1 files changed, 32 insertions, 1 deletions

diff --git a/src/libstore/realisation.hh b/src/libstore/realisation.hh
index a18cf2aa8..3922d1267 100644
--- a/src/libstore/realisation.hh
+++ b/src/libstore/realisation.hh
@@ -13,9 +13,25 @@ namespace nix {
 
 class Store;
 
+/**
+ * A general `Realisation` key.
+ *
+ * This is similar to a `DerivedPath::Opaque`, but the derivation is
+ * identified by its "hash modulo" instead of by its store path.
+ */
 struct DrvOutput {
-    // The hash modulo of the derivation
+    /**
+     * The hash modulo of the derivation.
+     *
+     * Computed from the derivation itself for most types of
+     * derivations, but computed from the (fixed) content address of the
+     * output for fixed-output derivations.
+     */
     Hash drvHash;
+
+    /**
+     * The name of the output.
+     */
     std::string outputName;
 
     std::string to_string() const;
@@ -60,6 +76,21 @@ struct Realisation {
     GENERATE_CMP(Realisation, me->id, me->outPath);
 };
 
+/**
+ * Collection type for a single derivation's outputs' `Realisation`s.
+ *
+ * Since these are the outputs of a single derivation, we know the
+ * output names are unique so we can use them as the map key.
+ */
+typedef std::map<std::string, Realisation> SingleDrvOutputs;
+
+/**
+ * Collection type for multiple derivations' outputs' `Realisation`s.
+ *
+ * `DrvOutput` is used because in general the derivations are not all
+ * the same, so we need to identify firstly which derivation, and
+ * secondly which output of that derivation.
+ */
 typedef std::map<DrvOutput, Realisation> DrvOutputs;
 
 struct OpaquePath {