lix/src/nix/flake.cc

#include "command.hh"
#include "common-args.hh"
#include "shared.hh"
#include "eval.hh"
#include "eval-inline.hh"
#include "flake/flake.hh"
#include "get-drvs.hh"
#include "store-api.hh"
#include "derivations.hh"
#include "attr-path.hh"
#include "fetchers.hh"
#include "registry.hh"
#include "json.hh"
#include "eval-cache.hh"

#include <nlohmann/json.hpp>
#include <queue>
#include <iomanip>

using namespace nix;
using namespace nix::flake;

class FlakeCommand : virtual Args, public MixFlakeOptions
{
    std::string flakeUrl = ".";

public:

    FlakeCommand()
    {
        expectArgs({
            .label = "flake-url",
            .optional = true,
            .handler = {&flakeUrl},
            .completer = {[&](size_t, std::string_view prefix) {
                completeFlakeRef(getStore(), prefix);
            }}
        });
    }

    FlakeRef getFlakeRef()
    {
        return parseFlakeRef(flakeUrl, absPath(".")); //FIXME
    }

    Flake getFlake()
    {
        auto evalState = getEvalState();
        return flake::getFlake(*evalState, getFlakeRef(), lockFlags.useRegistries);
    }

    LockedFlake lockFlake()
    {
        return flake::lockFlake(*getEvalState(), getFlakeRef(), lockFlags);
    }

    std::optional<FlakeRef> getFlakeRefForCompletion() override
    {
        return getFlakeRef();
    }
};

static void printFlakeInfo(const Store & store, const Flake & flake)
{
    logger->cout("Resolved URL:  %s", flake.resolvedRef.to_string());
    logger->cout("Locked URL:    %s", flake.lockedRef.to_string());
    if (flake.description)
        logger->cout("Description:   %s", *flake.description);
    logger->cout("Path:          %s", store.printStorePath(flake.sourceInfo->storePath));
    if (auto rev = flake.lockedRef.input.getRev())
        logger->cout("Revision:      %s", rev->to_string(Base16, false));
    if (auto revCount = flake.lockedRef.input.getRevCount())
        logger->cout("Revisions:     %s", *revCount);
    if (auto lastModified = flake.lockedRef.input.getLastModified())
        logger->cout("Last modified: %s",
            std::put_time(std::localtime(&*lastModified), "%F %T"));
}

static nlohmann::json flakeToJSON(const Store & store, const Flake & flake)
{
    nlohmann::json j;
    if (flake.description)
        j["description"] = *flake.description;
    j["originalUrl"] = flake.originalRef.to_string();
    j["original"] = fetchers::attrsToJSON(flake.originalRef.toAttrs());
    j["resolvedUrl"] = flake.resolvedRef.to_string();
    j["resolved"] = fetchers::attrsToJSON(flake.resolvedRef.toAttrs());
    j["url"] = flake.lockedRef.to_string(); // FIXME: rename to lockedUrl
    j["locked"] = fetchers::attrsToJSON(flake.lockedRef.toAttrs());
    if (auto rev = flake.lockedRef.input.getRev())
        j["revision"] = rev->to_string(Base16, false);
    if (auto revCount = flake.lockedRef.input.getRevCount())
        j["revCount"] = *revCount;
    if (auto lastModified = flake.lockedRef.input.getLastModified())
        j["lastModified"] = *lastModified;
    j["path"] = store.printStorePath(flake.sourceInfo->storePath);
    return j;
}

struct CmdFlakeUpdate : FlakeCommand
{
    std::string description() override
    {
        return "update flake lock file";
    }

    std::string doc() override
    {
        return
          #include "flake-update.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        /* Use --refresh by default for 'nix flake update'. */
        settings.tarballTtl = 0;

        lockFlake();
    }
};

static void enumerateOutputs(EvalState & state, Value & vFlake,
    std::function<void(const std::string & name, Value & vProvide, const Pos & pos)> callback)
{
    state.forceAttrs(vFlake);

    auto aOutputs = vFlake.attrs->get(state.symbols.create("outputs"));
    assert(aOutputs);

    state.forceAttrs(*aOutputs->value);

    for (auto & attr : *aOutputs->value->attrs)
        callback(attr.name, *attr.value, *attr.pos);
}

struct CmdFlakeInfo : FlakeCommand, MixJSON
{
    std::string description() override
    {
        return "list info about a given flake";
    }

    std::string doc() override
    {
        return
          #include "flake-info.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        auto flake = getFlake();

        if (json) {
            auto json = flakeToJSON(*store, flake);
            logger->cout("%s", json.dump());
        } else
            printFlakeInfo(*store, flake);
    }
};

struct CmdFlakeListInputs : FlakeCommand, MixJSON
{
    std::string description() override
    {
        return "list flake inputs";
    }

    std::string doc() override
    {
        return
          #include "flake-list-inputs.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        auto flake = lockFlake();

        if (json)
            logger->cout("%s", flake.lockFile.toJSON());
        else {
            logger->cout("%s", flake.flake.lockedRef);

            std::unordered_set<std::shared_ptr<Node>> visited;

            std::function<void(const Node & node, const std::string & prefix)> recurse;

            recurse = [&](const Node & node, const std::string & prefix)
            {
                for (const auto & [i, input] : enumerate(node.inputs)) {
                    bool last = i + 1 == node.inputs.size();

                    if (auto lockedNode = std::get_if<0>(&input.second)) {
                        logger->cout("%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s",
                            prefix + (last ? treeLast : treeConn), input.first,
                            *lockedNode ? (*lockedNode)->lockedRef : flake.flake.lockedRef);

                        bool firstVisit = visited.insert(*lockedNode).second;

                        if (firstVisit) recurse(**lockedNode, prefix + (last ? treeNull : treeLine));
                    } else if (auto follows = std::get_if<1>(&input.second)) {
                        logger->cout("%s" ANSI_BOLD "%s" ANSI_NORMAL " follows input '%s'",
                            prefix + (last ? treeLast : treeConn), input.first,
                            printInputPath(*follows));
                    }
                }
            };

            visited.insert(flake.lockFile.root);
            recurse(*flake.lockFile.root, "");
        }
    }
};

struct CmdFlakeCheck : FlakeCommand
{
    bool build = true;

    CmdFlakeCheck()
    {
        addFlag({
            .longName = "no-build",
            .description = "do not build checks",
            .handler = {&build, false}
        });
    }

    std::string description() override
    {
        return "check whether the flake evaluates and run its tests";
    }

    std::string doc() override
    {
        return
          #include "flake-check.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        settings.readOnlyMode = !build;

        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)
                throw Error("'%s' is not a valid system type, at %s", system, pos);
        };

        auto checkDerivation = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                auto drvInfo = getDerivation(*state, v, false);
                if (!drvInfo)
                    throw Error("flake attribute '%s' is not a derivation", attrPath);
                // FIXME: check meta attributes
                return store->parseStorePath(drvInfo->queryDrvPath());
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the derivation '%s'", attrPath));
                throw;
            }
        };

        std::vector<StorePathWithOutputs> drvPaths;

        auto checkApp = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                #if 0
                // FIXME
                auto app = App(*state, v);
                for (auto & i : app.context) {
                    auto [drvPathS, outputName] = decodeContext(i);
                    store->parseStorePath(drvPathS);
                }
                #endif
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the app definition '%s'", attrPath));
                throw;
            }
        };

        auto checkOverlay = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                state->forceValue(v, pos);
                if (!v.isLambda() || v.lambda.fun->matchAttrs || std::string(v.lambda.fun->arg) != "final")
                    throw Error("overlay does not take an argument named 'final'");
                auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
                if (!body || body->matchAttrs || std::string(body->arg) != "prev")
                    throw Error("overlay does not take an argument named 'prev'");
                // FIXME: if we have a 'nixpkgs' input, use it to
                // evaluate the overlay.
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the overlay '%s'", attrPath));
                throw;
            }
        };

        auto checkModule = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                state->forceValue(v, pos);
                if (v.isLambda()) {
                    if (!v.lambda.fun->matchAttrs || !v.lambda.fun->formals->ellipsis)
                        throw Error("module must match an open attribute set ('{ config, ... }')");
                } else if (v.type() == nAttrs) {
                    for (auto & attr : *v.attrs)
                        try {
                            state->forceValue(*attr.value, *attr.pos);
                        } catch (Error & e) {
                            e.addTrace(*attr.pos, hintfmt("while evaluating the option '%s'", attr.name));
                            throw;
                        }
                } else
                    throw Error("module must be a function or an attribute set");
                // FIXME: if we have a 'nixpkgs' input, use it to
                // check the module.
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the NixOS module '%s'", attrPath));
                throw;
            }
        };

        std::function<void(const std::string & attrPath, Value & v, const Pos & pos)> checkHydraJobs;

        checkHydraJobs = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                state->forceAttrs(v, pos);

                if (state->isDerivation(v))
                    throw Error("jobset should not be a derivation at top-level");

                for (auto & attr : *v.attrs) {
                    state->forceAttrs(*attr.value, *attr.pos);
                    if (!state->isDerivation(*attr.value))
                        checkHydraJobs(attrPath + "." + (std::string) attr.name,
                            *attr.value, *attr.pos);
                }

            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the Hydra jobset '%s'", attrPath));
                throw;
            }
        };

        auto checkNixOSConfiguration = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                Activity act(*logger, lvlChatty, actUnknown,
                    fmt("checking NixOS configuration '%s'", attrPath));
                Bindings & bindings(*state->allocBindings(0));
                auto vToplevel = findAlongAttrPath(*state, "config.system.build.toplevel", bindings, v).first;
                state->forceAttrs(*vToplevel, pos);
                if (!state->isDerivation(*vToplevel))
                    throw Error("attribute 'config.system.build.toplevel' is not a derivation");
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the NixOS configuration '%s'", attrPath));
                throw;
            }
        };

        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.addTrace(pos, hintfmt("while checking the template '%s'", attrPath));
                throw;
            }
        };

        auto checkBundler = [&](const std::string & attrPath, Value & v, const Pos & pos) {
            try {
                state->forceValue(v, pos);
                if (!v.isLambda())
                    throw Error("bundler must be a function");
                if (!v.lambda.fun->formals ||
                    v.lambda.fun->formals->argNames.find(state->symbols.create("program")) == v.lambda.fun->formals->argNames.end() ||
                    v.lambda.fun->formals->argNames.find(state->symbols.create("system")) == v.lambda.fun->formals->argNames.end())
                    throw Error("bundler must take formal arguments 'program' and 'system'");
            } catch (Error & e) {
                e.addTrace(pos, hintfmt("while checking the template '%s'", attrPath));
                throw;
            }
        };

        {
            Activity act(*logger, lvlInfo, actUnknown, "evaluating flake");

            auto vFlake = state->allocValue();
            flake::callFlake(*state, flake, *vFlake);

            enumerateOutputs(*state,
                *vFlake,
                [&](const std::string & name, Value & vOutput, const Pos & pos) {
                    Activity act(*logger, lvlChatty, actUnknown,
                        fmt("checking flake output '%s'", name));

                    try {
                        state->forceValue(vOutput, pos);

                        if (name == "checks") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                state->forceAttrs(*attr.value, *attr.pos);
                                for (auto & attr2 : *attr.value->attrs) {
                                    auto drvPath = checkDerivation(
                                        fmt("%s.%s.%s", name, attr.name, attr2.name),
                                        *attr2.value, *attr2.pos);
                                    if ((std::string) attr.name == settings.thisSystem.get())
                                        drvPaths.push_back({drvPath});
                                }
                            }
                        }

                        else if (name == "packages") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                state->forceAttrs(*attr.value, *attr.pos);
                                for (auto & attr2 : *attr.value->attrs)
                                    checkDerivation(
                                        fmt("%s.%s.%s", name, attr.name, attr2.name),
                                        *attr2.value, *attr2.pos);
                            }
                        }

                        else if (name == "apps") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                state->forceAttrs(*attr.value, *attr.pos);
                                for (auto & attr2 : *attr.value->attrs)
                                    checkApp(
                                        fmt("%s.%s.%s", name, attr.name, attr2.name),
                                        *attr2.value, *attr2.pos);
                            }
                        }

                        else if (name == "defaultPackage" || name == "devShell") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                checkDerivation(
                                    fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                            }
                        }

                        else if (name == "defaultApp") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                checkApp(
                                    fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                            }
                        }

                        else if (name == "legacyPackages") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs) {
                                checkSystemName(attr.name, *attr.pos);
                                // FIXME: do getDerivations?
                            }
                        }

                        else if (name == "overlay")
                            checkOverlay(name, vOutput, pos);

                        else if (name == "overlays") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs)
                                checkOverlay(fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                        }

                        else if (name == "nixosModule")
                            checkModule(name, vOutput, pos);

                        else if (name == "nixosModules") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs)
                                checkModule(fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                        }

                        else if (name == "nixosConfigurations") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs)
                                checkNixOSConfiguration(fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                        }

                        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 if (name == "defaultBundler")
                            checkBundler(name, vOutput, pos);

                        else if (name == "bundlers") {
                            state->forceAttrs(vOutput, pos);
                            for (auto & attr : *vOutput.attrs)
                                checkBundler(fmt("%s.%s", name, attr.name),
                                    *attr.value, *attr.pos);
                        }

                        else
                            warn("unknown flake output '%s'", name);

                    } catch (Error & e) {
                        e.addTrace(pos, hintfmt("while checking flake output '%s'", name));
                        throw;
                    }
                });
        }

        if (build && !drvPaths.empty()) {
            Activity act(*logger, lvlInfo, actUnknown, "running flake checks");
            store->buildPaths(drvPaths);
        }
    }
};

struct CmdFlakeInitCommon : virtual Args, EvalCommand
{
    std::string templateUrl = "templates";
    Path destDir;

    const Strings attrsPathPrefixes{"templates."};
    const LockFlags lockFlags{ .writeLockFile = false };

    CmdFlakeInitCommon()
    {
        addFlag({
            .longName = "template",
            .shortName = 't',
            .description = "the template to use",
            .labels = {"template"},
            .handler = {&templateUrl},
            .completer = {[&](size_t, std::string_view prefix) {
                completeFlakeRefWithFragment(
                    getEvalState(),
                    lockFlags,
                    attrsPathPrefixes,
                    {"defaultTemplate"},
                    prefix);
            }}
        });
    }

    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(attrsPathPrefixes), lockFlags);

        auto [cursor, attrPath] = installable.getCursor(*evalState);

        auto templateDir = cursor->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", "--force", "--" };
            for (auto & s : files) args.push_back(s);
            runProgram("git", true, args);
        }
    }
};

struct CmdFlakeInit : CmdFlakeInitCommon
{
    std::string description() override
    {
        return "create a flake in the current directory from a template";
    }

    std::string doc() override
    {
        return
          #include "flake-init.md"
          ;
    }

    CmdFlakeInit()
    {
        destDir = ".";
    }
};

struct CmdFlakeNew : CmdFlakeInitCommon
{
    std::string description() override
    {
        return "create a flake in the specified directory from a template";
    }

    std::string doc() override
    {
        return
          #include "flake-new.md"
          ;
    }

    CmdFlakeNew()
    {
        expectArgs({
            .label = "dest-dir",
            .handler = {&destDir},
            .completer = completePath
        });
    }
};

struct CmdFlakeClone : FlakeCommand
{
    Path destDir;

    std::string description() override
    {
        return "clone flake repository";
    }

    std::string doc() override
    {
        return
          #include "flake-clone.md"
          ;
    }

    CmdFlakeClone()
    {
        addFlag({
            .longName = "dest",
            .shortName = 'f',
            .description = "destination path",
            .labels = {"path"},
            .handler = {&destDir}
        });
    }

    void run(nix::ref<nix::Store> store) override
    {
        if (destDir.empty())
            throw Error("missing flag '--dest'");

        getFlakeRef().resolve(store).input.clone(destDir);
    }
};

struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
{
    std::string dstUri;

    CmdFlakeArchive()
    {
        addFlag({
            .longName = "to",
            .description = "URI of the destination Nix store",
            .labels = {"store-uri"},
            .handler = {&dstUri}
        });
    }

    std::string description() override
    {
        return "copy a flake and all its inputs to a store";
    }

    std::string doc() override
    {
        return
          #include "flake-archive.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        auto flake = lockFlake();

        auto jsonRoot = json ? std::optional<JSONObject>(std::cout) : std::nullopt;

        StorePathSet sources;

        sources.insert(flake.flake.sourceInfo->storePath);
        if (jsonRoot)
            jsonRoot->attr("path", store->printStorePath(flake.flake.sourceInfo->storePath));

        // FIXME: use graph output, handle cycles.
        std::function<void(const Node & node, std::optional<JSONObject> & jsonObj)> traverse;
        traverse = [&](const Node & node, std::optional<JSONObject> & jsonObj)
        {
            auto jsonObj2 = jsonObj ? jsonObj->object("inputs") : std::optional<JSONObject>();
            for (auto & [inputName, input] : node.inputs) {
                if (auto inputNode = std::get_if<0>(&input)) {
                    auto jsonObj3 = jsonObj2 ? jsonObj2->object(inputName) : std::optional<JSONObject>();
                    auto storePath =
                        dryRun
                        ? (*inputNode)->lockedRef.input.computeStorePath(*store)
                        : (*inputNode)->lockedRef.input.fetch(store).first.storePath;
                    if (jsonObj3)
                        jsonObj3->attr("path", store->printStorePath(storePath));
                    sources.insert(std::move(storePath));
                    traverse(**inputNode, jsonObj3);
                }
            }
        };

        traverse(*flake.lockFile.root, jsonRoot);

        if (!dryRun && !dstUri.empty()) {
            ref<Store> dstStore = dstUri.empty() ? openStore() : openStore(dstUri);
            copyPaths(store, dstStore, sources);
        }
    }
};

struct CmdFlakeShow : FlakeCommand
{
    bool showLegacy = false;

    CmdFlakeShow()
    {
        addFlag({
            .longName = "legacy",
            .description = "show the contents of the 'legacyPackages' output",
            .handler = {&showLegacy, true}
        });
    }

    std::string description() override
    {
        return "show the outputs provided by a flake";
    }

    std::string doc() override
    {
        return
          #include "flake-show.md"
          ;
    }

    void run(nix::ref<nix::Store> store) override
    {
        auto state = getEvalState();
        auto flake = std::make_shared<LockedFlake>(lockFlake());

        std::function<void(eval_cache::AttrCursor & visitor, const std::vector<Symbol> & attrPath, const std::string & headerPrefix, const std::string & nextPrefix)> visit;

        visit = [&](eval_cache::AttrCursor & visitor, const std::vector<Symbol> & attrPath, const std::string & headerPrefix, const std::string & nextPrefix)
        {
            Activity act(*logger, lvlInfo, actUnknown,
                fmt("evaluating '%s'", concatStringsSep(".", attrPath)));
            try {
                auto recurse = [&]()
                {
                    logger->cout("%s", headerPrefix);
                    auto attrs = visitor.getAttrs();
                    for (const auto & [i, attr] : enumerate(attrs)) {
                        bool last = i + 1 == attrs.size();
                        auto visitor2 = visitor.getAttr(attr);
                        auto attrPath2(attrPath);
                        attrPath2.push_back(attr);
                        visit(*visitor2, attrPath2,
                            fmt(ANSI_GREEN "%s%s" ANSI_NORMAL ANSI_BOLD "%s" ANSI_NORMAL, nextPrefix, last ? treeLast : treeConn, attr),
                            nextPrefix + (last ? treeNull : treeLine));
                    }
                };

                auto showDerivation = [&]()
                {
                    auto name = visitor.getAttr(state->sName)->getString();

                    /*
                    std::string description;

                    if (auto aMeta = visitor.maybeGetAttr("meta")) {
                        if (auto aDescription = aMeta->maybeGetAttr("description"))
                            description = aDescription->getString();
                    }
                    */

                    logger->cout("%s: %s '%s'",
                        headerPrefix,
                        attrPath.size() == 2 && attrPath[0] == "devShell" ? "development environment" :
                        attrPath.size() == 3 && attrPath[0] == "checks" ? "derivation" :
                        attrPath.size() >= 1 && attrPath[0] == "hydraJobs" ? "derivation" :
                        "package",
                        name);
                };

                if (attrPath.size() == 0
                    || (attrPath.size() == 1 && (
                            attrPath[0] == "defaultPackage"
                            || attrPath[0] == "devShell"
                            || attrPath[0] == "nixosConfigurations"
                            || attrPath[0] == "nixosModules"
                            || attrPath[0] == "defaultApp"
                            || attrPath[0] == "templates"))
                    || ((attrPath.size() == 1 || attrPath.size() == 2)
                        && (attrPath[0] == "checks"
                            || attrPath[0] == "packages"
                            || attrPath[0] == "apps"))
                    )
                {
                    recurse();
                }

                else if (
                    (attrPath.size() == 2 && (attrPath[0] == "defaultPackage" || attrPath[0] == "devShell"))
                    || (attrPath.size() == 3 && (attrPath[0] == "checks" || attrPath[0] == "packages"))
                    )
                {
                    if (visitor.isDerivation())
                        showDerivation();
                    else
                        throw Error("expected a derivation");
                }

                else if (attrPath.size() > 0 && attrPath[0] == "hydraJobs") {
                    if (visitor.isDerivation())
                        showDerivation();
                    else
                        recurse();
                }

                else if (attrPath.size() > 0 && attrPath[0] == "legacyPackages") {
                    if (attrPath.size() == 1)
                        recurse();
                    else if (!showLegacy)
                        logger->cout("%s: " ANSI_YELLOW "omitted" ANSI_NORMAL " (use '--legacy' to show)", headerPrefix);
                    else {
                        if (visitor.isDerivation())
                            showDerivation();
                        else if (attrPath.size() <= 2)
                            // FIXME: handle recurseIntoAttrs
                            recurse();
                    }
                }

                else if (
                    (attrPath.size() == 2 && attrPath[0] == "defaultApp") ||
                    (attrPath.size() == 3 && attrPath[0] == "apps"))
                {
                    auto aType = visitor.maybeGetAttr("type");
                    if (!aType || aType->getString() != "app")
                        throw EvalError("not an app definition");
                    logger->cout("%s: app", headerPrefix);
                }

                else if (
                    (attrPath.size() == 1 && attrPath[0] == "defaultTemplate") ||
                    (attrPath.size() == 2 && attrPath[0] == "templates"))
                {
                    auto description = visitor.getAttr("description")->getString();
                    logger->cout("%s: template: " ANSI_BOLD "%s" ANSI_NORMAL, headerPrefix, description);
                }

                else {
                    logger->cout("%s: %s",
                        headerPrefix,
                        attrPath.size() == 1 && attrPath[0] == "overlay" ? "Nixpkgs overlay" :
                        attrPath.size() == 2 && attrPath[0] == "nixosConfigurations" ? "NixOS configuration" :
                        attrPath.size() == 2 && attrPath[0] == "nixosModules" ? "NixOS module" :
                        ANSI_YELLOW "unknown" ANSI_NORMAL);
                }
            } catch (EvalError & e) {
                if (!(attrPath.size() > 0 && attrPath[0] == "legacyPackages"))
                    throw;
            }
        };

        auto cache = openEvalCache(*state, flake);

        visit(*cache->getRoot(), {}, fmt(ANSI_BOLD "%s" ANSI_NORMAL, flake->flake.lockedRef), "");
    }
};

struct CmdFlakePrefetch : FlakeCommand, MixJSON
{
    CmdFlakePrefetch()
    {
    }

    std::string description() override
    {
        return "download the source tree denoted by a flake reference into the Nix store";
    }

    std::string doc() override
    {
        return
          #include "flake-prefetch.md"
          ;
    }

    void run(ref<Store> store) override
    {
        auto originalRef = getFlakeRef();
        auto resolvedRef = originalRef.resolve(store);
        auto [tree, lockedRef] = resolvedRef.fetchTree(store);
        auto hash = store->queryPathInfo(tree.storePath)->narHash;

        if (json) {
            auto res = nlohmann::json::object();
            res["storePath"] = store->printStorePath(tree.storePath);
            res["hash"] = hash.to_string(SRI, true);
            logger->cout(res.dump());
        } else {
            notice("Downloaded '%s' to '%s' (hash '%s').",
                lockedRef.to_string(),
                store->printStorePath(tree.storePath),
                hash.to_string(SRI, true));
        }
    }
};

struct CmdFlake : NixMultiCommand
{
    CmdFlake()
        : MultiCommand({
                {"update", []() { return make_ref<CmdFlakeUpdate>(); }},
                {"info", []() { return make_ref<CmdFlakeInfo>(); }},
                {"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>(); }},
                {"prefetch", []() { return make_ref<CmdFlakePrefetch>(); }},
            })
    {
    }

    std::string description() override
    {
        return "manage Nix flakes";
    }

    std::string doc() override
    {
        return
          #include "flake.md"
          ;
    }

    void run() override
    {
        if (!command)
            throw UsageError("'nix flake' requires a sub-command.");
        settings.requireExperimentalFeature("flakes");
        command->second->prepare();
        command->second->run();
    }
};

static auto rCmdFlake = registerCommand<CmdFlake>("flake");