The reference type in ML/OCaml is an opt-in feature, a container with it's own special type. In Haskell, you can also capture IORefs or similar, but any reads/w

The reference type in ML/OCaml is an opt-in feature, a container with it's own special type. In Haskell, you can also capture IORefs or similar, but any reads/writes to them are effectful and captured in the types. The point is that Python does this all the time as the default. Any captured variable could potentially change under your feet. It is rarely useful and most beginners do not expect it, hence it is commonly considered a "gotcha".