Concurrency is hard. Immutability skates around the whole problem. A parser should be immutable, parsing state should be limited to a particular invocation of t

Concurrency is hard. Immutability skates around the whole problem. A parser should be immutable, parsing state should be limited to a particular invocation of the parser. Of course, mutable state cannot be abolished everywhere (often for efficiency reasons). In such cases, you should indeed understand the concurrency primitives.