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Make the Derived Path family of types inductive for dynamic derivations

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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>
This commit is contained in:
John Ericson 2023-01-15 17:39:04 -05:00
parent d00fe5f225
commit 60b7121d2c
48 changed files with 1136 additions and 267 deletions
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10
src/libexpr/eval.hh
View file

@ -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: