[evincarofautumn]

I think I see what you’re getting at. If I create an empty mutable list and send a reference to it to two different threads A and B, where A adds an integer to the list and B adds a string, then it’s nondeterministic which thread raises a type error.

And degrading the type to a union (int ∨ string) as I mentioned is still nondeterministic even though unions form a lattice—I can recover determinism when writing to the list in both A and B, but supposing A writes first, B writes second, and then A reads expecting only integers, it gets a type error because it hasn’t checked for the string case.

But I’d argue that 1. this is bad form and you want a type error somewhere and 2. you have this problem already with type-changing mutable references in a dynamically typed multithreaded language. (Say just “int” and “string” instead of “list of int” and “list of string”.)

[catnaroek]

I need a container type that can be empty, because I need the monomorphic type of the cell to be undecided until the reference has been handed out to the worker threads.

[johncolanduoni]

If your program doesn't have a race condition, then you're fine. And if it does, then the list being monomorphic is not going to help (try your scenario with a C++ vector or Java ArrayList).