Allow using RegisterPrimop to define constants.

This enables plugins to add new constants, as well as new primops.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libexpr/primops.hh b/src/libexpr/primops.hh
index 39d23b04a5ce..31bf3f84f6c7 100644
--- a/src/libexpr/primops.hh
+++ b/src/libexpr/primops.hh
@@ -9,6 +9,9 @@ struct RegisterPrimOp
 {
     typedef std::vector<std::tuple<std::string, size_t, PrimOpFun>> PrimOps;
     static PrimOps * primOps;
+    /* You can register a constant by passing an arity of 0. fun
+       will get called during EvalState initialization, so there
+       may be primops not yet added and builtins is not yet sorted. */
     RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun);
 };