Allow external code using libnixexpr to add types

Code that links to libnixexpr (e.g. plugins loaded with importNative, or
nix-exec) may want to provide custom value types and operations on
values of those types. For example, nix-exec is currently using sets
where a custom IO value type would be more appropriate. This commit
provides a generic hook for such types in the form of tExternal and the
ExternalBase virtual class, which contains all functions necessary for
libnixexpr's type-polymorphic functions (e.g. `showType`) to be
implemented.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 3934a83eec25..3cecc33dc299 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -144,12 +144,23 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
         }
 
+        case tExternal:
+            v.external->printValueAsXML(state, strict, location, doc, context, drvsSeen);
+            break;
+
         default:
             doc.writeEmptyElement("unevaluated");
     }
 }
 
 
+void ExternalValueBase::printValueAsXML(EvalState & state, bool strict,
+    bool location, XMLWriter & doc, PathSet & context, PathSet & drvsSeen)
+{
+    doc.writeEmptyElement("unevaluated");
+}
+
+
 void printValueAsXML(EvalState & state, bool strict, bool location,
     Value & v, std::ostream & out, PathSet & context)
 {