            || attrPath[0] == "templates"))
                     || ((attrPath.size() == 1 || attrPath.size() == 2)
                         && (attrPath[0] == "checks"
                             || attrPath[0] == "packages"
@@ -714,6 +867,14 @@ struct CmdFlakeShow : FlakeCommand
                     logger->stdout("%s: app", headerPrefix);
                 }
 
+                else if (
+                    (attrPath.size() == 1 && attrPath[0] == "defaultTemplate") ||
+                    (attrPath.size() == 2 && attrPath[0] == "templates"))
+                {
+                    auto description = visitor.getAttr("description")->getString();
+                    logger->stdout("%s: template: " ANSI_BOLD "%s" ANSI_NORMAL, headerPrefix, description);
+                }
+
                 else {
                     logger->stdout("%s: %s",
                         headerPrefix,
@@ -743,6 +904,7 @@ struct CmdFlake : virtual MultiCommand, virtual Command
                 {"list-inputs", []() { return make_ref<CmdFlakeListInputs>(); }},
                 {"check", []() { return make_ref<CmdFlakeCheck>(); }},
                 {"init", []() { return make_ref<CmdFlakeInit>(); }},
+                {"new", []() { return make_ref<CmdFlakeNew>(); }},
                 {"clone", []() { return make_ref<CmdFlakeClone>(); }},
                 {"archive", []() { return make_ref<CmdFlakeArchive>(); }},
                 {"show", []() { return make_ref<CmdFlakeShow>(); }},
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index f471319be..4b171dcba 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -237,7 +237,7 @@ App Installable::toApp(EvalState & state)
 }
 
 std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
-Installable::getCursor(EvalState & state, bool useEvalCache)
+Installable::getCursors(EvalState & state, bool useEvalCache)
 {
     auto evalCache =
         std::make_shared<nix::eval_cache::EvalCache>(false, Hash(), state,
@@ -245,6 +245,15 @@ Installable::getCursor(EvalState & state, bool useEvalCache)
     return {{evalCache->getRoot(), ""}};
 }
 
+std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
+Installable::getCursor(EvalState & state, bool useEvalCache)
+{
+    auto cursors = getCursors(state, useEvalCache);
+    if (cursors.empty())
+        throw Error("cannot find flake attribute '%s'", what());
+    return cursors[0];
+}
+
 struct InstallableStorePath : Installable
 {
     ref<Store> store;
@@ -474,7 +483,7 @@ std::pair<Value *, Pos> InstallableFlake::toValue(EvalState & state)
 }
 
 std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
-InstallableFlake::getCursor(EvalState & state, bool useEvalCache)
+InstallableFlake::getCursors(EvalState & state, bool useEvalCache)
 {
     auto evalCache = openEvalCache(state,
         std::make_shared<flake::LockedFlake>(lockFlake(state, flakeRef, lockFlags)),
diff --git a/src/nix/installables.hh b/src/nix/installables.hh
index a2db71389..1e6623f88 100644
--- a/src/nix/installables.hh
+++ b/src/nix/installables.hh
@@ -56,6 +56,9 @@ struct Installable
     }
 
     virtual std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+    getCursors(EvalState & state, bool useEvalCache);
+
+    std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
     getCursor(EvalState & state, bool useEvalCache);
 
     virtual FlakeRef nixpkgsFlakeRef() const
@@ -109,7 +112,7 @@ struct InstallableFlake : InstallableValue
     std::pair<Value *, Pos> toValue(EvalState & state) override;
 
     std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
-    getCursor(EvalState & state, bool useEvalCache) override;
+    getCursors(EvalState & state, bool useEvalCache) override;
 
     std::shared_ptr<flake::LockedFlake> getLockedFlake() const;
 
diff --git a/src/nix/local.mk b/src/nix/local.mk
index 9ed55f1f6..43b7754e3 100644
--- a/src/nix/local.mk
+++ b/src/nix/local.mk
@@ -29,5 +29,3 @@ $(eval $(call install-symlink, $(bindir)/nix, $(libexecdir)/nix/build-remote))
 src/nix-env/user-env.cc: src/nix-env/buildenv.nix.gen.hh
 
 src/nix/develop.cc: src/nix/get-env.sh.gen.hh
-
-$(d)/flake.cc: $(d)/flake-template.nix.gen.hh
diff --git a/src/nix/search.cc b/src/nix/search.cc
index bbac56fcb..65a1e1818 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -177,7 +177,7 @@ struct CmdSearch : InstallableCommand, MixJSON
             }
         };
 
-        for (auto & [cursor, prefix] : installable->getCursor(*state, true))
+        for (auto & [cursor, prefix] : installable->getCursors(*state, true))
             visit(*cursor, parseAttrPath(*state, prefix));
 
         if (!json && !results)
diff --git a/tests/flakes.sh b/tests/flakes.sh
index 6a550ef32..fdf31f5c1 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -20,12 +20,14 @@ flake2Dir=$TEST_ROOT/flake2
 flake3Dir=$TEST_ROOT/flake3
 flake4Dir=$TEST_ROOT/flake4
 flake5Dir=$TEST_ROOT/flake5
+flake6Dir=$TEST_ROOT/flake6
 flake7Dir=$TEST_ROOT/flake7
+templatesDir=$TEST_ROOT/templates
 nonFlakeDir=$TEST_ROOT/nonFlake
 flakeA=$TEST_ROOT/flakeA
 flakeB=$TEST_ROOT/flakeB
 
-for repo in $flake1Dir $flake2Dir $flake3Dir $flake7Dir $nonFlakeDir $flakeA $flakeB; do
+for repo in $flake1Dir $flake2Dir $flake3Dir $flake7Dir $templatesDir $nonFlakeDir $flakeA $flakeB; do
     rm -rf $repo $repo.tmp
     mkdir $repo
     git -C $repo init
@@ -145,13 +147,22 @@ cat > $registry <<EOF
         "type": "indirect",
         "id": "flake1"
       }
+    },
+    { "from": {
+        "type": "indirect",
+        "id": "templates"
+      },
+      "to": {
+        "type": "git",
+        "url": "file://$templatesDir"
+      }
     }
   ]
 }
 EOF
 
 # Test 'nix flake list'.
-[[ $(nix registry list | wc -l) == 6 ]]
+[[ $(nix registry list | wc -l) == 7 ]]
 
 # Test 'nix flake info'.
 nix flake info flake1 | grep -q 'URL: .*flake1.*'
@@ -392,18 +403,61 @@ nix build -o $TEST_ROOT/result flake4/removeXyzzy#sth
 
 # Testing the nix CLI
 nix registry add flake1 flake3
-[[ $(nix registry list | wc -l) == 7 ]]
+[[ $(nix registry list | wc -l) == 8 ]]
 nix registry pin flake1
-[[ $(nix registry list | wc -l) == 7 ]]
+[[ $(nix registry list | wc -l) == 8 ]]
 nix registry remove flake1
-[[ $(nix registry list | wc -l) == 6 ]]
+[[ $(nix registry list | wc -l) == 7 ]]
 
 # Test 'nix flake init'.
+cat > $templatesDir/flake.nix <<EOF
+{
+  description = "Some templates";
+
+  outputs = { self }: {
+    templates = {
+      trivial = {
+        path = ./trivial;
+        description = "A trivial flake";
+      };
+    };
+    defaultTemplate = self.templates.trivial;
+  };
+}
+EOF
+
+mkdir $templatesDir/trivial
+
+cat > $templatesDir/trivial/flake.nix <<EOF
+{
+  description = "A flake for building Hello World";
+
+  outputs = { self, nixpkgs }: {
+    packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hello;
+    defaultPackage.x86_64-linux = self.packages.x86_64-linux.hello;
+  };
+}
+EOF
+
+git -C $templatesDir add flake.nix trivial/flake.nix
+git -C $templatesDir commit -m 'Initial'
+
+nix flake check templates
+nix flake show templates
+
 (cd $flake7Dir && nix flake init)
+(cd $flake7Dir && nix flake init) # check idempotence
 git -C $flake7Dir add flake.nix
 nix flake check $flake7Dir
+nix flake show $flake7Dir
 git -C $flake7Dir commit -a -m 'Initial'
 
+# Test 'nix flake new'.
+rm -rf $flake6Dir
+nix flake new -t templates#trivial $flake6Dir
+nix flake new -t templates#trivial $flake6Dir # check idempotence
+nix flake check $flake6Dir
+
 # Test 'nix flake clone'.
 rm -rf $TEST_ROOT/flake1-v2
 nix flake clone flake1 --dest $TEST_ROOT/flake1-v2
@@ -663,4 +717,4 @@ git -C $flakeB commit -a -m 'Foo'
 [[ $(nix eval --update-input b $flakeA#foo) = 1912 ]]
 
 # Test list-inputs with circular dependencies
-nix flake list-inputs $flakeA
\ No newline at end of file
+nix flake list-inputs $flakeA