Make the Derived Path family of types inductive for dynamic derivations

We want to be able to write down `foo.drv^bar.drv^baz`:
`foo.drv^bar.drv` is the dynamic derivation (since it is itself a
derivation output, `bar.drv` from `foo.drv`).

To that end, we create `Single{Derivation,BuiltPath}` types, that are
very similar except instead of having multiple outputs (in a set or
map), they have a single one. This is for everything to the left of the
rightmost `^`.

`NixStringContextElem` has an analogous change, and now can reuse
`SingleDerivedPath` at the top level. In fact, if we ever get rid of
`DrvDeep`, `NixStringContextElem` could be replaced with
`SingleDerivedPath` entirely!

Important note: some JSON formats have changed.

We already can *produce* dynamic derivations, but we can't refer to them
directly. Today, we can merely express building or example at the top
imperatively over time by building `foo.drv^bar.drv`, and then with a
second nix invocation doing `<result-from-first>^baz`, but this is not
declarative. The ethos of Nix of being able to write down the full plan
everything you want to do, and then execute than plan with a single
command, and for that we need the new inductive form of these types.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

1 files changed, 5 insertions, 5 deletions

diff --git a/src/nix/build.cc b/src/nix/build.cc
index ad1842a4e..479100186 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -9,18 +9,18 @@
 
 using namespace nix;
 
-nlohmann::json derivedPathsToJSON(const DerivedPaths & paths, ref<Store> store)
+static nlohmann::json derivedPathsToJSON(const DerivedPaths & paths, Store & store)
 {
     auto res = nlohmann::json::array();
     for (auto & t : paths) {
-        std::visit([&res, store](const auto & t) {
+        std::visit([&](const auto & t) {
             res.push_back(t.toJSON(store));
         }, t.raw());
     }
     return res;
 }
 
-nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWithResult> & buildables, ref<Store> store)
+static nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWithResult> & buildables, const Store & store)
 {
     auto res = nlohmann::json::array();
     for (auto & b : buildables) {
@@ -125,7 +125,7 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
             printMissing(store, pathsToBuild, lvlError);
 
             if (json)
-                logger->cout("%s", derivedPathsToJSON(pathsToBuild, store).dump());
+                logger->cout("%s", derivedPathsToJSON(pathsToBuild, *store).dump());
 
             return;
         }
@@ -136,7 +136,7 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
             installables,
             repair ? bmRepair : buildMode);
 
-        if (json) logger->cout("%s", builtPathsWithResultToJSON(buildables, store).dump());
+        if (json) logger->cout("%s", builtPathsWithResultToJSON(buildables, *store).dump());
 
         if (outLink != "")
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())