Document the derived path types.

src/libstore/derived-path.hh

@@ -2,7 +2,6 @@
 
 #include "util.hh"
 #include "path.hh"
-#include "path.hh"
 
 #include <optional>
 
@@ -12,6 +11,13 @@ namespace nix {
 
 class Store;
 
+/**
+ * An opaque derived path.
+ *
+ * Opaque derived paths are just store paths, and fully evaluated. They
+ * cannot be simplified further. Since they are opaque, they cannot be
+ * built, but they can fetched.
+ */
 struct DerivedPathOpaque {
     StorePath path;
 
@@ -20,6 +26,18 @@ struct DerivedPathOpaque {
     static DerivedPathOpaque parse(const Store & store, std::string_view);
 };
 
+/**
+ * A derived path that is built from a derivation
+ *
+ * Built derived paths are pair of a derivation and some output names.
+ * They are evaluated by building the derivation, and then replacing the
+ * output names with the resulting outputs.
+ *
+ * Note that does mean a derived store paths evaluates to multiple
+ * opaque paths, which is sort of icky as expressions are supposed to
+ * evaluate to single values. Perhaps this should have just a single
+ * output name.
+ */
 struct DerivedPathBuilt {
     StorePath drvPath;
     std::set<std::string> outputs;
@@ -33,6 +51,16 @@ using _DerivedPathRaw = std::variant<
     DerivedPathBuilt
 >;
 
+/**
+ * A "derived path" is a very simple sort of expression that evaluates
+ * to (concrete) store path. It is either:
+ *
+ * - opaque, in which case it is just a concrete store path with
+ *   possibly no known derivation
+ *
+ * - built, in which case it is a pair of a derivation path and an
+ *   output name.
+ */
 struct DerivedPath : _DerivedPathRaw {
     using Raw = _DerivedPathRaw;
     using Raw::Raw;
@@ -48,6 +76,11 @@ struct DerivedPath : _DerivedPathRaw {
     static DerivedPath parse(const Store & store, std::string_view);
 };
 
+/**
+ * A built derived path with hints in the form of optional concrete output paths.
+ *
+ * See 'DerivedPathWithHints' for more an explanation.
+ */
 struct DerivedPathWithHintsBuilt {
     StorePath drvPath;
     std::map<std::string, std::optional<StorePath>> outputs;
@@ -61,6 +94,21 @@ using _DerivedPathWithHintsRaw = std::variant<
     DerivedPathWithHintsBuilt
 >;
 
+/**
+ * A derived path with hints in the form of optional concrete output paths in the built case.
+ *
+ * This type is currently just used by the CLI. The paths are filled in
+ * during evaluation for derivations that know what paths they will
+ * produce in advanced, i.e. input-addressed or fixed-output content
+ * addressed derivations.
+ *
+ * That isn't very good, because it puts floating content-addressed
+ * derivations "at a disadvantage". It would be better to never rely on
+ * the output path of unbuilt derivations, and exclusively use the
+ * realizations types to work with built derivations' concrete output
+ * paths.
+ */
+// FIXME Stop using and delete this, or if that is not possible move out of libstore to libcmd.
 struct DerivedPathWithHints : _DerivedPathWithHintsRaw {
     using Raw = _DerivedPathWithHintsRaw;
     using Raw::Raw;