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/libexpr/eval.hh b/src/libexpr/eval.hh
index 29d0f05a1..0268a2a12 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -22,7 +22,7 @@ namespace nix {
 class Store;
 class EvalState;
 class StorePath;
-struct DerivedPath;
+struct SingleDerivedPath;
 enum RepairFlag : bool;
 
 
@@ -532,12 +532,12 @@ public:
     StorePath coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
     /**
-     * Part of `coerceToDerivedPath()` without any store IO which is exposed for unit testing only.
+     * Part of `coerceToSingleDerivedPath()` without any store IO which is exposed for unit testing only.
      */
-    std::pair<DerivedPath, std::string_view> coerceToDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx);
+    std::pair<SingleDerivedPath, std::string_view> coerceToSingleDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx);
 
     /**
-     * Coerce to `DerivedPath`.
+     * Coerce to `SingleDerivedPath`.
      *
      * Must be a string which is either a literal store path or a
      * "placeholder (see `DownstreamPlaceholder`).
@@ -551,7 +551,7 @@ public:
      * source of truth, and ultimately tells us what we want, and then
      * we ensure the string corresponds to it.
      */
-    DerivedPath coerceToDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
+    SingleDerivedPath coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
 
 public: