Add some explanation to src/eval/fixpoint.rs

src/eval/fixpoint.rs

@@ -88,6 +88,25 @@ pub fn rec_env<'a, I: Iterator<Item = (&'a Ident, &'a Field)>, C: Cache>(
                 let with_ctr_applied = PendingContract::apply_all(
                     RichTerm::new(Term::Var(id_value), value.pos),
                     field.pending_contracts.iter().cloned().flat_map(|ctr| {
+                        // This operation is the heart of our preliminary fix for
+                        // [#1161](https://github.com/tweag/nickel/issues/1161). Whenever we detect
+                        // the presence of free type variables in a contract, by witnessing a nonempty
+                        // type environment in the label, we need to not just apply the original
+                        // contract but also its dual. The rationale here is that a record field that
+                        // recursively depends on another of type `T`, say, should be considered a
+                        // function with domain `T`. Consequently, the same contract that would be a
+                        // applied to the argument of a function of type `T -> Dyn` should be applied
+                        // to the recursive reference.
+                        //
+                        // Thus, the recursive reference must satisfy the contract for `T` as well as
+                        // the "dual" contract `T.dualize()`; the latter is defined to be the domain
+                        // contract for a function of type `T -> Dyn`. This sublety only matters if
+                        // `T` contains free type variables because only then does `T.dualize()` differ
+                        // from `T` at all.
+                        //
+                        // We expect to implement a way of solving this dilemma without essentially
+                        // applying every contract twice. This will likely involve a rewriting of
+                        // contracts corresponding to free variables which is yet to be proved sound.
                         if ctr.label.type_environment.is_empty() {
                             vec![ctr]
                         } else {