Use __toString when coercing sets to strings.

For example, "${{ foo = "bar"; __toString = x: x.foo; }}" evaluates
to "bar".

With this, we can delay calling functions like mkDerivation,
buildPythonPackage, etc. until we actually need a derivation, enabling
overrides and other modifications to happen by simple attribute set
update.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index eb55f6d4d431..2d7b7bcdcb62 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -69,7 +69,7 @@ public:
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
-        sFile, sLine, sColumn, sFunctor;
+        sFile, sLine, sColumn, sFunctor, sToString;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they