mark it

src/Init/Prelude.lean

@@ -4083,7 +4083,7 @@ Actions in the resulting monad are functions that take the local value as a para
 ordinary actions in `m`.
 -/
 def ReaderT (ρ : Type u) (m : Type u → Type v) (α : Type u) : Type (max u v) :=
-  ρ → m α
+  (a : @&ρ) → m α
 
 /--
 Interpret `ρ → m α` as an element of `ReaderT ρ m α`.