diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index bf55a9c9c..660f474b8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -979,10 +979,6 @@ void EvalState::mkSingleDerivedPathString(const SingleDerivedPath & p, Value & v
         });
 }
 
-/* Create a thunk for the delayed computation of the given expression
-   in the given environment.  But if the expression is a variable,
-   then look it up right away.  This significantly reduces the number
-   of thunks allocated. */
 Value * Expr::maybeThunk(EvalState & state, Env & env)
 {
     Value * v = state.allocValue();
diff --git a/src/libexpr/include/nix/expr/nixexpr.hh b/src/libexpr/include/nix/expr/nixexpr.hh
index 414eb5116..aa62760d8 100644
--- a/src/libexpr/include/nix/expr/nixexpr.hh
+++ b/src/libexpr/include/nix/expr/nixexpr.hh
@@ -99,7 +99,16 @@ struct Expr
     virtual ~Expr() {};
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
     virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+
+    /** Normal evaluation, implemented directly by all subclasses. */
     virtual void eval(EvalState & state, Env & env, Value & v);
+
+    /**
+     * Create a thunk for the delayed computation of the given expression
+     * in the given environment. But if the expression is a variable,
+     * then look it up right away. This significantly reduces the number
+     * of thunks allocated.
+     */
     virtual Value * maybeThunk(EvalState & state, Env & env);
     virtual void setName(Symbol name);
     virtual void setDocComment(DocComment docComment) {};