[dkfellows]

Tcl uses a memory model internally where each piece of memory belongs strictly to the thread that allocated it, with this enforced in lots of places. There are a few loopholes past it (for process-wide concepts such as the current working directory cache, or for inter-thread messaging) but by and large you write what appears to be single-threaded code.

There's support for deep coroutines and non-blocking I/O so it isn't limiting. You only really need multiple threads when dealing with compute-heavy code or a particularly crufty API.