Type of a DLS key

new_key f returns a new key bound to initialiser f for accessing domain-local variables.

If split_from_parent is provided, spawning a domain will derive the child value (for this key) from the parent value.

Note that the split_from_parent call is computed in the parent domain, and is always computed regardless of whether the child domain will use it. If the splitting function is expensive or requires client-side computation, consider using 'a Lazy.t key.

get k returns v if a value v is associated to the key k on the calling domain's domain-local state. Sets k's value with its initialiser and returns it otherwise.

set k v updates the calling domain's domain-local state to associate the key k with value v. It overwrites any previous values associated to k, which cannot be restored later.