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, 2 deletions

diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 8b3468009..bfc731744 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -106,7 +106,10 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
                 contextInfos[std::move(d.drvPath)].allOutputs = true;
             },
             [&](NixStringContextElem::Built && b) {
-                contextInfos[std::move(b.drvPath)].outputs.emplace_back(std::move(b.output));
+                // FIXME should eventually show string context as is, no
+                // resolving here.
+                auto drvPath = resolveDerivedPath(*state.store, *b.drvPath);
+                contextInfos[std::move(drvPath)].outputs.emplace_back(std::move(b.output));
             },
             [&](NixStringContextElem::Opaque && o) {
                 contextInfos[std::move(o.path)].path = true;
@@ -222,7 +225,7 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
             for (auto elem : iter->value->listItems()) {
                 auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
                 context.emplace(NixStringContextElem::Built {
-                    .drvPath = namePath,
+                    .drvPath = makeConstantStorePathRef(namePath),
                     .output = std::string { outputName },
                 });
             }